**4. Operation and results as preparation for the sheath studies**

#### **4.1. Sequence of a typical discharge**

**Figure 7.** Triplet He-I 4<sup>3</sup>

component at 4471.6 Å.

D-2<sup>3</sup>

continuous spectrum is obtained.

P line profile simulated with the EZSSS code with no external electric field (reference

scenario) shows discrete and continuous results, with the main component at wavelength λ = 4471.49 Å and the second

perturbed by an electric field, in the presence of a background magnetic field, the Explicit Zeeman Stark Spectral Simulator (EZSSS) [17] was used. This code generates the discrete spectrum by solving the Schrödinger equation in electric dipole approximation, with external electric and magnetic fields as perturbations. In the second step, by convoluting the discrete spectra with Gaussian and/or Lorentzian profiles to mimic the broadening mechanisms, the

tion with no electric field externally imposed on the system. The discrete spectra, calculated with the EZSSS code is shown as a set of lines in the mirror image of the intensity scale. The continuous spectra, presented in the positive part of the intensity axes is convoluted with a Gaussian distribution corresponding to the Doppler broadening with a temperature of the radiator of 0.7 eV. The distinct feature of these spectra is the occurrence of the second spectral line red-shifted from the main component, corresponding to the fine structure of the He triplet transition. This separation of the components allows estimating the E-field in the plasma.

An alternative, but more complicated approach, is to use Doppler-free saturation spectroscopy (DFSS) to eliminate disturbing effects such as the Doppler broadening and to highlight the E-field influence [17, 21]. The basic principle is to create a cross section in the plasma of overlapping pump and probe beams from the same laser source tuned to specific absorption

D - 2<sup>3</sup>

P He-I transi-

**Figure 7** depicts the modelled triplet line profile corresponding to the 4<sup>3</sup>

158 Plasma Science and Technology - Basic Fundamentals and Modern Applications

A typical discharge sequence is presented in **Figure 8a**, as it is seen by the different diagnostics; one camera view is shown in **Figure 8b**. The vessel is prefilled at the operating pressure before the start of the sequence. The RF power is matched for plasma with magnetic field. Therefore the coils are usually activated before the antenna. However, the reversed sequence, as used on the discharge in **Figure 8a** shows how the magnetic field affects the plasma and the measurements of the Langmuir probe. We notice that the ion saturation current starts to ramp up only when the field is activated. On the cameras, the visible light shows that the plasma is first confined in the plasma source and then, when the coils are powered, a plasma tube develops in the main vessel and is displaced towards the centre, further away from the Langmuir probe, following the field lines represented in **Figure 3**. The minimum amount of power required to ignite the plasma was evaluated at 50 W (with and without magnetic field). During the phase without the main field, arcing is noticed in the main vessel, which is then wiped out with the start-up of the coils. No arcing has been noticed on the helicon antenna

**Figure 8.** (a) Evolution of the main parameters for an injected helicon power of 700 W. (b) Plasma inside the main vessel in front of the ICRF antenna.

with the camera. However, post-experiment inspection revealed that the copper layer has been damaged on some parts of the line.

RF fields were measured with the manipulator over several discharges and are presented in **Figure 10** for two different magnetic fields at constant injected power of 2.5 kW and a neutral pressure of 10−4 mbar. The magnetic field strengths of 0.12 and 0.06 T, indicated in the figure, refer to the centre of the main vessel. The limit of 0.1 T, which was mentioned before, is for the helical plasma source. It is caused by the 1kA current limit in the small coils. The large coils around the main vessel can generate fields up to 0.4 T. The density profile has a Gaussian shape with a mean width of 10 cm, half the length of the plasma source geometrical radius. The profile does not change with the magnetic field, which corresponds to the "saturation" effect described above. Optimisation of the profile shape and peak density is still possible by adapting the magnetic field strength and topology. It seems to be connected to the helicon wave dispersion relation. A detailed study is on-going in order to create the best possible conditions at the location of the ICRF antenna to be relevant for the intended sheath studies. In the last set of experiments high-resolution passive optical emission spectroscopy was used to estimate an E-field in typical conditions of the IShTAR device operation [18]. To detect the expected Stark-effect-induced changes on the spectral line profile two experimental scenarios have been performed to obtain the results presented here. The reference data were recorded without the voltage applied to the electrode placed directly in the Helicon source, while the second set was obtained for the set of experiments with the electrode biased to a positive

A Test Facility to Investigate Sheath Effects during Ion Cyclotron Resonance Heating

DC-voltage of Uel = 1 kV. The obtained line profiles corresponding to the 4<sup>3</sup>

He-I 4<sup>3</sup>

vessel.

