Progress in High Density Experiments with Potential Confinement on GAMMA 10

After the last Novosibirsk Conference (1998), GAMMA 10 experiments have advanced in long sustainment of confining potential and higher density experiments. An experiment which attained doubling of density due to potential confinement with a 50 ms duration was reported before [1]. Experiments for long sustainment of potential confinement were carried out in order to study problems of steady state operation of a tandem mirror reactor. A confining potential was sustained for 150 ms by sequentially injecting two electron cyclotron resonance heating (ECRH) powers in the plug region. It was difficult before to increase the central cell density higher than about 2.5 × 10¹²cm⁻³ with and/or without potential confinement due to some density limiting mechanism. In order to overcome this problem, a new higher frequency ion cyclotron range of frequency (ICRF) system (RF3: 36–76 MHz, ω/ω_ci~6-10) has been installed. A higher density plasma has been produced with RF3. In addition to RF3, neutral beam injection (NBI) in the anchor cell became effective by reducing neutral gas from beam injectors. The plasma density in the anchor cell increased 70% by NBI for 20 ms, and the central cell density increased 20% with the density increase in the anchor cell. Potential confinement experiments at higher central cell densities up to 4 × 10¹² cm⁻³ were carried out with RF3 and NBI and 15% density increase due to the potential confinement was obtained in the high density experiments. The ion temperature on the axis and particle confinement time during potential confinement were 3 keV and 10 ms, respectively. We expect a larger density increase at a higher initial density by optimization of heating scenario with respect to ECRH, ICRF heating and NBI.