**3.3. External plasma source**

The plasma source is based on the concept of ionisation by helicon waves, which offer the possibility for large volumes to reach high densities (1018 m−3 and above) with uniform radial profiles


**Table 1.** Characteristics of the large and small coils.

[10–12]. The price for this objective is the high level of injected power (several kW), which can lead to dramatic damages if the flow of the power in the source is not controlled. To reduce the risks the operations will be carried out first in inductive mode and, by progressively increasing the available power on the generator, we will try to reach the mode transition to the helicon mode. The source includes a glass tube as vacuum vessel, four magnetic coils (the helicon wave is a magnetised wave), a helical antenna with its own power supply and a gas injector.

The most risk prone component is the glass tube, which faces the following dangers:

can be used to connect the RF transmission line for the ICRF antenna and pressure gauges and valves, and, on the opposite side, one large horizontal port (D = 225 mm) and two smaller ports (D = 100 mm) angled at approximately 30 degrees with respect to the horizontal line. All ports have the same axial position, at 391 mm from the back end of the cylinder. On both ends of the vessel, there are two flanges with the following glass windows: at the back end, three large windows (D = 160 mm) and two small panels (D = 105 mm) at one end; at the front end, two large ports (D = 400 mm) and two small windows (D = 105 mm). The plasma source is connected to one of the front large ports. In the first months of operation it was not centred with respect to the axis of the main vessel, which resulted in instable operating conditions. Therefore later on the connecting flange was changed and now the helical source and main vessel are aligned. The vacuum system is connected to the back flange and consists of a pre-vacuum pump to reach a pressure of 10−2 mbar and a turbo molecular pump to create a high vacuum at 10−6 mbar. These pumps have a large flow rate making; the minimal pressure can be reached in 30 min time.

152 Plasma Science and Technology - Basic Fundamentals and Modern Applications

The main vessel is equipped with two magnetic coils in a Helmholtz configuration. The distance between the coils is about 815 mm. Some of the characteristics are given in **Table 1**. The outer/inner diameter is 2200/1200 mm and the thickness varies between 120 and 180 mm. The coils are connected to the central high current supply of the institute, which uses two modules connected either in parallel or in series using regular electrical voltage at 10.5 kV at 50 Hz. The magnetic (B) field in the centre of the vacuum vessel is expected to be 0.1 T for a current of 2.4 kA in the main coils. Higher currents and fields are possible. The current generator can sustain 4 kA for 10s and 12kA for 1 s, corresponding with B-fields up to 0.275 and 0.4 T respectively.

The plasma source is based on the concept of ionisation by helicon waves, which offer the possibility for large volumes to reach high densities (1018 m−3 and above) with uniform radial profiles

Inductance L 0.4 mH 1.74 mH Resistance R 0.86 mΩ 1.54 mΩ Maximal current 47 kA 10 kA Maximal voltage 3 kV 12 V Time constant τ 0.5 s n.a. Pulsed current 4 kA 1 kA Nominal current 1.5 kA n.a. Nominal voltage 600 or 340 kV n.a. Pulse length 10 s 10 s Duty cycle 5.6% n.a.

**Large coils Small coils**

**3.2. Large magnetic field coils**

**3.3. External plasma source**

**Table 1.** Characteristics of the large and small coils.


The characteristics of the helicon magnetic coils are displayed in **Table 1**. These small coils are fed with a power supply DC10, with current between 0 and 1 kA, and voltage of 10 V. The maximum field inside the source is 0.1 T. In standard operation, this field is superimposed on the field generated by the large coils. The structure with the helicon in its central position is presented in **Figure 3** for a current of 1kA in the large coils and 0.45 kA in the small coils. The resulting B-field in the centre of the vessel is around 40 mT. By adapting the ratio in the field in small and large coils the plasma performance can be influenced. The optimisation of the performance is the subject of on-going research.

The test-bed flexibility makes it possible to test different types of antennas. The presently mounted antenna is a Shoji III half turn [14]. In other experiments, it proved to have a better coupling, especially with the mode m = 1. It has a length L<sup>a</sup> = 1 m and diameter d<sup>a</sup> = 0.6 m. The dispersion relation of the helicon wave sets a relation between the minimal density for which the wave can propagate and the magnetic field [14]:

$$kk\_z = \frac{e\,\mu\_0\,\text{m}\_\circ\,\text{\omega}}{B\_0} \,\text{\textdegree} \tag{2}$$

*k* is the total wave number, *kz* is the longitudinal wave number, *e* is the electron charge, *μ0* is the vacuum permeability, *ne* is the electron density, *ω* is the pulsation of the generator and *B0* is the static magnetic field.

**Figure 3.** Example of a typical magnetic field topology for IShTAR (in mT). It is generated with currents of 1 kA in the large coils and 0.45 kA in the small coils.

*kz* is determined by the geometry of the antenna:

$$k\_z = \left(2\kappa + 1\right) \frac{\pi}{L\_A'} \tag{3}$$

current) of the test-bed. The control is based on a Simatic system for the hardware and WinCC applications for the software with a time step of 10 ms; it controls the automatic opening and closing of the valves and pumps during the vacuum build-up and re-pressurisation. It also enables the operator to define ramps in the coil current. It ensures the safety of operation by keeping the access doors locked below a predefined vacuum level in the vessel, checking the

**Figure 4.** Minimum density required for the helicon mode *m* = 0 to propagate as a function of the magnetic field for the

= 0.3 m.

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

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

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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

**i.** One ASDEX Upgrade RF generator (30–120 MHz) with a power up to 2 MW at 30–80 MHz

**ii.** A broadband amplifier with an output power of 1 kW in the frequency range 100 kHz to

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

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

vacuum window. Two RF power sources are presently available for the ICRF system:

flow of coolant in the different components.

chosen antenna parameters *f* = 11.7 MHz, length = 1 m, radius r<sup>a</sup>

and 1 MW at 80–120 MHz.

network more suited to low levels of power.

**3.5. ICRF system**

100 MHz.

**3.6. ICRF antenna**

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

The radial wavenumber *kr* is determined from the calculation of the electrical field in a cylindrical 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 simple dielectric) to minimise the electric fields and the risk of arcing.

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 different feeding lines.

#### **3.4. Control system**

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,

**Figure 4.** Minimum density required for the helicon mode *m* = 0 to propagate as a function of the magnetic field for the chosen antenna parameters *f* = 11.7 MHz, length = 1 m, radius r<sup>a</sup> = 0.3 m.

current) of the test-bed. The control is based on a Simatic system for the hardware and WinCC applications for the software with a time step of 10 ms; it controls the automatic opening and closing of the valves and pumps during the vacuum build-up and re-pressurisation. It also enables the operator to define ramps in the coil current. It ensures the safety of operation by keeping the access doors locked below a predefined vacuum level in the vessel, checking the flow of coolant in the different components.
