Table of contents

The attached PDF contains, the full preface, a list of the scientific committee, former LAPD chairmen, local organizers, previous locations of LAPD meetings, participants email contacts and a list of the contributed papers.

The Fourteenth International Symposium on Laser-Aided Plasma Diagnostics (LAPD14), was held from 21–24 September 2009 in Castelbrando, Treviso, Italy. The series of LAPD symposia was originally started at Kyushu University in 1983, and since then it has been organized every two years alternately in Japan, Europe and the United States, traveling around the world five times.

Each LAPD Symposium brings together scientists working in different disciplines all related to the diagnostics of any type of plasma by laser or similar techniques. Researchers working on nuclear fusion, industrial process, low temperature plasma chemistry, laser development and material science, are invited to present prominent new diagnostic developments, with the aim of synergetic discussions. The broad spectrum of contributions represents one of the strengths of this symposium, which is an important, unique and fruitful source of cross-fertilization between these fields and a forum of discussions.

The scope of LAPD14 was very broad, including many techniques related to laser probing of plasmas: incoherent and coherent Thomson scattering, polarimetry, interferometry, reflectometry, laser induced fluorescence, laser absorption spectroscopy, laser photodetachment spectroscopy, cavity ringdown spectroscopy, Raman scattering, reflectometry, microwave diagnostics and related laser and hardware developments. LAPD14 was attended by 66 researchers, from 15 different countries who presented a total of 57 papers (13 general, 12 topical, 10 short talks and 23 poster contributions). It is a tradition of LAPD that the first lecture of each meeting, which is more general and aims to review prominent new developments, is called 'the Akazaki lecture' in honor of Professor Masanori Akazaki of Kyushu University, who was the first Chairman of LAPD from 1983 through 1989. The Akazaki lecture of LAPD14 was given by John Sheffield, who presented his work for the new edition of his worldwide known book Plasma Scattering of Electromagnetic Radiation.

The site of LAPD14 was chosen for its comfortable environment and the superb location: Castelbrando is an ancient castle whose origin goes back to the Roman age, a fortress built to watch over one of the ancient routes crossing the Alps. Completely restored 10 years ago it is now a high class resort for business meetings and residential holidays, with conference and meeting rooms, restaurants, shops, spa and a beauty farm. The quality of the location and the quietness and beauty of the surroundings made Castelbrando an ideal site for LAPD14 and contributed to the success of the Symposium.

I hope that this volume of Journal of Physics: Conference Series will serve as a valuable record of the Symposium to all workers in the field. On behalf of the Local Organizing Committee, I would like to thank all participants for their contributions to this Symposium. I also acknowledge all members of the international scientific committee and Professor Francesco Gnesotto, director of Consorzio RFX, for their support and cooperation.

Leonardo Giudicotti (Chairman of the Local Organizer Commitee)

INTERNATIONAL SCIENTIFIC COMMITTEE:

• A. J. H. Donné, FOM-Institute voor Plasmafysica, The Netherlands (Chairman)

• U. Czarnetzki, Ruhr University, Bochum, Germany

• B. Graham, The Queen's University, Belfast, UK

• G. Hebner, Sandia National Laboratories, Albuquerque, USA

• K. Kawahata, National Institute for Fusion Science, Japan

• N. C. Luhmann, Jr., University of California, Davis, USA

• S. Sasaki, Nagoya University, Nagoya, Japan

• H. Soltwisch, Ruhr University, Bochum, Germany

• P. P. Woskov, Massachusetts Institute of Technology, Cambridge, USA

• M. Akazaki (1983 - 1989)

• D. E. Evans (1989 - 1993)

• H. F. Döbele (1993 - 1999)

• K. Muraoka (1999 - 2003)

• A. J. H. Donné (2003 - )

• Leonardo Giudicotti, Consorzio RFX and Padova University (Chairman)

• Roberto Pasqualotto, Consorzio RFX

• Margherita Basso, Consorzio RFX

• Santolo De Benedictis, Institute of Inorganic Methodologies and Plasmas, CNR

• Paolo Innocente, Consorzio RFX

• Alberto Alfier, Consorzio RFX

• Enrico Scek Osman, Consorzio RFX

1983- Fukuoka, Japan: K. Muraoka (Organizer) 1985- Oxford, U.K.: D. Evans (Organizer) 1987- Lake Arrowhead, USA: N. C. Luhmann, Jr. (Organizer) 1989- Fukuoka, Japan: K. Muraoka (Organizer) 1991- Bad Honnef, Germany: F. Döbele (Organizer) 1993- Bar Harbor, USA: P. Woskov (Organizer) 1995- Fukuoka, Japan: K. Muraoka (Organizer) 1997- Doorwerth, Netherlands: A. J. H. Donné (Organizer) 1999- Lake Tahoe, USA: N. C. Luhmann, Jr. (Organizer) 2001- Fukuoka, Japan: K. Muraoka (Organizer) 2003- Les Houches, France: N. Sadeghi (Organizer) 2005- Snowbird, USA: N. C. Luhmann, Jr. (Organizer) 2007- Takayama, Japan: K. Kawahata (Organizer) 2009- Castelbrando, Italy: L. Giudicotti (Organizer)

These are the abstracts of ten contributions presented at the conference, but not submitted for publication in these proceedings.

