29th Symposium on Fusion Technology (SOFT 2016)

P2.034 A Metamateral Load For The LH Range Of Frequency In Magnetized Plasmas

6 Sep 2016, 14:20

1h 40m

Foyer 2A (2nd floor), 3A (3rd floor) (Prague Congress Centre)

Board: 34

Poster B. Plasma Heating and Current Drive P2 Poster session

Speaker

Julien Hillairet (IRFM)

Description

The coupling of lower hybrid (LH) range of frequencies waves to strongly magnetized plasmas is a critical issue on tokamaks as the RF power which can be transferred from the antenna to the plasma is often limited by the quality of this coupling. Development of new types of antennas aiming at improving the ability of the antenna to handle large powers in stationary conditions, as it will be requested on a fusion reactor, is hampered by the long and costly delay between the design and the feedback one gets from experiments on large facilities such as tokamaks. Moreover, the three-dimensional nature of the electron density in the vicinity of an LH antenna makes the analysis of the coupling difficult.  Hyperbolic Metamaterials, based on piling up two-dimensional periodic structures, are characterized by a dielectric tensor which has a negative permittivity constant in one direction epsilon// and therefore can mimic an homogeneous plasma whose electron density n is given by n/nc=1-epsilon//, where nc is the cut-off density of the wave (1.7×1E19m-3 at 3.7GHz).  A fishnet load, composed of 26 grids layers, was designed with epsilon//=-3, which corresponds to the optimal coupling conditions for a LH antenna. Each layer is a thin metallic grid (width 0.2mm, periods 28mm and 5.1mm) deposited on a 50mm-thick polyimide film. A low density foam (epsilon=1.05) is used as a spacer between the layers. The load was coupled to a 3.7GHz multi-junction type antenna module composed of 6 rectangular waveguides (8mm×72mm) aligned along the magnetic field. For an optimal distance of the load to the antenna, the power reflection coefficient measured in the waveguides (~20% on average) and at the input of the module (~1.5%) are consistent with the values computed with a full-wave code. Measurements of the electric field pattern in the load are also presented.