An overview of FTU results
Since the 2010 IAEA-FEC Conference, FTU has exploited improvements in
cleaning procedures and in the density control system to complete a
systematic exploration of access to high-density conditions in a wide
range of plasma currents and magnetic fields. The line-averaged
densities at the disruptive limit increased more than linearly with the
toroidal field, while no dependence on plasma current was found; in
fact, the maximum density of 4.3 × 10<SUP>20</SUP>
m<SUP>-3</SUP> was reached at B = 8 T even at the minimum current
of 0.5 MA, corresponding to twice the Greenwald limit. The lack of
plasma current dependence was due to the increase in density peaking
with the safety factor. Experiments with the 140 GHz electron cyclotron
resonance heating (ECRH) system were focused on the sawtooth (ST) period
control and on the commissioning of a new launcher with real-time
steering capability that will act as the front-end actuator of a
real-time system for ST period control and tearing mode stabilization.
Various ECRH and electron cyclotron current-drive modulation schemes
were used; with the fastest one, the ST period synchronized with an 8 ms
modulation period. The observed period variations were simulated using
the JETTO code with a critical shear model for the crash trigger. The
new launcher was of the plug-in type, allowing quick insertion and
connection to the transmission line. Both beam characteristics and
steering speed were in line with design expectation. Experimental
results on the connection between improved coupling of lower hybrid
waves in high-density plasmas and reduced wave spectral broadening were
interpreted by fully kinetic, non-linear model calculations. A
dual-frequency, time-of-flight diagnostic for the measurement of density
profiles was developed and successfully tested. Fishbone-like
instabilities driven by energetic electrons were simulated by the hybrid
MHD-gyrokinetic XHMGC code.