**3.7. Diagnostics**

In the present configuration, each diagnostic dedicated to physics applications has its own acquisition system. A centralised acquisition system is being built with a NI Chassis and a set of digitizer boards to offer a common time reference, shared data storage and network access. The instrumentation is still under development. At present the two main tools to diagnose plasma parameters and electric fields are probes and spectroscopy. In future other diagnostics will be added, for example an interferometer for density measurements has already been designed, in order to bench mark the data obtained by the Langmuir probes. In addition, more RF compensated probes will be added in the helical plasma source to study better the helicon physics and the effect of the magnetic topology.

The probes are depicted in **Figure 6**. Different Langmuir and B-dot probes have been installed. **Figure 6a** shows an inside view of the back flange, with an array of three probes in the middle, to the right a planar non-compensated Langmuir probe with a diameter of 2 cm can be seen. A manipulator arm (**Figure 6b**) can carry up to four probes simultaneously, which allows a partly 2D scan of the plasma profile on a shot-to-shot basis. In **Figure 6b** also the ICRF A Test Facility to Investigate Sheath Effects during Ion Cyclotron Resonance Heating http://dx.doi.org/10.5772/intechopen.76730 157

**Figure 6.** Inside view of the main vessel with the different probes: (a) on the back flange and (b) on a movable manipulator arm. The helical plasma source is visible on the right.

retained geometry (with initial dimensions in mm) of the IShTAR antenna is visible in **Figure 5**. The strap is fed from the top feed through by a coaxial transmission line connected to a vertical plate inserted into the strap. Unlike usual ICRF antennas the IShTAR antenna is not designed to couple as much power as possible to the plasma, but rather to generate the typical electric fields structures expected in a tokamak. The first simulations with MWS clearly demonstrated some issues due to the presence of sharp angles in the lateral limiters. Even by further smoothing these angles (see **Figure 5c**) the electric fields maps were dominated by peaked radiations at the level of the limiters, which could also be partly due to numerical issues (meshing). We therefore decided to replace these profiled limiters by rectangular plates: see **Figure 5d**, resulting in a more regular electric field distribution. This change also considerably simplifies the building of the antenna box [16]. The antenna was installed in

**Figure 5.** MicWs model of the ICRF antenna and its feeding line; (a) lateral cut view of the initial geometry; (b) front

view; (c) zoom of the angular part of the limiter; (d) simplified geometry; All dimensions are given in mm.

156 Plasma Science and Technology - Basic Fundamentals and Modern Applications

In the present configuration, each diagnostic dedicated to physics applications has its own acquisition system. A centralised acquisition system is being built with a NI Chassis and a set of digitizer boards to offer a common time reference, shared data storage and network access. The instrumentation is still under development. At present the two main tools to diagnose plasma parameters and electric fields are probes and spectroscopy. In future other diagnostics will be added, for example an interferometer for density measurements has already been designed, in order to bench mark the data obtained by the Langmuir probes. In addition, more RF compensated probes will be added in the helical plasma source to study better the

The probes are depicted in **Figure 6**. Different Langmuir and B-dot probes have been installed. **Figure 6a** shows an inside view of the back flange, with an array of three probes in the middle, to the right a planar non-compensated Langmuir probe with a diameter of 2 cm can be seen. A manipulator arm (**Figure 6b**) can carry up to four probes simultaneously, which allows a partly 2D scan of the plasma profile on a shot-to-shot basis. In **Figure 6b** also the ICRF

IShTAR and is visible in **Figure 6b**.

helicon physics and the effect of the magnetic topology.

**3.7. Diagnostics**

antenna can be seen, it has a single antenna strap, which is curved to follow the plasma shape. In this picture the antenna box was not yet closed and the connection to the feeding line on the top is visible. For the sheath physics, an important parameter to investigate is the electric (E) field created at the antenna and plasma facing components. The ions accelerated in the sheath potential can damage the components by creating a local overheating (hot spots) and or sputtering, as it is has been observed during ICRF experiments in different fusion machines.

After a theoretical study, two different spectroscopic methods have been retained and are presently being developed in parallel.

Passive optical spectroscopy is selected as the first approach to directly measure electric fields in the vicinity of an ICRF antenna, without disturbing the plasma environment [17–20]. This technique enables studying the perturbation of the electronic structure of an atom caused by an external electric field, the Stark effect. These perturbations are detectable as a shift of the central wavelength of a spectral line, and the occurrence of forbidden components of the fine structure in a spectral line profile. We have focused our research on the Stark effect on 43 D - 2<sup>3</sup> P transitions in helium. The method requires a high-resolution spectrometer with high dynamic range detector capable to resolve the allowed and forbidden lines of the transition under the study. To this end a 0.75 m Andor spectrometer equipped with an Andor ICCD camera has been installed. In the test work the He n = 4<sup>3</sup> D - 2<sup>3</sup> P transition has been selected for the E-field measurements, since the signal-to-noise ratio of the recorded spectra was acceptable and the line Stark broadening is strongly affected by the E-field. Once the time-averaged spectra are recorded, the spectral line profiles are compared to the simulated ones and the electric field amplitude is extracted with the method of least squares. To simulate the spectra 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 continuous spectrum is obtained.

transition of He or H atoms. The pump beam depletes the ground state; the probe beam passes the plasma with reduced absorption. By depleting the ground state, the fine structure of the spectral line should become more clearly visible, in the form of local dips in the Doppler broadened absorption line. This allows measuring line profile with eliminated effect of the

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

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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

Doppler broadening and precisely estimate E-field with high sensitivity.

**4.1. Sequence of a typical discharge**

in front of the ICRF antenna.

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

**Figure 7** depicts the modelled triplet line profile corresponding to the 4<sup>3</sup> D - 2<sup>3</sup> P He-I transition 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

**Figure 7.** Triplet He-I 4<sup>3</sup> D-2<sup>3</sup> 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 component at 4471.6 Å.

transition of He or H atoms. The pump beam depletes the ground state; the probe beam passes the plasma with reduced absorption. By depleting the ground state, the fine structure of the spectral line should become more clearly visible, in the form of local dips in the Doppler broadened absorption line. This allows measuring line profile with eliminated effect of the Doppler broadening and precisely estimate E-field with high sensitivity.
