Alpha-Particle-Driven Toroidal Alfvén Eigenmodes in the Tokamak Fusion Test Reactor

Abstract

Alpha-particle-driven toroidal Alfvén eigenmodes (TAEs) have been observed for the first time in deuterium-tritium (D-T) plasmas on the tokamak fusion test reactor (TFTR). These modes are observed 100-200 ms following the end of neutral beam injection in plasmas with reduced central magnetic shear and elevated central safety factor ͓q͑0͒ . 1͔. Mode activity is localized to the central region of the discharge ͑r͞a , 0.5͒ with magnetic fluctuation levelB Ќ ͞B k ϳ 10 25 and toroidal mode numbers in the range n 2 4, consistent with theoretical calculations of a-TAE stability in TFTR.[S0031-9007 (97)02857-3] PACS numbers: 52.55.Fa, 52.35.Bj, 52.35.Py, 52.55.PiDeuterium-tritium (D-T) plasma operation on the tokamak fusion test reactor (TFTR) provides the first opportunity to investigate the interaction of fusion alpha particles with plasma waves under reactor relevant conditions. Such investigations are crucial for assessing the impact of plasma instabilities on the confinement of energetic alpha particles, which are required to sustain ignition in a D-T reactor. One candidate instability with the potential for affecting alpha particle confinement in tokamaks is the toroidal Alfvén eigenmode (TAE) [1]. This Letter describes the first observation of purely alpha-particledriven TAEs in TFTR with central b a as low as 0.02% (b a ϵ alpha particle pressure͞magnetic pressure), well below that expected in the International Thermonuclear Experimental Reactor (ITER) [central b a ϳ ͑0.5 1͒%].TAEs are discrete frequency modes occurring inside toroidicity induced gaps in the shear Alfvén spectrum which can be destabilized by the pressure gradient of energetic ions. These modes can potentially cause internal redistribution and enhanced loss of energetic alpha particles in a D-T reactor due to their extended radial structure, relatively low instability threshold, and resonant interaction with 3.5 MeV alpha particles near the Alfvén velocity [2,3]. The characteristics of TAEs and associated fast ion losses have been studied in experiments utilizing circulating neutral beam ions ͑E b # 100 keV͒ [4-6], deeply trapped minority ions in the MeV range of energy [7-9], nonlinear beat wave excitation using fast magnetosonic waves [10], and external excitation using saddle coils [11]. Until recently alpha-driven TAEs had not been observed in TFTR, even in the highest fusion power D-T "supershot" plasmas ͑P fus ഠ 10.7 MW͒ with b a ഠ 0.3% in the core of the discharge [12]. These results were consistent with theoretical calculations of alpha-driven TAE stability in TFTR, after taking into account beam ion Landau damping and radiative damping due to coupling to the kinetic Alfvén wave (KAW) [13]. However, a better comparison with theory requires the actual observation of purely alpha particle driven TAEs in D-T plasmas, as described in this Letter. The present experiment was motivated by recent theoretical calculations for low-n modes ͑n , 6͒ in TFTR indicating a significant reduction in the central b a required for destabilizing TAEs under condi...

Chapter 5: Physics of energetic ions

“…These findings led to a re-interpretation of TFTR-DT data using reflectometry to reconstruct the internal structure of alpha particle driven modes [138]. These modes were originally considered to be TAEs [139], with a caveat associated with the anti-ballooning structure of a low frequency n = 2 mode [140]. The observation of an anti-ballooning density mode structure for the low frequency n = 2 mode has been highly controversial for some time but was resolved when the NOVA-K code suggested that the modes could be RSAEs, which near q min should exhibit density peaking on the high field side of the magnetic axis ( figure 6).…”

Fast Ion Induced Shearing of 2D Alfvén Eigenmodes Measured by Electron Cyclotron Emission Imaging

“…A host of fast-ion driven instabilities may be excited by fusion-born alpha particles [1,2] in future devices such as ITER [3], as well as by confined fast ions created during neutral beam injection and RF heating in contemporary machines [1,4]. These instabilities have been shown to affect the energetic particle population leading to enhanced transport, losses, and modification of the pressure and current profiles [1].…”

Movable magnetic probe system in the T-10 tokamak

“…[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] For example, a small set of coils wound from a bare copper wire on a grooved rectangular ceramic forms are used for high-frequency modes measurements in DIII-D (bandwidth of about 1.1 MHz). 4 In order to avoid the shielding effects of a metal cover, the probes are protected from the plasma by a housing of boron nitride.…”