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

The monograph Plasma Scattering of Electromagnetic Radiation was published by Academic Press in 1975. A Russian edition, Atomidzat, came out in 1978. An updated version is being prepared by D. Froula, S. Glenzer. N Luhmann, and J. Sheffield for publication in 2010 by Elsevier. The new version will discuss the broader applications of Thomson scattering, which include the full range of plasmas used in research and industry. The expansion of the field has been made possible by the growing number of powerful radiation sources (from X-rays to microwaves), detectors, and innovative techniques. When the book was published, the highest temperatures in laboratory plasmas were around 2 keV for the electrons. Compare this to today's 25 keV where the relativistic effects are dramatic. The application to low temperature plasmas with T_e in the range of 1 − 30+ eV, important in industry, has grown. Important capabilities have been developed in the areas of energetic particle, micro-instability, and high energy density plasma measurements. For the future, we look forward to the use of scattering as a diagnostic on the large new fusion facilities–NIF, LMJ, and ITER.

We have been studied the advanced Thomson scattering diagnostics from viewpoints of new concepts, laser technology and spectrum analysis. This paper summarizes results of development on technologies for advanced Thomson scattering diagnostics.

In this paper we review our recent work based on Laser Induced Fluorescence (LIF) diagnostics applied to Dielectric Barrier Discharges (DBD) at atmospheric pressures. Emphasis is given to the main issues that have to be faced in this application through the detailed description of the most difficult case, that of CH LIF detection. The application to kinetic studies of LIF diagnostics is then illustrated through the application of LIF to the CN radical and of Optical-Optical Double Resonance (OODR) LIF to the measurement of N₂(A³Σ⁺_u) metastable.

The problem of degraded energy confinement due to excessive transport across the magnetic field remains one of the critical issues for magnetically confined plasma. Radial transport of ion energy is relatively well understood, but that of the electrons has remained as anomalous, greatly exceeding the neoclassical prediction. It has been suggested that the anomalous electron thermal transport is explained by an electron gyro-scale turbulence driven by the electron temperature gradient (ETG) instability. A 280-GHz collective Thomson scattering system has been employed to address the electron gyro-scale fluctuations in National Spherical Torus Experiment (NSTX) plasmas. The spatial resolution of the targeting wavenumber is greatly affected by the configuration of the magnetic field since the radial fluctuation is perpendicular to the local magnetic field line. The effect of the toroidal field curvature and magnetic shear on the spatial resolution of the scattering system is investigated numerically. An absolute power calibration was performed to determine the power response of the heterodyne detection system. These spatial resolution studies and absolute power calibration were applied to estimate the normalized density fluctuations from the measured scattering signals in NSTX plasmas.

The recent development of quantum cascade lasers (QCLs) offers an attractive new option for the monitoring and control of industrial plasma processes and for trace-gas analysis as well as for highly time-resolved studies on the kinetics of plasma processes. The contribution reviews selected examples of the application of QCLs for infrared absorption studies in basic research and for plasma monitoring and control in industry.

In magnetically confined fusion experiments interferometry is commonly used to measure plasma electron density. This because interferometry is a very reliable technique, not affected by calibration problem. The main drawbacks of interferometers are the integral characteristic of the measurement and the fringe loosing problem. Scanning beam interferometry can always overcome the former problem and in some cases the second as well. The advantages of the scanning beam technique can better exploited in two colours vibration compensated medium infra-red (MIR) interferometers. Indeed the short wavelength of the probing beams provide small beam sizes allowing plasma density measurement along many non-overlapping paths, furthermore the vibration compensation system relaxes the requirement of massive interferometer structures embracing the fusion device, actually allowing to install back-reflecting mirror directly attached to the experimental machine.

As an example of application of this technique the two-color medium-infra-red-compensated scanning beam interferometer installed on the Frascati Tokamak Upgrade (FTU) experiment is presented. We present also a preliminary design of a scanning interferometer for the new proposed Fusion Advanced Studies Torus (FAST) experiment. The interferometer uses retroreflectors installed inside the vessel to back reflect the scanning beams; this will allow realizing scanning beams measuring from the plasma edge. This feature in principle can solve the fundamental problem of interferometers: the fringe loosing problem.

The motional Stark effect (MSE) diagnostic is the worldwide standard technique for internal magnetic field pitch angle measurements in magnetized plasmas. Traditionally, it is based on using polarimetry to measure the polarization direction of light emitted from a hydrogenic species in a neutral beam. As the beam passes through the magnetized plasma at a high velocity, in its rest frame it perceives a Lorentz electric field. This field causes the H-alpha emission to be split and polarized. A new technique under development adds laser-induced fluorescence (LIF) to a diagnostic neutral beam (DNB) for an MSE measurement that will enable radially resolved magnetic field magnitude as well as pitch angle measurements in even low-field (<1 T) experiments. An MSE-LIF system will be installed on the National Spherical Torus Experiment (NSTX) at the Princeton Plasma Physics Laboratory. It will enable reconstructions of the plasma pressure, q-profile and current as well as, in conjunction with the existing MSE system, measurements of radial electric fields.

