The Lower Hybrid (LH) wave is a plasma wave of frequency between the ion and electron cyclotron resonance frequencies. An LH launched from the plasma boundary can accelerate electrons along the field lines and thereby drive current in the plasma. This method, called Lower Hybrid Current Drive (LHCD) has the higher current drive efficiency and is used to control the edge plasma current and access advanced plasma regimes.

The Frascati Tokamak Upgrade (FTU) is preparing a test, to start in September, which is the first stage of experimental exploration of a new concept considered for the ITER LH wave launcher: the Passive-Active Multijunction (PAM). The PAM is a structure in which the active wave guides are separated by passive wave guides that allow enough space for the cooling system (see photo) and neutron shielding. Since LHCD could contribute to the production of advanced plasma scenarios on ITER, the PAM test is crucial to check the viability of LHCD in steady-state operation. The FTU prototype, developed within a collaboration between the Association Euratom-ENEA and Euratom-CEA, is aimed at verifying the power handling capability of the launcher at ITER relevant densities and magnetic field values (a unique feature of FTU), the coupling flexibility (i.e. the operational range of density in front of the structure) and at validating numerical codes developed for modelling coupling and propagation. Comparing the number of faster than average electrons produced by the PAM with that produced by a conventional grill indicated the effectiveness of the wave launcher. For that purpose, FTU has been equipped with two new fast electron Bremsstrahlung cameras measuring the X-rays generated by the fast electrons produced by lower hybrid waves. The PAM module, which has been successfully characterised in the laboratory during 2002, is presently being assembled on a test bed for testing at high power and will be installed in the tokamak in July. Subsequently, a fully equipped prototype with the cooling system foreseen for ITER will be tested on Tore Supra (Cadarache, France) in long pulse operation.

ENEA (Ente per le Nuove Tecnologie, l’Energia e l’Ambiente):

CEA (Commissariat à l’Energie Atomique)