Observation of beta-induced Alfvén eigenmodes in the DIII-D tokamak

Abstract

Energetic ions can drive Alfven gap modes unstable, causing large losses of fast ions. Toroidicityinduced Alfven eigenmodes (TAE) were expected to disappear into the shear Alfven continuum and become stable as the plasma beta increased. Although TAE modes may disappear, another dangerous instability with similar properties but approximately half the TAE frequency appears in a spectral gap that is created by finite beta effects. The measured frequency of the new mode agrees with the theoretical frequency of beta-induced Alfven eigenmodes.PACS numbers: 52.55.Fa, 52.35.Bj, 52.55.Pi An ignited magnetic fusion reactor must confine charged fusion products while they thermalize. Collective modes driven by alpha particles may degrade the alpha confinement and prevent ignition in a deuteriumtritium tokamak reactor. One way to study the physics of alpha-driven instabilities is to inject deuterium neutral beams into low toroidal field deuterium plasmas. In previous experiments, beam ions destabilized toroidicityinduced Alfven eigenmodes (TAE) in TFTR [1] and in DIII-D [2,3]. These modes are dangerous because they cause large, concentrated losses of the resonant fast ions that clamp the beam beta near the point of marginal stability [1,4]. ("Beta" is the ratio of kinetic to magnetic energy.) TAE modes appear in a gap in the Alfven continuum that is caused by the toroidal curvature of the plasma. The center of this "TAE gap" occurs at a frequencywhere VA ^B/^jAnntmi is the Alfven speed, q is the safety factor, R is the tokamak major radius, B is the magnetic field, and /!,-and m, are the ion density and mass [5], (An example of the gap structure appears in Fig. 4.) Analysis of the TAE mode suggested that its frequency would decrease into the Alfven continuum (where it would be heavily damped) as the plasma beta approached the stability limit for ideal ballooning modes [6]. But recent calculations found a new gap underneath the Alfven continuum caused by the compressional response of the plasma to shear Alfven waves in the presence of finite pressure and curvature [7]. The energy associated with this compression produces a frequency shift that raises the Alfven continuum, thus opening a low-frequency gap. Global modes in this gap with the dominant polarization of shear Alfven waves were discovered numerically [8]; we call these modes beta-induced Alfven eigenmodes (BAE). In this Letter, experimental evidence of destabilization of BAE modes by energetic beam ions is reported for the first time.The experiments are performed in the DIII-D tokamak (/?-1.8 m, a =0.65 m) in relatively pure (Z e fr^2) deuterium plasmas. Near tangential (tangency radius /? t an -1.10 m), -75 keV deuterium neutrals are injected in the direction of the plasma current; in some plasmas, near-perpendicular beams G?tan -0.74 m) are also injected. The normalized beta, p^=p t aB t /I p , is usually near the nominal limit of Z^ -3.5, although larger values can be obtained with current ramping. (Here p t is the toroidal beta in percent, a is the minor radius...

Beam-driven energetic particle modes in advanced tokamak plasmas

“…Beam-driven instabilities with frequencies below the frequency of toroidicity-induced Alfvén eigenmodes (TAEs) but above the frequency of tearing modes and fishbones have been observed in DIII-D since 1991 [1]. Subsequent analysis failed to establish conclusively the identity of this instability [2], which was called the β-induced Alfvén eigenmode (BAE) in the initial publication.…”

Alpha particle-driven toroidal Alfvén eigenmodes in Tokamak Fusion Test Reactor deuterium–tritium plasmas: Theory and experiments

“…TAE KTAE are two t ypes of many Alfven Eigenmodes. Other types include the Betainduced Alfven Eigenmode BAE 25 , the Ellipticity-induced Alfven Eigenmode EAE 26 , the Non-circularity triangularity-induced Alfven Eigenmode NAE 26 31 RTAE . However, TAE KTAE are most susceptible to alpha particle destabilization for two reasons.…”

NBI-driven Alfvénic modes at ASDEX Upgrade

“…Finally, also the non-linear interaction with other Alfvénic instabilities, such as TAEs has been investigated [10]. In this paper, we aim to describe and model another, closely connected Alfvénic mode in the same frequency regime as the RSAE, the beam-driven beta induced Alfvén eigenmode (BAE) [11,12,13,16,17,18,19]. Although a lot of literature is available for BAE modes they have so far not been investigated in the current ramp-up phase of a tokamak discharge.…”