The radial flow of neutral particles in an electron-cyclotron-resonance (ECR) argon plasma has been measured by using a newly developed high resolution laser induced fluorescence (LIF) measurement system. The flow velocity is determined by the Doppler shift of the LIF spectrum. A very high accuracy calibration of an excitation laser has been achieved by installing a saturated absorption spectroscopy unit into the LIF system. We utilized the Lamb dip, which is obtained by the saturated absorption spectroscopy, as a frequency standard for the determination of the flow velocity. The use of Lamb dip is particularly appropriate for the flow velocity measurement, since the position of Lamb dip in the frequency scale is Doppler-shift free and is not disturbed by the motion of reference medium. By utilizing the Lamb dip as the frequency standard, the reliability and stability of the laser frequency calibration is increased. From the radial measurements of LIF spectra of metastable argon atoms at the microwave power of 250 W and 5 kW, it is found that there exists an inward flow of neutral particles in the both plasmas. Both of the radial flow velocity profiles peak around 4 cm from the center, which is comparable with the radius of the boundary of the E×B rotation and anti-E×B rotation in the 5 kW discharge. The maximum flow velocity increases with the microwave power.

Interaction of a plasma with a surface results in chemical and physical restructuring of the surface as well as the plasma in the vicinity of the surface. Studying such a reorganization of the atoms and molecules in the surface layer requires optical tools that can penetrate the plasma environment. At the same time, surface specificity is required. Sum Frequency Generation (SFG) is an optical method that fulfills these requirements. SFG has been developed into a surface specific probe during the eighties and nineties. Nowadays SFG is routinely applied in the research of complex interfaces. In such experiments, liquid/gas, solid/gas, solid/liquid, or liquid/liquid interfaces are probed, and the chemical surface composition, orientational distribution, order and chirality can be retrieved. An application to investigate plasma-wall interactions is feasible too.

Experimental knowledge of the fast ion physics in magnetically confined plasmas is essential. The collective Thomson scattering (CTS) diagnostic is capable of measuring localized 1D ion velocity distributions and anisotropies dependent on the angle to the magnetic field. The CTS installed at ASDEX-Upgrade (AUG) uses mm-waves generated by the 1 MW dual frequency gyrotron. The successful commissioning the CTS at AUG enabled first scattering experiments and the consequent milestone of first fast ion distribution measurements on AUG presented in this paper. The first fast ion distribution results have already uncovered some physics of confined fast ions at the plasma centre with off-axis neutral beam heating. However, CTS experiments on AUG H-mode plasmas have also uncovered some unexpected signals not related to scattering that required additional analysis and treatment of the data. These secondary emission signals are generated from the plasma-gyrotron interaction therefore contain additional physics. Despite their existence that complicate the fast ion analysis, they do not prevent the diagnostic's capability to infer the fast ion distribution function on AUG.

Laser scattering experiments were performed in high pressure (100s of Torr) parallel-plate, slot-type DC microdischarges operating in argon or nitrogen. Laser Thomson Scattering (LTS) and Rotational Raman Scattering were employed in a novel, backscattering, confocal configuration. LTS allows direct and simultaneous measurement of both electron density (n_e) and electron temperature (T_e). For 50 mA current and over the pressure range of 300 − 700 Torr, LTS yielded T_e = 0.9 ± 0.3 eV and n_e = (6 ± 3)·10¹³ cm⁻³, in reasonable agreement with the predictions of a mathematical model. Rotational Raman spectroscopy (RRS) was employed for absolute calibration of the LTS signal. RRS was also applied to measure the 3D gas temperature (T_g) in nitrogen DC microdischarges. In addition, diode laser absorption spectroscopy was employed to measure the density of argon metastables (1s5 in Paschen notations) in argon microdischarges. The gas temperature, extracted from the width of the absorption profile, was compared with T_g values obtained by optical emission spectroscopy.

We developed a system of cavity-ringdown absorption spectroscopy employing a cw diode laser for measuring the absolute density of N₂(A³Σ⁺_u) in plasmas. We achieved a sensitive detection limit of 10−⁷ for the absorbance. The saturation of absorption was avoided simply by switching off the laser beam when the cavity length was detuned slightly from the length corresponding to the perfect resonance. The absolute N₂(A³Σ⁺_u) density was deduced from the absorbance of the B³Π_g(v' = 2) − A³Σ⁺_u(v" = 0) band by comparing the absorption spectrum with spectral simulation, where we assumed the same values for the translational and rotational temperatures.

Development of a high-power, sub terahertz pulse gyrotron has started in FIR FU for application to CTS on LHD. A new electron gun was designed to produce a laminar electron beam with a good quality. Second harmonic TE_6,5 and TE_8,5 modes were selected as oscillation modes well isolated from other competing modes. Single mode oscillation has been confirmed for both TE_6,5 and TE_8,5 modes. The maximum power is larger than 50 kW for TE_6,5 mode (0.349 THz) and about 40 kW for TE_8,5 mode (0.390 THz). These powers are new records of second harmonic gyrotron in this frequency range. Feasibility study of CTS with a 0.4 THz pulse gyrotron from a high density plasma in LHD was carried out. The CTS condition is satisfied for a wide operation regime and scattering angles large enough for good spatial resolution. Ray tracing calculation shows that the launched scattered beams are propagated almost straight. CTS spectra calculated with a newly developed code indicates that a large signal to noise ratio can be obtained against ECE for use of a 100 kW gyrotron.

