ICRF plasma production at hydrogen minority regime in LHD

This study aim is to develop further an ion cyclotron range of frequencies (ICRF) method of plasma production in stellarators based on the minority heating. The previous studies demonstrate production of low density plasma (9.5 × 1017m−3) at low power of up to 0.2 MW. The higher ICRF heating power experiments become possible after introducing a programmable ICRF power ramp up at the front of the ICRF pulse. With this trick, all the shots went with the antenna voltage within the safe range. Increase of the ICRF power predictably results in increase of the density of produced plasma. Without pre-ionization the plasma density achieved was 6 × 1018 m−3 which is 6 times higher than in previous experiments. However, the electron temperature was not high, the light impurities were hot fully stripped, and there were no recombination peaks after termination of the ICRF pulse. Plasma density is too low to provide good conditions for efficient plasma heating. For the reference, the ICRF heating of high density cold plasma prepared by electron cyclotron resonance heating is performed. Both electrons and ions were heated to high temperatures, and this plasma state is sustained. The antenna–plasma coupling was much better which result in larger heating power with the lower antenna voltage.


Introduction
The majority of the stellarators are equipped with electron cyclotron resonance heating (ECRH) systems (see, e.g. [1]) which are versatile in sense that they can be used both for plasma heating and for plasma production. The physical base for ECRH is strong interaction of the electrons gyrating in the magnetic field with the resonant electromagnetic fields. This interaction takes place both in very low and high plasma densities that provides a possibility of plasma density increase up to values determined by the neutral gas burn-up.
Both ECRH plasma heating and plasma production are only possible when the electron cyclotron resonance condition ω = nω ce is met in the plasma confinement volume (here n = 1, 2). This requirement restricts the range of the magnetic field possible for the device operation. So, if there is a need to widen this range, one should look for other than ECRH plasma production scenarios.
Plasma production is practiced at the ion cyclotron range of frequencies (ICRF) in pure gases (He, H 2 , D 2 and others) [2,3]. It was tried at Large Helical Device (LHD) [4]. In present times it is used at the Uragan-2M (U-2M) machine only [5][6][7]. It should be noticed that for big machines the plasma production frequency should be chosen much lower than the ion cyclotron frequency [8]. So, this scenario is hard to reproduce using existing ICRH equipment.
Low density plasma production in high pressure gases is demonstrated in tokamaks using the existing ICRH equipment (see, e.g. [9]). In spite of poor of plasma parameters, such plasmas fit well requirements for wall conditioning discharges. Also in LHD, such a scenario of plasma production has been demonstrated [10].
A scenario of plasma production aimed to use the ICRH equipment had been developed at U-2M [5][6][7] for providing a fully ionized target plasma. The scenario use two-ion component plasma and requires the cyclotron zone for light plasma ions to be present in the plasma confinement volume. The scenario can be realized with the standard strap antennas.
The physical mechanisms which this scenario involves are described in [11]. Within this scenario, the RF heating of electrons is provided in the range of plasma densities from very low at the initial stage to high at the final stage. At high plasma densities, the electron heating is made by the slow wave originated at the mode conversion zone [12]. At low plasma densities, the direct slow wave excitation by the antenna is necessary to heat the plasma electrons. The ICRF antennas with the straps oriented across the magnetic field lines are efficient for the fast wave (FW) excitation. The direct slow wave excitation, which is a parasitic effect, does also present. Since the necessary plasma production power is proportional to the plasma density (see, e.g. [13]), even small antenna coupling to the slow wave could be in practice sufficient for successful plasma production at the initial stage.
The scenario could be considered for use at Wendelstein 7-X (W7-X) at 1.7 T magnetic field [14] with the hydrogen light ion species (heating frequency is about 26 MHz). In-depth studies of this scenario at U-2M could be a background for such a project, but a small size of U-2M and low magnetic field impose some uncertainties. At LHD, only few shots were made in this regime [15]. In those experiments, the ICRF plasma production was for the first time demonstrated in neutral gas with pressures suitable for further plasma heating. The experiments were made under constraint for the RF power to be lower than 0.2 MW. The low plasma density (9.5 × 10 17 m −3 ) was obtained, and the antenna-plasma coupling was not high.
