**3. Design and setup**

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 have been retained for the final design.

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].

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

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 explained in the next subsections.
