Observation of a high-confinement regime in a tokamak plasma with ion cyclotron resonance heating

Abstract

The H mode in ion cyclotron-resonance-heated plasmas has been investigated with and without additional neutral-beam injection. Ion cyclotron-resonance heating can cause the transition into a highconfinement regime (H mode) in combination with beam heating. The H mode, however, has also been realized -for the first time -with ion cyclotron-resonance heating alone in the hydrogen minority scheme in deuterium plasma at an absorbed rf power of 1.1 MW.PACS numbers: 52.50.Gj, 52.55.Fa, 52.55.Pi Confinement studies on many tokamak plasmas all over the world have revealed an unfavorable phenomenon, called L mode. The good confinement of resistively heated plasmas could not be maintained at high plasma temperatures.With increasing auxiliary heating power, confinement was found to deteriorate severely, obstructing the successful approach of the ignition conditions. Therefore, the discovery of a high-confinement regime, called H mode, ' which combines the virtues of good global and central confinement even with additional heating was of paramount importance for fusion research.The characteristic and directly observable signatures of the H mode are a distinct drop of the H, radiation in the divertor (which is mostly a measure of the reduced power flow across the separatrix into the divertor chamber), a rising electron density (indicating an improved particle confinement), an enhanced plasma energy content (due to the improved energy confinement), a significantly enhanced edge electron temperature, and the existence of an MHD phenomenon, called ELMs (edge localized modes). ELMs are short bursts of energy leaving the plasma, representing short transitions back to degraded confinement accompanied by enhanced power loss.Since the H mode has been observed so far only in plasmas of divertor tokamaks, heated by neutral-beam injection (NI), it is important to know whether this good-confinement regime is restricted to NI heating alone or whether it is achievable for heating in the ion cyclotron range of frequencies [ion cyclotron-resonance heating (ICRH)], too. Additionally, the realization of the H mode with different auxiliary heating methods will give some insight into the global confinement structure of tokamak plasmas. In particular, ICRH decouples heating and particle refueling, and ICRH changes the ion energy distribution of the plasma differently from NI. Thus, heating by ICRH introduces additional properties extending the range of parameters to study plasma confinement.Our investigations, where ICRH has been applied alone and together with NI, were based on an H-mode recipe which has been developed during NI experiments.Best conditions have been reported for clean deuterium plasmas (Z,tt-1 -1.5 ) in the single-null divertor configuration with the plasma shifted up by about a tenth of the minor radius, corresponding to 2 times the density and temperature falloff lengths of the scrapeoff layer. It was theoretically expected that the ion grad-8-drift direction should have a large impact on the formation of the H mode, which ...

Preliminary results of ELMy H-mode experiments on the HL-2A tokamak

“…The H-mode was reproduced on PDX with a NBI power of 6 MW in 1984 [2], on DIII-D with a NBI power of 6 MW in 1986 [3] and on JET with a NBI power of 10 MW in 1987 [4]. Later, the H-mode was also realized with a pure ion cyclotron resonance heating (ICRH) power of 1.1 MW on ASDEX [5], and with a sole ECRH power of 0.7 MW on DIII-D [6]. The first H-mode in a stellarator was achieved on W7-AS with an electron cyclotron resonance heating (ECRH) power of 0.4 MW in 1993 [7].…”

High non-inductive fraction H-mode discharges generated by high-harmonic fast wave heating and current drive in the National Spherical Torus Experiment

“…2 Several years later, Hmode plasmas were generated in DIII-D (Ref. 3) by direct fast-wave heating of electrons, via Landau damping and transit time magnetic pumping.…”

On the breakdown modes and parameter space of ohmic tokamak start-up

“…Later, in order to reduce the toroidal electric field, electron cyclotron resonance heating (ECRH) (Erckmann & Gasparino 1994), ion cyclotron resonance heating (ICRH) (Steinmetz et al. 1987) and low hybrid wave heating (LHWH) (Yoshino & Seki 1997; Shinya et al. 2017) started being employed to help initiate the discharge.…”

“…Early theoretical and experimental investigations on initial discharge in a toroidal plasma can be found in Dimock et al (1973), Sand, Waelbroeck & Waidmann (1973), Papoular (1976), Strachan (1976), Sometani & Fujisawa (1978). Later, in order to reduce the toroidal electric field, electron cyclotron resonance heating (ECRH) (Erckmann & Gasparino 1994), ion cyclotron resonance heating (ICRH) (Steinmetz et al 1987) and low hybrid wave heating (LHWH) (Yoshino & Seki 1997;Shinya et al 2017) started being employed to help initiate the discharge. Experiments (Gilgenbach et al 1981;Holly et al 1981;Toi et al 1988;Yoshino & Seki 1997;Lloyd 1998) have proved that these auxiliary heating methods can lower the toroidal voltage needed to initiate start-up, reduce the volt-second consumption in start-up for more rapid burn through and even allow a large range for impurity content.…”