In continuation of such studies, a series of new experiments was carried on at LHD with the results presented in this paper. The major distinctive feature of this series is higher RF power. In the RF plasma production scenarios, RF generator operation without load is unavoidable. This happens at the initial stage of plasma production and could result in arcing. When the RF power is higher, the voltage on the RF system elements is higher which increase the risks of hardware damage by arc discharge on its elements. However, since plasma can be produced at low power, and higher power is only necessary to increase its density, the high power could apply later, after some plasma is already created. Such a regime is realized at U-2M where the RF power increased stepwise [5]. In the series of experiments described below a gradual increase of the RF power was used.

Experimental setup
The LHD is a large-scale heliotron-type device at Toki, Japan [16][17][18]. The superconducting magnetic system includes one pair of helical coils and three pairs of poloidal coils [16]. The maximum magnetic field at the magnetic axis is B t = 3 T. The poloidal and toroidal period field period numbers are l/m = 2/10. The major plasma radius is R ax = 3.6 m, minor radius is a p = 0.6 m and plasma volume reaches 30 m 3 .
Three different heating systems are used in LHD: the ECRH, the neutral beam (NB) injection and the ICRF heating [18]. The LHD ECRH equipment includes two 77 GHz high power gyrotrons with a maximum power of 1.2 MW each [19,20], three 154 GHz gyrotrons with a maximum power of 1 MW each [20] and one 56 GHz gyrotron (maximum power of 0.5 MW) [21]. The NB injection was used for plasma heating and sustain. Tangentially injecting negative ion-based NB injectors (NB#1-3) can produce plasma [22], while perpendicularly injecting NB injectors (NB#4, 5) cannot. In this experiment, NB#4 (see figure 1) was used to enable a possibility to measure the bulk ion temperature by charge-exchange spectroscopy. The injection energy was 40 keV, and injection power was up to 6 MW. The ICRFs heating system in the LHD consists of the generator, amplifiers, transmission lines, impedance-matching devices, and two pairs of antennas. The maximum output power is 3 MW [23]. The ICRF heating is used with a fixed frequency of 38.47 MHz [24]. There are two types of antennas on the LHD: a Hand-Shake form antennas (HAS) [25] and a Field-Aligned-Impedance Transforming (FAIT) antennas [26]. The HAS and FAIT antennas are located at ports 3.5 and 4.5, respectively (see figure 1). The antennas occupy the upper (U) and lower (L) ports of the LHD vacuum chamber. The strap elements of both antennas are oriented perpendicular to the magnetic field. All the straps are placed at the outer part of the torus and are shielded by the Faraday screens. The HAS antenna straps occupy the same poloidal positions, but different toroidal ones. For the FAIT antenna it is vice versa: its straps occupy the same toroidal positions, but different poloidal ones. In ICRF plasma experiments there is a possibility to use the straps independently.

Experimental details
In the experiments, two values of the magnetic field are used, 2.55 T and 2.75 T. The hydrogen cyclotron zone crosses the plasma column (see figures 2(a) and (b)) in both cases. For the injection of fast neutral particles only perpendicular NB#4 was enabled. ECRH was used to produce the pre-plasma at the frequencies of 77 GHz and 154 GHz. The gas is helium with a hydrogen minority. To determine the gas composition and partial pressures, a mass spectrometer system is used, MKS e-Vision2.
The time dependence of the line averaged electron density was measured using a 13-channel far-infrared (FIR) laser interferometer [27,28]. The temperature and electron density profiles along the major radius were measured by the Thomson scattering (TS) system [29,30]. The central ion temperature has been measured from the Doppler broadening of soft x-ray lines of impurities [31,32]. The time-resolved optical emission spectroscopy was used to detect the spectral lines H α and He I and ratio of helium to hydrogen [33]. The intensities of the spectral lines of plasma impurities were also measured, in particular for C III, C IV, O V, O VI and Fe XVI lines. Total radiated power was measured by wide resistive metal film bolometer [34].