D - 2<sup>3</sup>

He-I were recorded over the whole duration of a discharge, with an exposure time of 0.5 s, in a kinetic series of 24 scans per discharge. The processed data shows a reproducible shift of the

front of the DC-biased electrode. For a rough estimate of the electric field expected in a thermal sheath in front of the electrode we can assume that it originates from the difference in potentials between the electrode and the plasma, over the distance corresponding to the sheath thickness. Compared to the potential applied on the electrode, Uel = 1 kV, the plasma potential is Upl = 0 kV. The sheath thickness corresponding to the density of helium plasma in IShTAR of ne = 1016 m−3, and the electron temperature of T<sup>e</sup> = 5 eV is proportional to the Debye length of λD = 5×10<sup>4</sup> m. Therefore the electric field in the vicinity of the electrode biased to Uel = 1 kV, can be estimated to be the applied voltage drop over the sheath thickness. To complete the

**Figure 10.** Radial profile measurements of the plasma density. The magnetic field strengths refer to centre of the main

P line (**Figure 11**) when the external electric field is present in the thermal sheath in

D - 2<sup>3</sup>

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P transition in

#### **4.2. Characterisation and optimisation of the plasma**

The operating parameters (injected RF power, neutral gas pressure and magnetic field) were scanned to observe the effect on the plasma density and temperature measured in the centre of the main vessel. The purpose is to select the set of parameters that optimises the density. The results of the parametric studies are presented in **Figure 9** [22–24]. At a constant magnetic field, the density increases almost linearly with the power. A small drop around 1.8 kW is observed, which may be due to an internal resonance. The nature of the resonance is not yet clear and needs further investigation. The optimisation of the IShTAR plasma operations is subject of on-going research. However no step in the scan is seen, which would correspond to a change of regime from inductive to helicon mode. This is probably due to a too low level of power with respect to the volume to ionise. The electron temperature does not feature any evolution inside a band between 4 and 7 eV, except for a peak at the same assumed resonance at 1.8 kW. This would confirm that the increased injected power is used to generate denser plasmas and not to heat. At constant injected power, two different trends are observed when the magnetic field is ramped up. At low pressure, the density reaches a plateau at 0.06 T and saturates before starting to drop at 0.14 T. At higher pressure, the density increases almost linearly with the magnetic field. The temperature presents a slight trend towards an increase, but there is not much difference between the high- and low-pressure cases. The cause for the saturation of the density is not yet clear and will require comparison with theoretical models, which are presently not yet available. The radial profiles of the plasma density and of the

**Figure 9.** (a) and (c) Scan of argon plasma parameters as a function of the injected power at constant magnetic field (0.1 T) and neutral gas pressure P (10<sup>4</sup> mbar). (b) and (d) Plasma parameters as a function of the magnetic field at constant injected power (2.5 kW) for two levels of neutral gas pressure P (8×10–4 and 3×10–3 mbar).

RF fields were measured with the manipulator over several discharges and are presented in **Figure 10** for two different magnetic fields at constant injected power of 2.5 kW and a neutral pressure of 10−4 mbar. The magnetic field strengths of 0.12 and 0.06 T, indicated in the figure, refer to the centre of the main vessel. The limit of 0.1 T, which was mentioned before, is for the helical plasma source. It is caused by the 1kA current limit in the small coils. The large coils around the main vessel can generate fields up to 0.4 T. The density profile has a Gaussian shape with a mean width of 10 cm, half the length of the plasma source geometrical radius. The profile does not change with the magnetic field, which corresponds to the "saturation" effect described above. Optimisation of the profile shape and peak density is still possible by adapting the magnetic field strength and topology. It seems to be connected to the helicon wave dispersion relation. A detailed study is on-going in order to create the best possible conditions at the location of the ICRF antenna to be relevant for the intended sheath studies.

with the camera. However, post-experiment inspection revealed that the copper layer has

The operating parameters (injected RF power, neutral gas pressure and magnetic field) were scanned to observe the effect on the plasma density and temperature measured in the centre of the main vessel. The purpose is to select the set of parameters that optimises the density. The results of the parametric studies are presented in **Figure 9** [22–24]. At a constant magnetic field, the density increases almost linearly with the power. A small drop around 1.8 kW is observed, which may be due to an internal resonance. The nature of the resonance is not yet clear and needs further investigation. The optimisation of the IShTAR plasma operations is subject of on-going research. However no step in the scan is seen, which would correspond to a change of regime from inductive to helicon mode. This is probably due to a too low level of power with respect to the volume to ionise. The electron temperature does not feature any evolution inside a band between 4 and 7 eV, except for a peak at the same assumed resonance at 1.8 kW. This would confirm that the increased injected power is used to generate denser plasmas and not to heat. At constant injected power, two different trends are observed when the magnetic field is ramped up. At low pressure, the density reaches a plateau at 0.06 T and saturates before starting to drop at 0.14 T. At higher pressure, the density increases almost linearly with the magnetic field. The temperature presents a slight trend towards an increase, but there is not much difference between the high- and low-pressure cases. The cause for the saturation of the density is not yet clear and will require comparison with theoretical models, which are presently not yet available. The radial profiles of the plasma density and of the