The collective Thomson scattering (CTS) technique has been utilized with the backscattering configuration in the collective scattering regime to diagnose the velocity distribution functions in the Large Helical Device (LHD). The receiver was equipped with 16 channels and the first test has been carried out using the eight channels for scattered radiation and these channels cover a few GHz frequency shift from the 76.95 GHz probe beam. During the discharge, the electron density and temperature at the central region of the LHD are 1×10¹⁹m⁻³, and 1.0 keV, respectively. The probing beam with rectangular wave modulation is injected by 50 Hz in order to be distinct from the background electron cyclotron emission (ECE). The scattered radiation is resolved successfully at each channel of CTS receiver system. The detected signals of bulk ion and electron components are by a factor of 10 ~ 10² larger than the background ECE signal. We found that the measured spectra are in reasonably agreement with the theoretical spectra calculated by using the reliable measured electron temperature and density for input parameters. The CTS receiver system will be improved to obtain more accurate velocity distributions in high temperature plasmas.

Pulsed polarimetry techniques are described for the determination of the local magnetic field distribution, B(r), in magnetically confined plasmas. Pulsed polarimetry is a non-perturbative Lidar-like technique that exploits both the Thomson scattering and magneto-optic Faraday effects to measure B_|| at the position and along the sightline of a polarized light pulse propagating in the plasma. The implementation of pulsed polarimetry on high performance magnetized plasmas of relevance to magnetic fusion falls naturally into three categories based on the size and optical activity of the plasma and present-day laser and detector options: 1) large tokamak plasmas typified by ITER and DEMO, 2) meter-sized high energy density plasmas typified by the Magnetized Target Fusion (MTF) program and 3) cm-sized plasmas typified by wire Z-pinches. Plausible pulsed polarimetry systems are presented for each category along with the current interest in obtaining the local field measurement for the respective programs.

A new plasma diagnostic tool of terahertz time-domain spectroscopy (THz-TDS) has been developed. The THz-TDS system allows one to obtain the electron density and collision frequency of a plasma simultaneously from the phase shift and absorption rate of THz waves that are transmitted through the plasma. The line-averaged electron densities of Ar inductively coupled plasmas (ICPs) and CH₄/Ar capacitively coupled plasmas (CCPs) were evaluated by the system and found to be 10¹⁷−10¹⁸ m⁻³ for Ar ICPs and 10¹⁵−10¹⁶ m⁻³ for CH₄/Ar CCPs, depending on the discharge conditions. These results have demonstrated the feasibility of electron density measurements by THz-TDS for reactive plasmas.

Ion phase-space resolved fluctuations carry detailed information pertaining to transport as well as to the plasma degrees of freedom. Using a 500 mW diode laser that can scan over Doppler broadened Argon ion lines in less than 50 microseconds we observe fluctutations and correlations with an adjustable bandwidth by means of a comb-filter. The high laser intensity coupled with fast scanning makes optical pumping between the Zeeman sublevels observable. This opens a new strategy for optical tagging as well as observing correlations between fluctuations at different ion velocities. The experiments are performed in a 2 meter length 0.1 meter diameter CW Argon gas discharge created by a 10 MHz inductive plasma source in a uniform 1 kG magnetic field. The plasma density is typically 10⁹ cm⁻³, the electron temperature is 2 eV and the ions have a temperature of 0.1 eV. Under these conditions the ions are weakly collisional.

Electric field measurements at near-atmospheric pressure environments based on electric-field induced Raman scattering are applied to repetitively pulsed nanosecond discharges. The results have revealed that the peak electric field near the centre of the gap is almost independent of the applied voltage. Minimum sustainable voltage measurements suggests that, at each discharge pulse, charged particles that remain from the previous pulse serve as discharge seeds and play an important role for generation of uniform glow-like discharges.

Laser induced fluorescence by one (LIF) and two photon (TALIF) excitation has been employed to characterize NO, O species in the expanded stream of N₂-O₂ air like low pressure plasma jet. The gas, excited in a coaxial RF capacitive discharge at pressure P₁, expands through a de Laval nozzle into a vessel at P₂ = 0.25 Torr at expansion ratio P₁/P₂ of about 35. The multiple expansion– compression waves of the jet are traced by laser induced fluorescence of NO and O dissociation products expanding through the nozzle. The quantitative O and NO densities, obtained by in-situ calibration of TALIF and LIF signals are discussed.

Inside a miniaturized capacitively coupled cold atmospheric pressure plasma jet operated at a helium base gas flow with a minor molecular oxygen admixture atomic oxygen is created. The build up of atomic oxygen along the discharge channel and its further decay in the effluent is investigated by means of xenon calibrated two photon laser induced fluorescence spectroscopy (TALIF). The longitudinal and the transversal atomic oxygen distribution is measured from the discharge core through the transition area into the effluent. A particular emphasis is set on the influence of collisional quenching at elevated pressures.