ICRF plasma production
The experiments on ICRF plasma creation were carried out first in the magnetic field B 0 = 2.55 T (see figure 2(a)). Three scenarios were realized: ECRH + ICRH, ICRH + NB, ICRH. In the first scenario with pre-ionization, the initial plasma was created by the ECRH discharge. Then, on the plasma decay after the ECRH pulse, ICRF power was injected. The plasma parameters in this scenario are better than those obtained earlier in [15]. Plasma density is higher, but still lower than in the regular LHD regimes.
In the other two scenarios (ICRH + NB and ICRH), preionization was not used; the plasma was produced only by the ICRF discharge. In the ICRF + NB scenario, perpendicular NB injection was performed after the plasma was produced by the ICRF discharge. As an illustration of this scenario, the results for one of the shots are presented in figures 3-5. To prevent undesirable incidents arising on the antenna (arcing, RF breakdown on the antenna elements) the RF power was injected with smooth ramp-up (see figures 3 and 6).
The cut-off point for the wave propagation is important for antenna-plasma coupling. The plasma density in the cut-off point (critical density) could be estimated using the FW dispersion equation with assumption that the poloidal wavenumber component is small. The critical density for FW can be estimated from the relation [35]: where ω pi is the ion-plasma frequency, ω ci is the ion cyclotron frequency, ω RF is the frequency of the electromagnetic wave, N || is the parallel refractive index. The refractive index is defined as N ∥ = k ∥ /k 0 where respectively k ∥ and k 0 parallel wavenumber and wavenumber in vacuum. In helium plasma at k || ≈ 1 m −1 and B t = 2.75 T, depending on the magnitude of the magnetic field in front of the FAIT antenna (≈2.2-2.6 T), the critical density ranges is (1.9-2.3)× 10 16 m −3 . For other modes with higher k ∥ critical density scales as N ec, FW ∝ |k || | 2 . This means that more modes penetrate to plasma when the plasma density is higher. In the beginning, for a short period of time ≈30 ms (start of the RF pulse on the FAIT (U) antenna is ≈2.02 s), the power was increased to ≈150 kW. Then the power continued increase during ≈610 ms with lower trend and reached the value of ≈550 kW. The gas breakdown and production of plasma with density up to 1 × 10 17 m −3 (see figure 4) occurs in short time ∼20-26 ms from the start. The further time is characterized by appearance of spectral lines of hydrogen and helium atoms H I and He I (see figure 3), slight decrease of voltage V max from ≈16.5 kV to ≈14 kV and increase of antenna resistance R p from ≈2 Ω to ≈3.5 Ω (see figure 4).
In further discharge development the density then increases relatively slowly from ≈1 × 10 17 m −3 to ≈4 × 10 17 m −3 during ∼300 ms (see figures 3 and 4). The spectral lines of O V, O VI, and C III appeared with a some delay of ∼100 ms (see figure 3). The following increase in density from ≈4 × 10 17 m −3 to a maximum value of ≈2.6 × 10 18 m −3 occurred during ∼400 ms (see figures 3 and 4). The intensity of the spectral lines also increased at the beginning of this stage. At a maximum density value of 2.6 × 10 18 m −3 the RF power is ≈550 kW, the values of V max and R p are ≈22 kV and ≈6.1 Ω, respectively. Radiation losses also increase to ≈370 kW.
Then the density decreases to 2 × 10 18 m −3 and almost does not change until the end of the RF pulse. The intensity of the spectral line He I decreased. The intensity of other spectral lines almost did not change, as well as the values of V max and R p . As can be seen from figure 5, the density decreased most significantly in the central area of plasma column. Note here that the RF power density in this discharge reaches 19 kWm −3 while in the previous experiments on the production of ICRF plasma, it was ∼7 kWm −3 , and plasma with a density of ≈9.5 × 10 17 m −3 was generated [15].   [36] respectively. The intensity of the spectral line C III of the excited C 2+ * ion is high and the level of the C IV line emissivity is low. The ionization potential of the C 1+ ion in the ground state is U i = 24.4 eV [35], for the C 2+ ion U i = 47.9 eV [36]. The presence of O V, O VI line emission and absence of C IV may be explained by low concentration of carbon in the plasma core. The carbon presumably comes from the wall and is ionized in the peripheral layers of the plasma where the temperature is lower. Oxygen is ionized in the center of the plasma volume where the temperature is higher.
