Plasma homogeneity over the extraction beamlet groups at the half size ITER negative ion source at ELISE test facility

The negative ion source for neutral beam injection at ITER has to provide high-intensity and low-divergence negative hydrogen and deuterium ion beams. Extracting the negative ions from the plasma is inevitably accompanied by co-extraction of electrons, which can limit the source performance, especially in deuterium. Reducing the co-extracted electrons is done by applying a magnetic filter field. On the one hand, the filter field reduces the electron temperature and the amount of co-extracted electrons, on the other hand, it introduces × B→ drifts. The drifts create a vertical plasma inhomogeneity in front of the extraction area and, consequently, an asymmetry of the co-extracted electrons. This motivated detailed studies on the vertical plasma inhomogeneity over the beamlet groups in the ELISE ion source using a movable Langmuir probe in deuterium plasma. It is demonstrated that the vertical distribution of the plasma potential changes together with the sheath, forming at the plasma grid – from electron repelling to attracting sheath. A repelling sheath reduces the flux of electrons from the plasma to the grid surface and consequently, more electrons are co-extracted. The attracting sheath collects the majority of electrons on the grid and reduces the co-extracted electrons.


Introduction
The upcoming ITER fusion device plans to use two neutral hydrogen or deuterium beam injectors with a total power of 33 MW [1,2].The neutral beam will be formed by accelerating hydrogen negative ions to 870 keV and deuterium ones to 1 MeV and then neutralized.For this purpose, an inductively coupled plasma source will be used.It must be capable of providing a negative ion beam with an accelerated current density of 230 A/m 2 for pulse duration up to 1000 s in hydrogen and 200 A/m 2 for 3600 s in deuterium, at 0.3 Pa filling pressure and more than 90% beam uniformity.Hydrogen and deuterium are weakly electronegative gases and the volume produced negative ions are only a small fraction of the plasma charged particles.Evaporating caesium in the discharge reduces the work function of the surfaces [3] and the extracted negative ions are produced mainly by atom conversion on the first grid of the extraction system -the plasma grid (PG).Applying a magnetic filter field (FF) in the extraction region reduces the electron temperature (T e ) to ≈ 1 eV, which reduces the electron detachment losses of the negative ions in the plasma.[4,5] of the charged particles.The top/bottom plasma asymmetry can affect the co-extracted electrons and lead to high local heat load deposition on the EG.In general, the ratio of the coextracted electrons to the negative ions must stay below 1 to avoid damage to the grid.The coextracted electrons are higher with a stronger temporal increase during extraction in deuterium operation compared to hydrogen [6], which leads to lower achievable source performance.
The ion source at the ELISE (Extraction from a Large Ion Source Experiment) test facility [7] is a half-sized ITER ion source and aims to demonstrate ITER's ion source requirements in hydrogen and deuterium.Recently, ELISE was upgraded with CW extraction and it is capable of producing 1000 s pulse in hydrogen with extracted negative ion current of 18.6 A -56% of ITER target and with electron to ion ratio below one.In deuterium, however, 500 s pulse has been demonstrated with 12 A -42% of ITER target [8].Even with already well-known techniques for reducing the co-extracted electrons via biasing the first electrode of the extraction system against the source walls [9] and the bias plate (window-frame-structured surface) [10], the coextracted electrons have asymmetry top/bottom and they increase in time, reaching electron to ion ratios above one at the end of the pulse in deuterium.The aim of this study is to examine the vertical plasma homogeneity and the sheath structure over the top/bottom beamlet groups at different operational parameters.

Experimental setup
The ion source at the ELISE test facility (Fig. 1a)) has 4 drivers where the plasma is generated by RF-coupling (with power up to 75 kW/driver) and expands in a common chamber towards the extraction region.The ion source is electrically isolated and the equipped extraction system has the PG in contact with the plasma, the EG and an acceleration grid on ground potential.Each grid has 640 apertures (14 mm diameter) arranged in 8 beamlet groups.A window-framestructured bias plate (BP) is placed 7 mm away from the PG and it is electrically isolated.The BP can be set to source wall potential or let it on floating potential via a switch.The evaporation of caesium is via two ovens on the left and right sides of the source in the expansion area.The source is equipped with a diagnostic flange around the PG where an RF-compensated probe is mounted over one of the middle beamlet groups on the top side (Fig. 1 b)).The probe is fixed in 2 cm axial distance (towards the drivers) from the PG (1.3 cm from BP) and its tip can be vertically moved.The range of the probe measurements (∆) is from 2.5 cm above to the middle of the beamlet group.The magnetic FF [11,12] is created by a high-current flowing from bottom to top through the PG (I PG ), resulting in an effective drift up.In standard ion source operation, the FF is enhanced by permanent magnets, mounted on the right and left side of the source.Due to space restrictions, it is impossible to mount the probe to the bottom.Reversing the orientation of the I PG together with the permanent magnets generates the same magnetic field but with a negative sign, resulting in a drift down.This mirrors vertically the plasma and simulates measurements with the probe over a bottom beamlet group.
The vertical profile of the FF is assumed to be flat.As can be seen in Fig. 1c) the magnetic field in the measurements regions is with a maximum field at I PG = 4.6 kA ≈ 4.58 mT (I PG scale factor 0.88 plus 0.5 mT offset from the external magnets).The probe tip diameter (0.1 mm) is at least one order of magnitude smaller than the expected electron Larmor radius and thus the plasma density (n i ) is obtained from ABR theory [13], the plasma potential (U pl ) from the zero crossing of the second derivative of the volt-ampere trace and T e from the EEDF.Some of the measurements are with caesium, in a region with high negative ion density (measured with cavity ring down spectroscopy), however, the ratio of negative ion to electron density is below 2 and thus standard probe evaluation is used.The plasma pulse length is 20 s.The measurements with caesium and I PG ̸ = 0 kA are with an extraction phase of 10 s.The scan with 9 probe vertical positions is done for less than 2 s.

