Neutral-Beam Current-Driven High-Poloidal-Beta Operation of the DIII-D Tokamak

Abstract

Neutral-beam current-drive experiments in the DIII-D tokamak with a single null poloidal divertor are described. A plasma current of 0.34 MA has been sustained by neutral beams alone, and the energy confinement is of //-mode quality. Poloidal p values reach 3.5 without disruption or coherent magnetic activity suggesting that these plasmas may be entering the second stability regime.PACS numbers: 52.55.Fa, 52.50.GjThe tokamak magnetic fusion configuration requires a toroidal current within the plasma. Generally this current is inductively coupled. Tokamaks can therefore only operate for finite-duration pulses. Also, the current concentrates in regions of high electrical conductivity (regions of high electron temperature) and thereby does not necessarily produce an optimum radial current profile. Numerous noninductive current-drive methods 1 have been proposed, including injection of electromagnetic waves and neutral beams. These methods could allow steady-state tokamak operation and optimization of the radial current profiles to possibly improve confinement and provide access to the second stability region, in which increasing plasma pressure increases plasma stability.The concept of neutral-beam current drive was proposed by Ohkawa 2 and the basic principle was demonstrated in the Culham Levitron. 3 First tokamak results were obtained in DITE 4 and subsequently in TFTR 5 and JET. 6 This paper presents new results from the DIII-D 7 tokamak in which the plasma current was sustained entirely by neutral beams from 1.5 s without assistance from the Ohmic-heating transformer. After Ohmic startup, the Ohmic-heating-coil current was held constant so that the plasma current could freely adjust. This technique provides a striking demonstration of neutral-beam current drive. The poloidal beta, fi p , reached 3.5, raising the possibility that the plasma is entering the second stability region, as described later.The DIII-D tokamak 7 was operated with a singlenull-divertor configuration having a 1.70-m major radius, 0.6-m minor radius, 1.75 vertical elongation, and 2.1-T toroidal magnetic field. The experiments were carried out with a helium plasma having a line-averaged density n e =2x 10 19 m ~3. Eight hydrogen neutral beams, 8 consisting of 52% neutral power at 75 keV, 30% at 37 keV, and 18% at 25 keV, were injected in the same direction as the plasma current. Four beams intersected the vacuum system axis at 47° and four beams intersected at 63°.Plasma parameters are shown in Fig. 1 as functions of time. Initially, a 0.22-MA Ohmic discharge was estab-lished without sawteeth, indicating an on-axis safety factor <7o>L At 1.1 s the Ohmic-heating-primary-coil current was held constant, so that without beam injection the plasma current decayed, as shown by the dashed line of Fig. 1(a). With 10 MW of absorbed neutralbeam injection [ Fig. 1(b)], the plasma current increased to 0.34 MA. During the period when the current was sustained, the loop voltage [ Fig. 1(c)] was zero, except for periodic voltage spikes associated with edgel...

The fishbone instability in the DIII-D tokamak

“…In Fig. 6, fishbones were absent in the very high /3 p (/3 p > 3) current drive discharges [29]. 'These plasmas had q 95 > 20 (q 95 is the safety factor at the flux surface that encloses 95 % of the poloidal flux), so it is unlikely that the q = 1 surface was in the plasma.…”

Interaction of energetic particles with large and small scale instabilities

“…Beam drive of current has been predicted by theory (see e.g. [6][7][8] and observed in many experiments (e.g., in TFTR [9,10], DIII-D [11,12], JET [13,14], JT-60U [15][16][17], for an overview see [18]). In most of these experiments the beam driven current was located on-axis, including those with N-NBI [16,17].…”

Reduction in Neutral Beam Driven Current in a Tokamak by Tearing Modes

“…Neutral beams are used on a number of tokamaks to heat the plasma and drive current [1][2][3][4]. High energy (typically 75 to 140 keV) neutral atoms are able to penetrate across the confining magnetic field of the tokamak and into the dense core of the plasma, at which point they become ionized and confined by the magnetic field of the tokamak.…”