The plasma of the created ICRF discharge at time moment t = 4 s was subjected by the perpendicular NB with a power up to ≈730 kW (see figure 3). The energy of the neutral hydrogen atoms was ≈25 keV. As can be seen from figures 3 and 4, this practically does not lead to any significant changes in the plasma parameters. The value of N e L also changes insignificantly (see figure 5).
Starting with a time of 4.6 s, the intensity of the spectral lines O V and C III begins to increase (see figure 3). The increase in the intensity of the spectral lines can be explained by the influx of impurities into the plasma. The voltage V max decreases and the resistance R p increases (see figure 4).
At 5.65 s the RF power is switched off, but NB continues to operate. Regardless this, the plasma density gradually decrease and the discharge fades.
As can be seen from figure 3, NB injection cannot sustain the plasma. A similar situation was observed in LHD experiments [37], where 6 MW perpendicular NB was injected into plasma having the density of ≈2 × 10 19 m −3 . Calculations for LHD [38] indicate that the heating efficiency using perpendicular NB depends on the initial plasma density and is lower than for tangential NB. Thus the implementation of the ICRF + NB scenario is not fully successful and requires further studies.
Experiments on plasma start-up with perpendicular NB were also performed in this experimental series. The NB parameters were the same as in the above described ICRF + NB scenario. As a result, only the H I and He I lines were observed at the level of noise during the injection time of 2 s. No plasma was produced under these experimental conditions. A similar result was obtained at W-7X, where the NB pulse was 0.7 s in duration [39].
Next experiments on ICRF plasma production were carried out in the magnetic field B 0 = 2.75 T (see figure 2). Several scenarios were realized: ICRH, ECRH + ICRH and ECRH + ICRH + NB.
In the ICRH scenario several antennas were used simultaneously to increase the injected RF power. As before, the RF power was gradually ramped up at start (see figures 6 and 7). At time moment 2.01 s, the RF pulses of the FAIT (U) and HAS (U) antennas start (see figure 7). With a short delay of ≈300 ms the RF pulse on the FAIT (L) antenna goes. At 2.08 s the RF power is switched off at the HAS (U) antenna. Further on, only the FAIT (U and L) antenna operates. Breakdown and creation of plasma with low density occurs in time ∼30 ms and is characterized by growth of intensity of spectral lines of hydrogen and helium atoms H I and He I (see figure 6).
The plasma density of ≈1 × 10 17 m −3 is reached for a time of ∼60 ms after the start of the RF pulse and the intensity of spectral lines of oxygen and carbon ions begins to grow (see figures 6 and 7). The R p resistance at this stage practically does not change and is ∼4.6 Ω for HAS (U) and FAIT (U) antennas, ∼2 Ω for FAIT (L) antennas. The voltage V max also practically does not vary. On further rise in the plasma density the intensity of spectral lines increase and radiated power grows up to ≈500 kW, the resistance R p goes up to 5-10 Ω for FAIT  (L) and 7-10.8 Ω for FAIT (U) antennas and voltage V max decreases consequently. A correlation between He I and C III line intensities is observed (see figure 6). After this there is a decrease in the intensity of He I and an increase in the intensity of C III. Then the maximum intensity of C III is reached accompanied by the minimum of He I intensity, followed by a decrease in the intensity of C III and an increase in the intensity of He I. There is also a slight decrease in the intensity of H I, and for a short period of time OV line is irradiated and the C IV line appears. These line behaviors may be explained by an increase in the temperature of the electrons which leads to an increase in the degree of ionization and a decrease in the intensity of excited helium and hydrogen atoms. Supposedly, further influx of impurities to the plasma leads to decrease of temperature and increase of neutral atoms density.
