**3.5. ICRF system**

*kz*

is determined by the geometry of the antenna:

The radial wavenumber *kr*

large coils and 0.45 kA in the small coils.

different feeding lines.

**3.4. Control system**

*kz* <sup>=</sup> (2*<sup>κ</sup>* <sup>+</sup> 1) \_\_\_ *<sup>π</sup>*

154 Plasma Science and Technology - Basic Fundamentals and Modern Applications

where *κ* is the longitudinal mode number and *LA* is the length of the antenna.

simple dielectric) to minimise the electric fields and the risk of arcing.

*LA*

drical geometry (in a simple case with constant density). This relation is represented for the mode m = 0 in **Figure 4**. For a maximum field B = 0.1 T in the plasma source, a density n<sup>i</sup> ≈ 7 1016 m−3 is necessary for the helicon wave to propagate. The low frequency has been chosen based on simulations with the electromagnetic code MicroWave Studio (MWS) [15] (with a

**Figure 3.** Example of a typical magnetic field topology for IShTAR (in mT). It is generated with currents of 1 kA in the

The plasma source is equipped with a gas valve at the back of the tube with a flow rate range of 5–5000 sccm. Three gases can routinely be used: argon, helium and hydrogen with three

The control system automates the experiments, enables a fast start and remote operations, and it is possible to monitor the status of the different parameters (pressure, temperature,

is determined from the calculation of the electrical field in a cylin-

, (3)

A mock-up of an operational ICRF antenna can be installed on the wall of the vessel and connected on one of the side ports to the transmission line, which is equipped with a ceramic vacuum window. Two RF power sources are presently available for the ICRF system:


The matching of the ASDEX Upgrade generator is insured by a system of two stub tuners. The broadband amplifier requires the installation of an additional capacitor-base matching network more suited to low levels of power.

### **3.6. ICRF antenna**

An ICRF antenna was designed at the Laboratory for Plasma Physics in Brussels, Belgium (LPP-ERM/KMS) and installed in IShTAR. The MicroWave Studio (MWS) [15] model of the

**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.

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

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.

**Figure 6.** Inside view of the main vessel with the different probes: (a) on the back flange and (b) on a movable manipulator

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

http://dx.doi.org/10.5772/intechopen.76730

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After a theoretical study, two different spectroscopic methods have been retained and are

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

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

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

D - 2<sup>3</sup>

P transition has been selected for

presently being developed in parallel.

arm. The helical plasma source is visible on the right.

camera has been installed. In the test work the He n = 4<sup>3</sup>

43 D - 2<sup>3</sup>