Hot electron cyclotron emission (ECE) measurements and modeling calculations have been carried out for the Levitated Dipole Experiment (LDX)⁵. Heterodyne radiometers at 110, 137 and 165 GHz have been used to view the plasma from bottom and midplane locations. An intense environment of harmonic ECE of up to 1 keV at 110 GHz and afterglow decay times of up to 6.9 seconds were observed at lower pressures. Interpretation of the emission levels in terms of hot electron parameters requires integrating modeled emission over a large magnetic field range of 0.09 to 3.2 Tesla and a k to B angular view range of 0 to 90° within the field of view of each receiver as the hot electron density peak is followed around a given flux contour. Hot electron temperatures of the order of 100 keV, depending on knowledge of the electron density peak location, were determined by taking the ratio of radiometer signals at different frequencies. Abrupt radial movements of the highly peaked hot electron density where evident, particularly with ECRH turn on and turn off.

This paper describes an innovative laser diagnostic instrument, a two color FIR laser interferometer/polarimeter, under development at the National Institute for Fusion Science, aiming for the establishment of reliable electron density/current density profile measurement techniques in the next step magnetically confined fusion devices such as ITER. In the process of searching for new laser oscillation lines, we have successfully achieved the powerful laser lines simultaneously oscillating at 57.2 and 47.7 μm, which lead to the construction of a new type of two color laser interferometer. So far we have been developing a two color laser interferometer, and confirmed its original function, vibration subtraction, in a test stand. In this conference, development of laser diagnostics which is a combined system of a two color laser interferometer and a polarimeter including hardware development necessary for future fusion devices will be presented.

Two standard commercial flashlamp-pumped Nd:YAG lasers have been upgraded to "pulse-burst" capability. Each laser produces a burst of up to fifteen 2 J Q-switched pulses (1064 nm) at repetition rates 1–12.5 kHz. Variable pulse-width drive (0.15–0.39 ms) of the flashlamps is accomplished by IGBT (insulated gate bipolar transistor) switching of electrolytic capacitor banks. Direct control of the laser Pockels cell drive enables optimal pulse energy extraction, and up to four 2 J laser pulses during one flashlamp pulse. These lasers are used in the Thomson scattering plasma diagnostic system on the MST reversed-field pinch to study the dynamic evolution of the electron temperature.

Local values of the electron density and temperature in the edge of a fusion plasma can be derived with high space and time resolution by the use of line radiation of atomic helium beams. The accuracy of this method is mainly limited by the uncertainties in the collisional-radiative (CR) model which is needed in order to obtain both plasma parameters from the measured relative intensities of atomic helium lines. Laser-induced fluorescence spectroscopy on a thermal helium beam in the edge plasma of the tokamak TEXTOR in Jülich was applied to validate the CR model of helium. By use of a high-power, pulsed laser system (a dye laser pumped by an excimer laser) several laser excitation schemes starting from the n=2 levels have been tried. The fluorescence light was observed at the laser wavelength and elsewhere in the spectrum providing information on population densities of initial levels as well as on collisional population transfer between excited levels. This paper summarises the results of the measurements, showing principal limits and possible improvements of this experimental validation method of the CR model of the diagnostic helium beam.

The method of laser induced fluorescence (LIF) is applied to fluorescent lamps (FL) in order to investigate processes of electrode erosion in the vicinity of the electrodes. The life time of FLs which are ignited by instant start is mainly limited by sputtering of the coil electrodes and in final breaking. This sputtering of tungsten mainly occurs during the ignition in the glow discharge phase. Therefore, the density of W atoms is measured in the electrode region during ignition. Temporal and spatial resolved profiles were measured by LIF which has been combined with fast imaging. The life time of FLs which are started with preheated coils is also caused mainly by electrode failures. But the reason differs from the instant start case because here the loss is caused mainly by evaporation. End-of-lamp life is reached if the emitter material which is deposited at the coil to reduce the work function of the coil is lost completely. LIF is used to measure the density of the eroded emitter material, namely Barium atoms. First result of phase resolved absolute Ba atoms densities are presented.

We present a study of H₃⁺ recombination performed at 77 K on the two lowest rotational levels of this ion, which belong to its two different nuclear spin states of the studied ion. A near infrared cavity ring-down spectrometer (~1381 nm, CRDS arrangement) has been used to obtain the time evolution of concentration of both states. From the overall ion density decay during the afterglow we obtained the binary recombination rate coefficient α_bin (77 K) = 1.2×10⁻⁷ cm³s⁻¹. We have also observed ternary helium assisted recombination of both para and ortho H₃⁺. The process is very slow (at 77 K) and the obtained ternary recombination rate coefficient is in contradiction with the theoretical prediction. It is the first time that the binary and ternary H₃⁺ recombination rate coefficient was measured at a known population of para and ortho H₃⁺ ions in decaying plasma.