**Figure 9.** (a) and (c) Scan of argon plasma parameters as a function of the injected power at constant magnetic field (0.1 T)

injected power (2.5 kW) for two levels of neutral gas pressure P (8×10–4 and 3×10–3 mbar).

mbar). (b) and (d) Plasma parameters as a function of the magnetic field at constant

been damaged on some parts of the line.

and neutral gas pressure P (10<sup>4</sup>

**4.2. Characterisation and optimisation of the plasma**

160 Plasma Science and Technology - Basic Fundamentals and Modern Applications

In the last set of experiments high-resolution passive optical emission spectroscopy was used to estimate an E-field in typical conditions of the IShTAR device operation [18]. To detect the expected Stark-effect-induced changes on the spectral line profile two experimental scenarios have been performed to obtain the results presented here. The reference data were recorded without the voltage applied to the electrode placed directly in the Helicon source, while the second set was obtained for the set of experiments with the electrode biased to a positive DC-voltage of Uel = 1 kV. The obtained line profiles corresponding to the 4<sup>3</sup> D - 2<sup>3</sup> P transition in He-I were recorded over the whole duration of a discharge, with an exposure time of 0.5 s, in a kinetic series of 24 scans per discharge. The processed data shows a reproducible shift of the He-I 4<sup>3</sup> D - 2<sup>3</sup> P line (**Figure 11**) when the external electric field is present in the thermal sheath in front of the DC-biased electrode. For a rough estimate of the electric field expected in a thermal sheath in front of the electrode we can assume that it originates from the difference in potentials between the electrode and the plasma, over the distance corresponding to the sheath thickness. Compared to the potential applied on the electrode, Uel = 1 kV, the plasma potential is Upl = 0 kV. The sheath thickness corresponding to the density of helium plasma in IShTAR of ne = 1016 m−3, and the electron temperature of T<sup>e</sup> = 5 eV is proportional to the Debye length of λD = 5×10<sup>4</sup> m. Therefore the electric field in the vicinity of the electrode biased to Uel = 1 kV, can be estimated to be the applied voltage drop over the sheath thickness. To complete the

**Figure 10.** Radial profile measurements of the plasma density. The magnetic field strengths refer to centre of the main vessel.

in the future a much more powerful generator can be used, when it will be coupled to the

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163

Attention is also given to developing diagnostics for characterising the plasma parameters and electric fields, especially in the vicinity of the ICRF antenna, since they are the key ingredients for the sheath theories and modelling codes. Different probes and spectroscopic meth-

With the probes the behaviour of the plasma electron density and the three magnetic field components of the injected RF fields are measured under different operating conditions. Modelling is on-going using the COMSOL multi-physics environment [25, 26]. The measure-

For measuring electric fields in the plasma caused by the RF antenna sheaths two approaches are followed. Passive optical emission spectroscopy monitors Stark effects on spectral lines with a high-resolution spectrometer, provided that the local electric fields are strong enough to overcome the broadening of the lines. Doppler-free saturation spectroscopy is more powerful; a laser beam depletes the ground state, eliminates the line broadening effects and makes smaller electric fields visible. However, the more complicated set-up, with a careful alignment of laser beams, makes the measurements much more challenging. After a first test on a glow discharge plasma, the design of the optical path and the installation of the laser at IShTAR

The authors want to thank the technical staff at IPP - Garching, LPP-ERM/KMS and Ghent

The work received support from the Research Foundation Flanders (G0B3115N). This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of

University, in particular F. Fischer, G. Siegl, M. Berte and J. Peelman.

ments will also serve more advanced sheath and edge simulation codes in the future.

ASDEX RF system, with an available power up to the MW level.

ods are used.

have started.

**Acknowledgements**

**Conflict of interest**

There are no conflicts of interest.

the European Commission.

**Notes/Thanks/Other declarations**

**Figure 11.** Experimental 4<sup>3</sup> D-2<sup>3</sup> P He I line profile for EDC = 0 kV/cm and EDC = 2.7 kV/cm (black and red diamonds, respectively) fitted with the simulated theoretical spectra calculated with the EZSSS code (solid lines).

study, the He-I 4<sup>3</sup> D - 2<sup>3</sup> P transition is simulated with the EZSSS code for several values of the DC electric field, and the best match between the measured and simulated lines corresponds to a simulated spectral line exposed to the external electrical field of E = 2.7 kV/cm, as depicted in **Figure 3** with the solid lines. The estimated electrical field agrees with theoretical prediction and showing possibility of passive emission spectroscopy with high resolution to be used for E-field measurements [16, 17].
