  The non-solenoid start-up technique reported here relies on a double mode
conversion for electron Bernstein wave (EBW) excitation. It consists of the
mode conversion of the ordinary mode, entering the plasma from the low field
side of the tokamak, into the extraordinary (X) mode at a mirror-polarizer
located at the high field side. The X mode propagates back to the plasma,
passes through electron cyclotron resonance and experiences a subsequent X to
EBW mode conversion near the upper hybrid resonance. Finally the excited EBW
mode is totally absorbed at the Doppler shifted electron cyclotron resonance.
The absorption of EBW remains high even in cold rarefied plasmas. Furthermore,
EBW can generate significant plasma current giving the prospect of a fully
solenoid-free plasma start-up. First experiments using this scheme were carried
out on MAST [V. Shevchenko et al, Nuclear Fusion 50, 022004 (2010)]. Plasma
currents up to 33 kA have been achieved using 28 GHz 100kW 90ms RF pulses.
Recently experimental results were extended to longer RF pulses showing further
increase of plasma currents generated by RF power alone. A record current of
73kA has been achieved with 450ms RF pulse of similar power. The current drive
enhancement was mainly achieved due to RF pulse extension and further
optimisation of the start-up scenario.