An incoherent and infrared Thomson scattering diagnostic (ITS) was transferred from ISTTOK (Lisboa) and reconstructed on TCABR (S. Paulo). In the first phase of this international collaboration, the diagnostic uses a Neodymium:Glass laser with up to 10 Joules per laser pulse and a first generation polychromator with three pairs of interference filters and avalanche photodiodes. It measures 90° scattered radiation in a single volume of observation with a single laser pulse to obtain the instant plasma electron temperature. This paper reports the reconstruction activities already carried out and presents the first experimental results. These activities include: new data model performance, laser refurbishing, new laser delivery system, stray-light reduction in the vacuum vessel, new collection lens and relative diagnostic calibration. A long run of experiments with this diagnostic shows consistency and coherence with the other TCABR diagnostics and gives indications to be able to contribute effectively to the Alfven heating program of this tokamak.

Two lasers of different wavelengths could be used in the ITER LIDAR Thomson scattering system. This raises the possibility that absolute calibration from one laser could be used to calibrate the other laser system. Lower laser wavelengths have the advantage that the Raman cross section increases, σ_Raman∝ 1/λ⁴₀. However at lower laser wavelength the spectral width of the scattered spectrum decreases Δλ_Raman ∝ λ²₀ making measurement closer to the laser wavelength nessecary. The choice of calibrating gas presents a number of trade offs. Raman calibration using a hydrogenic molecule produces a broad spectrum, useful since it is not close to the laser wavelength. However, Raman calibration in Nitrogen or Oxygen produces a far greater number of scattered photons for the same calibrating gas pressure. The f-number of scattered light collected by a LIDAR system varies significantly with plasma major radius. The large change in collected solid angle changes the angle of incidence of the scattered light onto the detector and hence the spectral transmission of the optical filter. This change in spectral transmission must be very accurately measured to determine the detected cross section of Raman or Rayleigh scattered light. In the case of Raman scattering, uncertainty in the absolute calibration can be reduced by collecting both Stokes and anti- Stokes lines with a number of different optical filters.

The laser induced plasma in air produced by 6 ns, 532 nm Nd:YAG pulses with 25 mJ energy was studied using the Thomson scattering method and plasma imaging techniques. Plasma images and Thomson scattered spectra were registered at delay times ranging from 150 ns to 1 μs after the breakdown pulses. The electron density and temperature, as determined in the core of the plasma plume, were found to decrease from 7.4 × 10¹⁷ cm⁻³ to about 1.03 × 10¹⁷ cm⁻³ and from 100 900 K to 22 700 K. The highly elevated electron temperatures are the result of plasma heating by the second, probe pulse in the Thomson scattering experiments.

During the 2009 experimental campaign the Edge Thomson Scattering System started the operations, after a commissioning period. The main modification has been the adoption of interferential filters spectrometers with APD detectors, similar to those used on the main TS. Other improvements were the optimization of the laser input path and of the collection optics, which increased the signal level of about 50 %.

With the present setup, temperature and density measures show good compatibility with those of the main TS diagnostic; density data have been calculated after Rotational Raman Scattering calibration, a method which is not affected by impurity deposition on the collection window. Present operational limits of the diagnostic are discussed as well as short terms planned operations, focusing in particular on laser multi-pulse issues and laser sharing with the impurity injection system by laser blow-off.

A multichannel far-infrared (FIR) polarimeter has been recently installed and improved in RFX-mod to measure the Faraday rotation angle along vertical chords on a poloidal plasma section. Polarimetric data, associated with measurements of the electron density, permit the reconstruction of the poloidal magnetic field profile, B_θ. The entire diagnostic is described and its main sections outlined. Emphasis is placed on the work performed on the polarimeter to reduce the fluctuations affecting the old diagnostic signals and to increase the S/N ratio. In the recent installation of the polarimeter the optical line was more carefully designed and the mirror holders have been made in insulating material to avoid any interaction with the variable magnetic fields. Moreover all the optics have been fixed on an inertial granite platform. Examples of the first Faraday angle measurements performed on five chords are presented and discussed. The measured Faraday rotation angles are compared to a theoretically calculated value, based on the μ&p model, showing a good agreement between experimental and predicted data in the central region of the plasma. The comparison between experimental and predicted data is reported and discussed in the present work.

The interferometer diagnostics using Far Infrared (FIR) laser beams provide, by phase measurement techniques, precise line-electron density measurements of magnetic fusion plasmas. But the FIR beams still suffer the effect of refraction when traversing the plasma. This can cause, when the density gradients are strong, temporary losses of signal that induce the so called fringe jumps on the estimated phase. On the CEA Tore Supra tokamak, a Field Programmable Get Array (FPGA) electronics has been developed using a time delay method to calculate the phase and correct the fringe jumps at the frequency rate of the probe sinusoidal signal (100 KHz). To test the efficiency of the algorithm on various plasma scenarios, a prototype of the CEA electronics has been installed on the JET tokamak and the data have been compared with those issued from the present JET electronics, which calculates the phase by a different method. Statistical comparisons between the two methods on more than 1500 JET shots are reported in this article and show that the two methods give similar results but that none of them is 100% reliable as still some fringe jumps remain, in particular when Edge Localised Modes (ELMs), pellet injections or disruptions occur. To understand this phenomenon, an analysis of the fast changes of the 100 KHz raw input signals during ELMs and pellet injections has been done with a 1MHz numerical acquisition. The typical durations of signal losses have been found to be few hundreds of micro-seconds. Meanwhile, the line density can increase and then return to its original value. Simulations show that an algorithm that would block the phase calculation during a longer time (i.e. 500 μs) than the disturbed period would help to avoid fringe jumps.