The maximum density of ≈6 × 10 18 m −3 is reached in a time of ∼300 ms after the start of the RF pulse at an injected power of ≈800 kW (see figure 6). At a RF power density of ∼27 kWm −3 , the ICRF discharge creates a plasma with a density of ≈6 × 10 18 m −3 . Then the density decreases to 2.7 × 10 18 m −3 and practically does not vary until the end of the RF pulse. The resistances R p also decreases to 5.6 Ω for FAIT (L) and 2.8 Ω for FAIT (U) antennas (see figure 7). Increasing the injected RF power to 0.9-1 MW does not lead to significant changes in the plasma parameters (see figure 6). The formed plasma has a supposedly not a high temperature.

ECR + NB + ICRF scenario
In the ECRH + NB + ICRH scenario in the magnetic field B 0 = 2.75 T several stages can be separated. At the first stage, the plasma was produced and sustained only by the ECRH discharge. When a microwave power of ≈1 MW is injected during 2 s, the gas breakdown occurs and density increases to ≈1.3 × 10 19 m −3 in a short time of about ≈130 ms (see figure 8). The density then rapidly decreases to ≈6. 8    of the spectral lines are observed during plasma production, and later they keep constant levels after time moment 2.4 s. In the ECRH discharge, the temperature of the electrons in the center of the plasma column changes in the range of 4-8 keV (see figure 10). The ion temperature is apparently significantly lower than the electron temperature. The maximum energy content of the plasma is up to ≈0.18 MJ. At the second stage, a perpendicular NB with a power of up to ≈0.73 MW and with the energy of neutral hydrogen atoms of up to ≈25 keV was injected into the plasma after ECRH power off at 4.5 s. When NB is injected into the plasma, the plasma density slowly increases from ≈0.93 × 10 19 m −3 to ≈1.6 × 10 19 m −3 (see figure 8). But the electron temperature drops from ≈4.8 keV to ≈0.35 keV (see figure 10). The plasma energy content also decreases dramatically to ≈0.04 MJ. The spectral line intensities consistently increase for Fe XVI after ≈4.5 s, O V and O VI after ≈5 s, and C III and C IV after ≈5.3 s (see figure 9). This is due to a decrease of the electronic temperature and recombination of ions which are in higher charged states. Note that the C III line shows a decrease in intensity after ≈4.5 s (see figure 9). A similar pattern is observed for H I and He I (see figure 8). This may be due to that the main contribution to the intensity of C III is made by the ions that are at the periphery of the plasma, where the temperature is low and the charge of the ions is not high. Then the temperature decreases in the plasma volume, and the highly stripped carbon ions with a recombine with electrons. As a result, the observed intensity of C III increases.
As can be seen in this case, the injection of only perpendicular NB does not sustain a high temperature of the electrons and to heat the ions.
The third stage begins after ≈5.4 with switch on the ICRH and continuation of NB injection (see figure 8). At the beginning the FAIT (U) antenna starts and with a short delay the FAIT (L) starts. Then after ∼100 ms the HAS (U and L) antenna starts (see figure 11). The RF power ramps up to ≈1.7 MW during ∼300 ms. The maximum RF power at this stage was up to 1.9 MW. The injection of ICRF power increases the electron temperature to ≈1.7 keV and ion temperature to ≈2.2 keV (see figure 10). The plasma energy content also increases to ≈0.23 MJ. The intensity of the Fe XVI, O V, O VI, and C IV lines decreases, that could be explained by the electron temperature increase. As it can be seen, the heating of electrons and ions to a several keV occurs in a time of the order of ∼600 ms after the start of the ICRF discharge. In spite of the fact that ICRF wave is injected to the prepared plasma, fluctuations of R p resistance value are observed (see figure 11). Moreover, fluctuations in R p are observed both when the plasma has a relatively low temperature before ≈6 s and when the temperature is high (after ≈6 s). Fluctuations are also observed at the V max voltage. After ≈6.5 s, when the NB injection is stopped, these fluctuations disappear. The variations in R p and V max become smoother. This is probably due to the fast ions formed as a result of NB injection. The nature of these oscillation needs to be studied.
