**2.1. Functional requirements**

**1. Introduction**

diagnostic instrumentation.

IShTAR (Ion cyclotron Sheath Test ARrangement) is a linear magnetised plasma test facility for RF sheaths studies at the Max-Planck-Institut fuer Plasmaphysik in Garching (IPP-Garching), Germany. The test facility consists of a cylindrical vacuum vessel with a diameter of 1 m and length of 1.1 m. The plasma is created by an external cylindrical plasma source equipped with a helical antenna that has been designed to excite the m = 1 helicon mode. In contrast to a tokamak, a test stand provides more liberty to impose the parameters and gives better access for the instrumentation and antennas. The project also supports the development of diagnostic methods for

The present specific application is to analyse the formation of RF sheaths when waves in the Ion Cyclotron Range of Frequencies (ICRF) are injected in the plasma. In tokamaks, they have been successfully used to heat the plasmas to the nuclear fusion relevant temperatures of around 10 keV [1], but experimental and theoretical studies have shown that several spurious edge plasma interactions can prevent an optimal coupling if effects are forgotten or misunderstood. Particularly, the RF sheaths are created on the antenna surface and on limiters by the large E‖(parallel to the background magnetic field) component of the Slow Wave (SW) from the antenna [2, 3]. Under this field, the electrons with a lighter mass are more prone to be expelled from the plasma than the heavier ions: this creates a net positive DC voltage, called rectified sheath potential which conserves the charge ambipolarity. The additional DC potential raises the energy of the ions knocking the wall, the sputtering increases, hot spots are generated and more impurities are released into the plasma. At the same time the RF power losses become more pronounced [4–6]. Even though extensive studies have been carried out in the last years and the understanding has improved [7], a definitive solution is still pending because a systematic investigation cannot be done efficiently in the main fusion experiments. There are several reasons: the experimental time allocated to the topic is limited; the access, location and operations of diagnostics dedicated to RF sheath properties is constrained by other instruments, by the machine operation parameters and by the plasma itself. The application of corrective measures or the test of new antenna designs is constrained by the limited opening time of the tokamak and by the interfaces with the access port and the surrounding wall. Therefore, a dedicated test-bed offering more working time has been developed and assembled, in which it is possible to launch the waves using different antenna designs. More time can be devoted to the analysis of RF sheaths effects. New diagnostics to measure plasma parameters and electromagnetic fields in front of the ICRF antenna can be tested and the results can be compared with existing theoretical models of the sheath rectification effect. Different types of solutions emerging from modelling can be tested. In addition, other interesting phenomena related to antenna-plasma coupling can be analysed: sheath-plasma waves and resonances, effect of fast ions, plasma breakdown and wall conditioning by the ICRF antenna. In the first part, the constraints, which guided the initial choice and the operating parameters, are discussed. It is followed by a description of the resulting design and architecture of the facility. In the last part, first operations and measurements are illustrated, to conclude with the next steps and extension plans for the

characterising RF sheaths and validating and improving the theoretical predictions.

148 Plasma Science and Technology - Basic Fundamentals and Modern Applications

The main requirement for a test-bed dedicated to the analysis of ICRF antenna/plasma coupling, independent of the type of topic studied, is the geometry of the configuration, which has to be as similar as possible to the tokamak edge. This requires a vacuum vessel with a curved wall, a support to mount the ICRF antenna and a port for the RF feeding lines. A magnetised plasma is present and located only few centimetres away from the antenna (typically 15–20 cm). To simplify the configuration the magnetic lines are straight. Two main design factors have an important impact for the sheath studies: the plasma parameters and the antenna frequency. The former constrains the design of the plasma source and the second the choice of generator and matching system for the ICRF antenna. These factors determine the type of waves that are coupled and whether they are able to propagate, or not, in the plasma. The main parameters for the plasma are: the gas type, magnetic field, density and temperature. In a homogeneous, magnetised plasma, the wave equation reads:

$$
\overline{k} \times \{\overline{k} \times \overline{E}\} + k\_0^2 \overline{K} \cdot \overline{E} = 0,\tag{1}
$$

where *k*¯ is the wave vector, *E*¯ the electric field associated with the wave, ω the angular frequency of the wave in vacuum, k0 = ω /c the vacuum wavelength with *c* the speed of light and *K*¯ is the plasma dielectric tensor. A detailed study of the wave propagation in IShTAR can be found in [9]. The main purpose of the test-bed is to provide conditions where the dispersion relation of the fast wave (used for heating) and of the slow wave (which produces the RF sheath) are similar to the tokamak case. Cold plasma calculations have been done for the reference case of a tokamak. It is plotted in **Figure 1b**. The operational parameters (magnetic field, ICRF frequency) of the test-bed have been varied to get similar wave behaviour as for the tokamak case, where slow and fast waves are well distinguished (**Figure 1a**).

#### **2.2. Operational requirements**

The main operational constraint for the operation and maintenance of the machine is the flexibility and accessibility: this is what makes the difference between the tokamak and the test-bed. We will see in the design section that the requirements lead to a machine with an important level of complexity. It is necessary that it does not impede an easy access to the machine or impose long time of preparation before the experiments. Therefore we have the following requirements on the operations and on the maintenance:

**1.** Start-up sequence duration shorter than an hour: from initial sleep phase at atmospheric pressure to first plasma in vacuum with full instrumentation. This has an impact on the

The ICRF antenna itself can create plasma. The electrons are heated by the parallel component (parallel to the B-field) of the RF electric field. The advantage would be that no other means than the ICRF components are needed to create the plasma. But there are three disadvantages: first, the plan is to test different types of antenna and it will be difficult to reproduce the same plasma with different geometries. Second, the physics of plasma breakdown with ICRF antennas is still not completely understood. And third, the parallel electric field used for the breakdown is the same that creates the RF sheath: it may be difficult to disentangle both phenomena [10].

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

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

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An external plasma source solves this problem by separating the plasma creation from the plasma heating. The concept of a helicon discharge [11–13] has been retained: it is compatible with the magnetic field and it creates high densities in large volumes. However, the solution has a price: the physics of helicon sources is still an area of investigation; it requires large amount of power to ionise the plasma volume required; it adds a new wave inside the plasma, which can interact—depending on the frequency—with the ICRF waves. Therefore, a backup

The test-bed has therefore two large components: the main vessel where the ICRF antenna is installed and connected to the power transmission lines; and the plasma source, connected with an open port to the main vessel, which generates the plasma that will flow in front of the antenna. These systems are connected to power generators, gas feeding lines, DC current modules and real-time controllers to monitor the operations and the safety of the test-bed. An overview of the facility is depicted in **Figure 2**; the characteristics of different components are

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

solution exists with the use of a more classical inductive coil.

explained in the next subsections.

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

**3.1. Vacuum vessel**

**Figure 1.** *k* <sup>2</sup> <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 is deeply evanescent (slow wave SW).

speed of the vacuum pumps and on the control system, which has to be fully automatized. The purpose is to be able to do short iterations between experiments and analysis.


#### **2.3. Cost control**

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