Observation ofH-Mode Confinement in the DIII-D Tokamak with Electron Cyclotron Heating

Abstract

The first observation of //-mode confinement with electron cyclotron heating as the sole auxiliary heating method has been made in divertor discharges in the DIII-D tokamak. These discharges exhibit the usual characteristics of the H mode, including improved confinement of particles and energy, when electron cyclotron heating is added at a power level above 0.7 MW. The //-mode transition is accompanied by the development of an electron temperature pedestal of 0.25 keV and a dramatic steepening of the density gradient near the separatrix.PACS numbers: 52.55.Fa, 52.50.GjThe plasma energy-confinement time in tokamaks has generally been found to decrease when auxiliary power is added to the Ohmic heating. This phenomenon, L-mode scaling,' makes it difficult to heat a tokamak plasma to reactor temperatures. However, a regime of improved confinement, ^-mode scaling, has been observed in several tokamaks with neutral-beam injection (NBI) heating. ^"^ In the H mode, the energy confinement time may remain at the Ohmic level without degradation when auxiliary power is added, or the confinement time may degrade with power but remain a factor of about 2 larger than the confinement time under the L-mode scaling, as in the JET tokamak.^ Recently, the H mode has also been obtained with ion cyclotron heating,^ and, in the JFT-2M tokamak, with ion cyclotron heating,^ with a combination of NBI and electron cyclotron heating (ECH),^^ and with NBI in limiter discharges. ^^ The present work reports the first observation of the H mode with electron cyclotron heating as the sole source of auxiliary heating.These experiments have been carried out in the DIII-D tokamak operated in the expanded boundary divertor configuration under conditions which have facilitated the //-mode transition in discharges heated by NBI.^'^^ Typical plasmas had Ro = \.6S m, a-0.62 m, elongation = 1.8, with a single null divertor located in the direction of the ion \B drift, as preferred for the H mode. ^^ The gap between the separatrix flux surface and the nearest material wall was typically 3.5 cm. The plasma was developed from deuterium gas, and the plasma current was 0.48 MA.The ECH power was applied at a frequency of 60 GHz, for which the second-harmonic resonance occurs at a local magnetic field of 1.07 T. The antennas were located near the outside midplane of the plasma, and launched 85% of the incident power in the extraordinary mode and 15% in the ordinary mode. The total incident power, after allowance for waveguide losses, was up to 0.9 MW. The antennas launch power in the HEn mode at an angle ±17° to the radial with a Gaussian dispersion 11 ° in half width. When the resonance is at the plasma center, for the 17° launch angle, the local cutoff density for the extraordinary mode is 1.86x10^^ m"^, and for densities well below cutoff", the power is deposited in a volume with approximate minor radius of 20 cm in the midplane. The ECH power was typically a factor of 8 larger than the Ohmic power during the heating, and so the plasma power balance was domi...

Tests of causality: Experimental evidence that sheared E×B flow alters turbulence and transport in tokamaks

“…H-mode has been obtained in all auxiliary heated divertor tokamaks that have operated since 1982, in limiter discharges in several tokamaks, [53][54][55][56] in a current-free stellarator, 57,58 in a heliotron-torsatron 59,60 and in a linear tandem mirror machine. 61,62 H-mode has also been produced with a wide variety of techniques: Heating with neutral beam injection on many machines, electron cyclotron heating, 57,58,63,64 ion cyclotron heating, 53,65 lower hybrid heating, 66 and Ohmic heating. [67][68][69][70] Furthermore, H-mode has also been produced by biasing the plasma using an external electrode 68,71,72 or by biasing the limiter.…”

Observation of the limiter H mode in the JT-60 tokamak with lower-hybrid current drive

“…In fact, ECH has been reported to be more efficient than NBI in producing the H mode. 6 where the H mode with LHCD was destroyed by NBI. (b) Neutral-particle spectra from a charge-exchange analyzer whose chord normal to the magnetic field crosses the NB line at the plasma center.…”

Rapid change of the neutral hydrogen energy distribution at theL/Htransition in the JFT-2M tokamak

“…PACS numbers: 52.70.Nc, 52.25.Fi, 52.55.Fa Since the //-mode transition was first found in ASDEX [1,2], //-mode discharges have been obtained in almost all tokamaks regardless of the heating method and of the configuration [3][4][5][6][7][8][9][10][11][12]. The //-mode transition is now regarded as a general physics phenomenon in high-temperature toroidal plasmas.…”