Fourth stage ICRF discharge (after ≈6.5 s). As it is seen in figures 10 and 11, the discharge ICRF power up to 1.8 MW can maintain a dense plasma up to ≈1.2 × 10 19 m −3 and electron and ion temperatures up to ∼2 keV (see figure 10). The energy content of the plasma decreases slightly to ≈0.17 MJ at this stage.
At the third and fourth stages of discharge the RF power is the highest as compared to other pulses of this experimental series. However, the antenna voltages are not high, and antenna loading is quite good. This could be a result of better antennaplasma coupling to plasma with relatively high density.
The RF power shut down was carried out in steps (see figure 8). In the beginning, the FAIT (U and L) antenna was switched off at time moment ≈8.15 s (see figure 11). Accordingly, the total RF power was reduced to ≈0.92 MW. Then the HAS (U and L) antenna was switched off at ≈8.25 s. After switching off the FAIT antenna, the ion temperature decreases rapidly (see figure 10). The density and temperature of the electrons remain practically unchanged. After switching off the HAS antenna the electron temperature decreases (see figure 10). Plasma decay begins at ≈8.4 s. The intensity of the spectral lines of impurity ions begins to increase due to recombination (see figure 9). The intensity of the spectral lines of the neutral atoms H I and He I begins to increase with the beginning of the density decrease, respectively, after ≈8.4 s (see figure 8).
The measured hydrogen concentration at edge of plasma is shown in figure 8. For the ECRH discharge after ≈2.7 s and with NB injection, the hydrogen concentration is in the range 0.39-0.43. At the NB + ICRH stage, 0.47-0.51 correspondingly. For ICRF discharge in the range of 0.42-0.49. Figures 12 and 13 show the radial distribution of temperature and electron density at different stages of the discharge. As it can be seen from figure 12, the maximum electron temperature is observed at the ECRH discharge stage. The minimum appears at the NB injection stage. At all stages the maximum temperature is centrally peaked. At the NB + ICRH discharge the temperature in the central region is slightly lower than at the ICRH discharge. For all three discharges the temperatures at the plasma edge have close values. In the ECRH discharge and NB stages, the electron density has a maximum at radii ∼3-3.2 m and ∼4-4.2 m, and the density decreases toward the toroidal axis (see figure 13). In the NB + ICRF and ICRF  discharges, the maximum density is observed in the central region. In the ICRF discharge the density have a flat-top at radii of ∼3.2-4 m.

Conclusions
This study aim is to develop further an ICRF method of plasma production in stellarators based on the two-ion species plasma heating in mode conversion regime. The previous studies demonstrate production of low density plasma at low power levels. The higher RF power experiments become possible after introducing a programmable RF power ramp up at the front of the RF pulse. With this arrangement, all the shots went with the antenna voltage within the safe range. Increase of the RF power predictably results in increase of the density of produced plasma. Without pre-ionization the plasma density achieved in shot #179154 is 6 × 10 18 m −3 which 6 times more than in previous experiments. However, the electron temperature was not high, the light impurities were hot fully stripped, and there were no recombination peaks after termination of the RF pulse. Plasma density is probably too low to provide good conditions for efficient plasma heating after successful plasma production.
For the reference, the RF heating of high density cold plasma prepared by ECRH is performed. Both electrons and ions were heated to high temperatures, and this plasma state is sustained. The antenna-plasma coupling was much better which result in larger heating power with the low antenna voltage.
One can conclude from all these that the task of plasma density ramp-up from the achieved value to representative for LHD one may open a way for usage of ICRF heating for plasma production and heating during the same shot.

Data availability statement
The data supporting the findings of this study are available in the LHD experiment data repository at https://doi.org/10. 57451/lhd.