**3.1. Vacuum vessel**

speed of the vacuum pumps and on the control system, which has to be fully automatized.

<sup>⊥</sup> values for full and decoupled solutions as a function of the plasma density. (a) The reference case of *ISh*TAR

for typical operating conditions: *f* = 5 MHz, *Bt* = 0.1T, *Te* = 10 eV and plasma composition: 100% Ar. (b) A tokamak scenario with: *f* = 50 MHz, *Bt* = 2T, *Te* = 10 eV, plasma composition: 95% D, 5% H. The notation "log10" denotes sign (F) log10 (|F|) when |F| > 1 and 0 when |F| < 1. It is introduced to enable capturing the very different length scales of the modes the cold plasma supports while allowing to identify regions of wave propagation and evanescence by a mere glance of the sign of log10 (*k* ⊥ <sup>2</sup> ). In the low density region, both roots approach the vacuum limit, *k* ⊥ <sup>2</sup> = *k* 0 <sup>2</sup> − *k* // <sup>2</sup> , as expected, while at higher densities representative for the core one root is propagative (fast wave FW) and the other

**2.** Short vessel opening and closing durations: it should be possible to open the vessel, install new components (like an antenna) and close the vessel in a couple of days; here again, it helps to have short iteration times between the installation of new solutions, their test and

**3.** The "plug and play" instrumentation platform: the test-bed enables the analysis of many different phenomena requiring different types of diagnostics, either of in-house origin or provided by external teams. The control and acquisition system has to be universal enough to reduce the time between the installation of the instrument and the exploitation of its

The last constraint comes from the costs. To keep the costs under control, not all components were designed from scratch. Several have been refurbished from previous experiments. Among the most important ones is the main vacuum vessel with its 8 kA magnetic coils, the smaller coils and the ICRF generator, which comes from the WEGA stellarator. The downside is that their range of operation or geometry can limit the operational parameter space. This

The main design choice concerns the creation of the plasma in front of the ICRF antenna: either the ICRF itself can breakdown the gas or an external source can be used. Both solutions

overview of the requirements clarifies the guidelines for the design of the facility.

The purpose is to be able to do short iterations between experiments and analysis.

the analysis of their impact.

is deeply evanescent (slow wave SW).

**2.3. Cost control**

**Figure 1.** *k* <sup>2</sup>

**3. Design and setup**

have been retained for the final design.

data in the centralised discharge database.

150 Plasma Science and Technology - Basic Fundamentals and Modern Applications

The vacuum vessel has a cylindrical geometry. The length is 1126 mm and the diameter (D) is 1000 mm. There are five ports in the wall: two horizontal ports (D = 225 mm) on one side which

**Figure 2.** Overview of the different components of IShTAR.

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.

[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

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

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

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**i.** Arcing: the risk is to hit and break the glass tube. If only inductive discharges would be generated, it would be possible to install a shield to protect the tube (with the advantage to remove spurious capacitive effects in the plasma); however, with helicon plasmas, the electrostatic component is necessary as well. Therefore, before increasing the power, we need to investigate the distribution of RF fields in the plasma source and check that we are able to have a good absorption in the plasma (decreasing the voltage on the antenna). Even with that, an arc detection system may be necessary to prevent in a very

**ii.** Heat fluxes: the glass tube is made of borosilicate glass Duran with a safe maximum temperature of 150 and 500°C if it is submitted to homogeneous fluxes on its surface. Given its specific heat capacity Cp = 750 J/(kg °C), a mass of 17 kg and a discharge duration of

**iii.** Sputtering: this is the unknown and will require a better analysis. Previous experiments show an increase of the sputtering during helicon discharges, but variable from machine

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

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

*B*0

is the longitudinal wave number, *e* is the electron charge, *μ0*

is the electron density, *ω* is the pulsation of the generator and *B0*

, (2)

is

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:

short time the development of an arc.

8 s, it can sustain a flux of 250 W at 150°C.

performance is the subject of on-going research.

the wave can propagate and the magnetic field [14]:

*kkz* <sup>=</sup> *<sup>e</sup> <sup>μ</sup>*<sup>0</sup> *ne <sup>ω</sup>* \_\_\_\_\_\_

*k* is the total wave number, *kz*

the vacuum permeability, *ne*

is the static magnetic field.

to machine.
