Successful test of the ITER lower hybrid launcher prototype on FTU
The Frascati Tokamak Upgrade (FTU) has successfully tested a
prototype of the Lower Hybrid wave launcher presently considered for ITER: the
Passive-Active Multijunction (PAM). The PAM is a slow-wave structure in which
the active waveguides are separated by passive waveguides that allow enough
space for the cooling system and neutron shielding in the harsh environment
of a burning plasma (see photo). Since Lower Hybrid waves are essential for
producing advanced scenarios on ITER, the PAM test is crucial to test the viability
of Lower Hybrid waves in steady-state operation. The FTU prototype, developed
within a collaboration between ENEA and CEA, was aimed at testing the concept.
The power density achieved on the FTU experiment in quasi steady state conditions has been 75MW/m2, close to the design value of 80MW/m2 and above the value envisaged for the ITER launcher (corresponding to an equivalent 52MW/m2).
An important feature of the PAM is the good coupling capability far from the plasma, as anticipated in ITER conditions. This feature has been successfully tested on FTU by coupling LH power with the PAM located in a port a few millimetres behind the vacuum chamber: the reflected power in these conditions is below 1.6%.
Evident current drive effects have been observed, with a clear decrease in the loop voltage, which are comparable to those obtained by a conventional grill opportunely phased to inject a parallel phase velocity spectrum similar to that produced by the PAM.