Results and discussions
All measurements are done at 45 kW/driver RF power and in deuterium with a filling pressure of 0.3 Pa.The PG is biased against the source walls via an applied current of 5 A, defining the PG potential (U PG -the straight lines over the yellow area on the first plot of Fig. 2).It was investigated the effects of the FF, the BP bias, and the caesium on the plasma parameters and on the sheath structure in front of a beamlet group.Negative vertical direction in the figures means reversed FF (negative I PG and drift down) simulating measurements over the bottom beamlet group.
The measurements in a caesium-free plasma, BP potential (U BP ) set on a source wall and without a magnetic field (Fig. 2, black curves) show symmetric vertical behaviour of U pl .The measured U pl is lower over the BP region, with respect to value over the PG with a slight increase towards the middle of the source.U PG ≈ 10 V lower and U BP = 0 V, giving a negative U surface − U pl difference (U surface = U PG over the PG and U surface = U BP over the BP).Consequently, the electrons are repelled from the PG and BP surfaces, i.e. an electron-repelling sheath is present.Applying a magnetic field with I PG = 3.6 kA (3.7 mT with external magnets, blue curves), shifts U pl and U PG to higher values.Over the bottom beamlet group U PG is 1.5 V and U pl is ≈ 0.5 V higher with a flatter vertical profile compared to the top.These potential changes result in a decrease of U surface − U pl to values from 0.3 V to -3 V over the beamlet group, allowing electrons with less energy to be collected by the PG surface.Over the BP, U surface − U pl remains strongly negative, due to BP being on source wall potential.Setting the BP on a floating potential (the green curves), the potentials further increase and provide a more homogeneous top/bottom U pl vertical profile with values ≈ 41 V.However, U PG on top is 39.5 V and on bottom is 42 V, resulting in negative U surface − U pl over the top and positive over the bottom beamlet groups.The sheath structure changes in a vertical direction from electron repelling on top to electron attracting on bottom, allowing more electrons to impinge on the bottom and in the top about 3 eV over the BP and about 2 eV in the middle of the beamlet group.The plasma density increases 3 times on the bottom and 2 times on the top, providing a non-symmetric vertical profile with the FF.The observed inhomogeneities in T e and n i come from the × ⃗ B drifts which affect electron particle flux, but also the electron heat flux.It is known from modelling × ⃗ B drifts form a second axial maximum of n i in the expansion chamber of the source (the first is in the drivers) [5] and this could explain the observed increase of n i .
Setting the BP on floating potential gives no changes on the T e and n i on the bottom and a minor further decrease of T e on top.The plasma density on top, however, increases further and provides a more symmetric top/bottom profile.Adding caesium does not affect T e and n i .
Figure 3 shows the effect of the FF strength on the U surface − U pl and the co-extracted electrons, measured separately in the top and bottom part of the EG.The result with I PG = 0 kA is without extraction, due to the high heat load on the EG from the co-extracted electrons.The results show a positive U surface −U pl , allowing the electrons to impinge on the PG and BP and be collected prior to the extraction.The potential difference increases with the FF strength and the co-extracted electrons decrease.With bigger U surface − U pl difference are provided conditions -larger collection region -for electron collection from PG and BP prior extraction.There are many factors that determine the extraction, but the correlation between U surface − U pl and the co-extracted electron current is well-pronounced.

Summary and conclusions
The co-extracted electrons are the main factor limiting the performance of large-scale negative ion sources for fusion.The plasma inhomogeneity caused by × ⃗ B drifts can strongly affect the coextracted electrons, especially in deuterium operation.Therefore, a movable RF-compensated probe is used to study the plasma over top and bottom extraction beamlet groups at the ELISE ion source.The results with magnetic field show that a floating BP provides a flatter and more homogeneous U pl vertical profile over the beamlet group.Partial sheath reversal is observedelectron attracting over the bottom beamlet group and electron deflecting on top.The main result is that caesium in the plasma lowers U pl , becoming lower than U PG and more homogeneous over the beamlet group.This provides well-defined conditions for attracting the electron to impinge onto the PG and the BP surfaces and being collected prior to extraction.

Figure 1 .
Figure 1.ELISE ion source (a), PG view with the location of the movable probe measurements (b) and the horizontal magnetic filter field profile 2 cm away from PG for I PG = 2.5 kA without (blue curve) and with (green curve) additional external permanent magnets (c).

Figure 2 .Figure 3 .
Figure 2. The profiles of U pl , T e , n i and U surface − U pl over the beamlet groups.The black curves are for operation without magnetic field and U BP = U wall , blue curves for I PG = 3.6kA and U BP = U wall , the green curves for I PG = 3.6kA and U BP = floating.These three cases are in caesium-free plasma.The orange curves are with caesium in the plasma, I PG = 3.6kA and U BP = floating.Symbols represent probe measurements and the solid lines PG and BP potentials.The yellow area represents the beamlet groups and the white is BP.
The extracted negative ions are accompanied by co-extracted electrons which are deflected via embedded magnets in the extraction grid (EG) of the extraction system and collected by it.Introducing the FF creates vertical plasma inhomogeneity due to × ⃗