The Frascati Tokamak Upgrade (FTU) at the Association Euratom – ENEA in Frascati (Italy) 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 (which transmit the RF power) are separated by passive waveguides (which transmit no power, but contribute to the launched wave spectrum) that allow enough space for the cooling system (see photo) and neutron shielding. Lower Hybrid waves could contribute significantly to the production of externally driven current in steady-state (“advanced”) scenarios in ITER, so that the PAM test is crucial to test the viability of Lower Hybrid waves for such operational regimes. The FTU prototype, developed within a collaboration between ENEA and CEA, is aimed at testing the concept.

The power density achieved on the FTU experiment in almost steady state conditions has been 75 MW/m2, close to the design value of 80 MW/m2, and approximately 50% greater than the value required for ITER when allowance is made for the difference in RF frequencies between FTU (8 GHz) and ITER (5 GHz).

An important feature of the PAM is the good coupling capability at low density (below the cut-off value at the waveguide mouth), as will be the case in ITER. This feature has been successfully tested on FTU by coupling LH power with the PAM located close to (even 2 mm behind) the vacuum chamber inner surface: the reflected power in these conditions is below 1.6%, as shown in Fig. 1 for a discharge where the PAM grill was flush to the vacuum chamber wall, well behind the limiter.

Current drive effects are evidenced in Fig. 2, where a clear decrease in the loop voltage can be seen. A comparison is made between the PAM powered on its own, a conventional grill excited to launch about the same power with a quite similar spectrum of modes propagating in the plasma (as defined by their parallel refractive index, “N||”), and with a combined PAM and conventional grill. From the dash-dotted horizontal lines indicating the average value of Vloop during the different phases of the discharge, it can be seen that the current drive efficiency, characterised by the decrease in loop voltage, is comparable for the PAM and a conventional launcher. A more detailed analysis using a camera which measures Bremsstrahlung radiation from fast electrons is underway.

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