Author: André Messiaen (ERM/KMS)
For the RF heating of large tokamaks like JET or ITER, high power-density antenna arrays are required to provide the large amount of power needed to achieve the required central plasma temperature of 10 keV. In ITER an amount of 20 MW of RF power in the Ion Cyclotron Resonance (ICR) frequency band of 40-55 MHz has to be radiated through a port surface measuring about 1.5m by 1.9m. Furthermore, large Edge Localized Modes (ELMs) will produce abrupt changes of antenna loading, capable of causing a safety shut-down of the high-power generators. The ITER ICRH launcher must thus be capable of high power density, and the matching system of the antenna array must tolerate large load fluctuations (in the order of 3 to 5).

At the ERM/KMS, the main lines of the EFDA R&D program in support of ITER are the construction of the so-called ITER-like ICRH antenna for JET, and the parallel conceptual designs studies of possible ITER launchers in which the antenna matching is either made internally (i.e. before the vacuum window), or externally. Whereas the former is developed by CEA, the latter effort is led by LPP-ERM/KMS. In this article we describe the solution with external matching, and the test stand that was build to test the scheme.

Figure 1 shows the antenna lay-out. An array of 24 radiating straps should be capable of providing the large power density with an affordable antenna voltage. The main advantages of this design are the absence of in-vessel remotely operated components to achieve the matching, and the use of 4- ports passive junctions that provide more uniform RF current distribution among the straps and minimize the number of matching circuits. Indeed, these junctions combine the 24 straps in 8 triplets which are linked to “conjugate T” or hybrid matching circuits which should provide the load resilience needed in presence of ELMy discharges. The straps will unavoidably be coupled to each other as they are radiating in the same medium. The intricate theoretical expectations of this coupling on the load resilience have to be checked before the installation of such a complex antenna array in ITER.

The impedance matrix of the array remains identical when decreasing the scale length and increasing the working frequency by the same factor. It was therefore decided to construct, using a reduction scale factor of 5, a mock-up of the complete antenna array of 24 straps, grouped in 8 triplets by eight 4-ports junctions. This was achieved by the company TECHNIFUTUR (Sart- Tilman, Seraing, Belgium), through numerical machining starting from the original Catia 3-D drawings of the project. The frequency range corresponding to the full-scale system is 200- 275MHz in the reduced-scale model. Figure 2 shows the inner part of one 4-port junction.

Whereas tests in absence of plasma are already useful, they are not capable to simulate all the electromagnetic properties in presence of plasma or to properly test the tuning algorithm. However, tests with realistic plasma-like load conditions can be obtained when a medium having a large dielectric constant faces the strap array. This is essentially the case because in such a medium it is possible to achieve wave numbers that are typical for the fast Alfvén wave that is launched from the antenna in a magnetized plasma. Water can be advantageously used as such a load. To avoid the spurious effect of wave reflection on the walls of the water tank, salt can be added to the water to provide sufficient wave damping.

Figure 3 shows a picture of the complete ITER test stand with the mock-up and its water tank load. The array is mounted on a sliding support in order to adjust the distance between the array and the water tank, thus allowing to study the antenna loading as a function of this critical parameter. An extensive program has started in which various matching schemes are realized by means of external transmission line components and power sources, and their performances are checked and compared for various load conditions.

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