Role of magnetic island on the formation of internal transport barrier

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(1)Effect of perturbation coil, with which the m=2/n=1 magnetic island can be produced, on formation of the internal transport barrier (ITB) is investigated for EC H or NBI+EC H plasmas that have various plasma parameters in Compact Helical System (CHS). One of power supplies for poloidal field coils (inner vertical coil) is used to energize the perturbation coil for the magnetic island. When the electron density is low and the electron temperature is high, the electron temperature does not show any indication of the magnetic island under energizing the perturbation coil, while the density is high and the temperature is low the electron temperature shows the flattened profile indicating the presence of magnetic island.(2)This is called "healing of magnetic island" and is thought to be due to the bootstrap current in a finite beta plasma that flows to cancel the perturbation magnetic field in the radial direction. Usually the bootstrap current enhances the magnetic island in tokamaks, h … More owever, in helical system the bootstrap current cancels the magnetic island under some plasma parameters. In CT-IS the healing happens in the collisionless regime. This is unfavorable to study the effect of magnetic island on formation of ITB, because ITB is realized in the collisionless regime with the electron root.(3)The collisionless plasma with ITB is produced with the following parameters ; magnetic field strength B = 0.88Tesla, vacuum magnetic axis position = 0.92m, ECH power = 220kW, NBI power = 650kW x 2. Vacuum vessel wall is conditioned intensively with Ti-Bettering to suppress the influx from the wall. When the perturbation coil is energized the magnetic field configuration is slightly different from the configuration without the perturbation field, causing the electron density a little bit lower. The electron temperature profiles are compared for the plasmas with and without perturbation field. The central electron temperature is higher with perturbation field than that without perturbation field, however it is not shown clearly that the ITB is formed more easily with the perturbation field, that is, is formed at higher electron density. Less