An FIR interferometer/polarimeter system will be constructed in the Korea Superconducting Tokamak Advanced Research (KSTAR) device for electron density profile measurements. Due to the complex inner-wall geometry and long diagnostics ports, a specially designed in-vessel optic system is necessary to ensure a sufficient number of beam lines for inversion of the density profile. A single channel on the vertical center line and seven channels on the tangential plane will be placed using corner-cube reflectors mounted on the vacuum vessel. To avoid interferences with in-vessel structures such as neutral beam protection tiles and diagnostics systems, the positions of these beam lines are designed carefully using an in-vessel beam positioning module placed inside a narrow diagnostics port. Since these in-vessel optic components must be mounted on the vacuum vessel, a vibration compensation system needs to be integrated on the module. Designs of the corner-cube reflectors and the beam positioning modules have been performed for these purposes. A prototype corner-cube reflector will be installed on KSTAR before the 2010 campaign and vibration compensation performance and surface degradation due to plasma will be tested.

Reflectometry is considered to be one of the key diagnostics to measure density profiles and density fluctuations of fusion oriented plasmas. When an electromagnetic wave is launched into a plasma, the wave is reflected at the corresponding cutoff layer of the ordinary (O) mode or the extraordinary (X) mode. Reflectometry measures the time of flight (TOF) or group delay of the reflected wave. We have applied ultrashort-pulse reflectometry (USPR) to Large Helical Device (LHD) at National Institute for Fusion Science (NIFS). The highspatial analysis method called signal record analysis (SRA) is utilized to reconstruct the density profiles from the TOF signal. Also, it is noted that the remote control system using super science information network (super-SINET) has been introduced to the present USPR system. This remote system is exclusive, and it seems to be quite effective for collaborating experiment of large devices such as ITER.

A Helicon discharge in argon at low gas pressure of 0.1 Pa is generated with a flat coil antenna operated with 13.56 MHz. The power coupled into the discharge is around 1 kW which leads to an electron density of 10¹² cm⁻³. The velocity and temperature of the argon ions is measured by laser induced fluorescence (LIF) spatially resolved in radial and axial direction. Ambipolar diffusion accelerates the ions out of the heating region of the discharge with velocities up to 800 m/s. Due to homogenisation of the kinetic energy, the ion temperature increases from 0.15 eV in the centre to 0.4 eV at the edge of the plasma volume.

An optimized in-situ window cleaning system by laser blow-off through optical fiber has been developed on the basis of a feasibility study previously presented. The beam generated from a Q-switched Nd:YAG laser (up to 330mJ output energy, pulse duration 5ns FWHM with 10Hz repetition rate) is launched into a high damage threshold optical fiber (Ø=1mm) through an f=80mm lens kept in a sealed box at 1mbar pressure. The fiber output is focused on the coated surface of a vacuum window previously exposed to the plasma of the RFX-mod experiment. We investigate the energy density threshold necessary to ablate the impurity deposition substrate: above threshold a single laser pulse recovers ~95% of the window transmission before its exposure to the plasma, while below it the efficiency of the cleaning process is too poor. The system so conceived can clean completely the largest window on RFX-mod (10⁴mm² surface) in about 20minutes. We also present first results obtained firing the laser directly on a bundle of small core diameter fibers, showing performance similar to those attainable with commercial products.

Plasmas induced by UV laser ablation have been studied and analysed using time-of-flight measurements. A KrF laser beam of 23 ns FWHM time duration was focused into Si and Nb targets with a moderate laser fluence of 2 J/cm². A suitable theoretical expressions were derived for fitting the recorded ion current under the assumption of a "shifted" Maxwell-Boltzmann velocity distribution. The deconvolution of time resolved ion current signals, taking into account a multi-mode velocity distribution has revealed that the contribution of the different charge states of ions in the plasma induces electric field which accelerate the expanding plasma along the target normal.

In this work, we demonstrate the high potential of two-photon excitation of the 1S -2S transition of atomic hydrogen followed by optogalvanic detection, for measuring under identical experimental conditions, the kinetic temperature and the electric field strength in the cathode sheath region of a hollow cathode discharge. The first obtained results for both parameters are discussed in this paper.

Doppler-free two-photon optogalvanic spectroscopy has been applied to measure the strong electric field strength and the cathode fall characteristics of hollow cathode discharges operated in hydrogen and deuterium via the Stark splitting of the 2S level of atomic hydrogen isotopes. In this paper we show similarities and differences in the tendencies of the cathode fall characteristics of hydrogen and deuterium in a wide range of identical discharge parameters.

Electric fields are measured for the first time in molecular nitrogen at atmospheric pressures. Measurements are performed in either pure nitrogen or air. The laser spectroscopic technique applied here is based on a CARS-like four-wave mixing scheme originally developed for measurements in molecular hydrogen by Ochkin and Tskhai in 1995. The technique is ideal for investigation of microdischarges at atmospheric pressures. The frequencies of two focussed laser beams in the visible are tuned to match the energy difference between the two lowest vibrational levels in nitrogen. The presence of a static electric field then leads to the emission of coherent IR radiation at this difference frequency. The signal intensity scales with the square of the static electric field strength. Parallel to this process also anti-Stokes radiation by the standard CARS process is generated. Normalization of the IR signal by the CARS signal provides a population independent measurement quantity. Experimental results at various pressures and electric field strengths are presented.

