Progress in Long Sustainment of High Normalized Beta Plasma
I Research on the long sustainment of high beta plasmas has been proceeded in JT-60U. In the new series of experiments, the NBI power was carefully controlled in order to avoid the deterioration of plasma confinement caused by the neoclassical tearing mode. A multi injection of deuterium ice pellets was used to keep high pressure plasmas. The operational technique of JT-60U was also improved so as to keep a high triangularity configuration (δx = 0.45).
In a plasma with Ip = 1.0 MA, Bt = 1.8 T, high normalized beta βN ~ 2.7 has been successfully sustained for 7.4 s (Fig.), which is about 60 times longer than the energy confinement time τE and considerably exceeded the former record of 4 s with the same βN value. Although the magnetic fluctuations of the neoclassical tearing mode (m=3, n=2) appeared, its amplitude has saturated at low level and did not degrade the plasma confinement all through the NBI heating phase.
This result substantially exceeds the planned βN value of 1.8 in the standard operation of ITER and even leads to a possibility of operation with higher βN.
Fig. Waveforms of plasma current, Ip, neutral beam injection power, PNB, normalized beta, βN, magnetic fluctuations by the m=3/n=2 neoclassical tearing mode.