A Thomson scattering (TS) diagnostic based on the laser multi-pass intra-cavity plasma probing has been developed and successfully implemented in the TEXTOR tokamak [1, 2]. The laser system operates in a burst mode delivering up to 45 laser pulses 50 J of probing energy each at 5 kHz frequency. The diagnostic has provided the measurements of fast evolution of electron temperature and density at accuracy of 2% and 1% correspondingly at a spatial resolution <1 cm along the whole plasma diameter during a 9 ms time interval. The developed TS diagnostic combines unique features: large amount of collected photons, a high repetition rate of the measurements, a high spatial resolution along the whole plasma diameter and a high spectral resolution with hundred spectral points. Some of new advances of the TS diagnostics and its possible applications on the TEXTOR tokamak are briefly discussed in the report.

The design of Thomson scattering system for the Korean Superconducting Tokamak Advanced Research (KSTAR) device is described. The system includes a laser beam guiding system, laser input port, laser beam dump, collection lens, shutter, and cassette system. The laser guiding system has a collimating lens set for reducing the laser power loss. The laser beam dump will be attached inside of the vacuum vessel and hence it has been designed compactly. The preliminary design of collection lens, shutter system, and the cassette were done by PPPL and the engineering design is being executed by NFRI. The collection lens system has two sets of lenses, one set is designed for the core and the other set is designed for the edge. Two sets of pneumatic shutter systems are designed for independent remote control. Most of the KSTAR Thomson scattering system design is already finished and we are planning to install the whole system by March, 2010. We will measure the plasma parameters (T_e, n_e) in KSTAR during the third campaign.

The ITER design has highlighted the fundamental need to monitor the machine operation in more detail. The mission of the Thomson scattering diagnostics in the ITER divertor research/operation is discussed with due attention paid to challenges and capabilities of the existing diagnostic design.

The standard technique for the relative calibration in present Thomson scattering diagnostics, can be used in ITER only if the full collection path is included in the calibration: indeed, the in-vessel optics will be exposed to neutron and gamma irradiation and particle flux, that will likely cause a distortion of the spectral transmission curve. The standard scheme would ask to position a diffuser in front of the first collection mirror, e.g. on the protective shutter, and to illuminate it with a light source nearby. Alternatively, a back illumination scheme could be used, with a more efficient retroreflector array. A completely different approach is based on TS measurements from two lasers with different wavelengths: this self-calibrating method does not require any in-vessel tool and will provide both the electron temperature and the relative sensitivity of spectral channels.

One of the main challenges of the implementation of divertor Thomson scattering system on ITER is weak laser scattering signal to be detected against intense background plasma radiation. The paper review briefly the line and continuum radiation data from present magnetic fusion devices in the spectral range of interest to TS diagnostics. The results will form the basis of design and development of the TS diagnostics for the ITER divertor.

A pulsed-power experiment has been designed to produce arc-shaped magnetic flux tubes similar to ascending solar flares. The tubes are filled with hydrogen plasma (electron temperature ≤ 10 eV, electron density 2...3×10²⁰ m⁻³) and expand with a velocity of ~2.5 cm/μs, while keeping their cross section constant at a radius of about 1.5 cm. For measuring the spatial electron density distribution within the moving flux tube, a single cw laser beam can be used. The information taken from the laser beam, which traverses the vacuum vessel perpendicular to the plane of the plasma arch, can be either the phase shift or the beam deflection due to the density gradient. Assuming a parabolic distribution with a central electron density of 2 × 10²⁰ m⁻³, the maximum deflection angle occurring at an impact parameter of 0.7 amounts to γ_max/deg ≊ 10⁻⁵ × (λ/μm)². Hence, a FIR laser operating at λ = 433 μm would be deflected by γ_max = 1.9° only. Alternatively, a beam passing through the plasma centre would experience a plasma-induced phase shift of Δϕ_max/rad ≊ 10⁻² × (λ/μm), yielding 4.3 rad for a FIR laser (λ = 433 μm) and 0.1 rad for a CO₂ laser (λ = 10.6 μm). While the former is readily detectable in a standard interferometer, the latter requires a more advanced technique of measurement to achieve the necessary resolution. On the other hand, the short wavelength compared to FIR radiation allows for a very narrow beam and hence for a high spatial resolution. For these reasons a so-called coupled-cavity scheme for a CO₂ laser interferometer is presently under development.

An in-situ ellipsometer based on four-detector polarimeter(FDP) is under development at KSTAR. In-situ ellipsometer for tokamak discharges will measure the characteristics of thin films deposited onto a quartz window near the edge region in real time. These characteristics contain local deposition/erosion rates as well as hydrogen to carbon ratio, which have to be measured in-situ, for more clear insight view of plasma-wall interaction in tokamak edge plasmas. This paper reports the status of in-situ ellipsometer development for tokamak discharges at KSTAR, to study plasma-wall interaction and fuel retention. Basic concept, design and construction of the ellipsometer are described.