Observation of rapid frequency chirping instabilities driven by runaway electrons in a tokamak

Abstract

We report the first observation of chirping instabilities driven by runaway electrons (REs) in a tokamak. The instabilities are accessed during the post-disruption RE beam stage in a low density background plasma ( m−3) on DIII-D. The chirping instabilities are observed when a decelerating loop voltage is applied to the RE beam. The frequency chirping is detected in two distinct frequency bands: 0.1–10 MHz and 30–80 MHz. The mode frequency increases linearly when the toroidal magnetic field sensed by the RE beam increases. The frequency chirps by 0.3–2.4 MHz on a timescale of 1 ms. Modification of the RE distribution function is directly measured during the chirping in the low-frequency band consistent with the hole–clump model for frequency chirping. The low-frequency instabilities also correlate with an increase of intermittent RE loss from the plasma. These observations provide a novel experimental platform for fundamental studies of nonlinear chirping. They also support continued investigation of opportunities to utilize kinetic instabilities for RE mitigation in a tokamak reactor.

Alpha particle driven Alfvénic instabilities in ITER post-disruption plasmas

“…The interaction of runaways with plasma waves has been investigated both in theory [13][14][15][16][17][18][19][20] and through observations in multiple tokamaks [21][22][23][24][25][26][27]. Several tokamaks (such as DIII-D or TEXTOR) have reported runaway suppression in correlation with increased wave activity in the current quench of the disruption or the plateau phase of the runaway beam.…”

Detection of Alfvén Eigenmodes on COMPASS with Generative Neural Networks

“…These Alfvénic instabilities can, in turn, degrade confinement of the energetic particles and thus lower plasma performance and possibly endanger plasmafacing components of the tokamak. 1 Alfvén waves are typically excited by fast ions generated by auxiliary plasma heating or fusion reactions; however, magnetic measurements of the COMPASS tokamak 2 and DIII-D tokamak 3 have recently revealed Alfvénic modes that are driven by runaway electrons (REs). Since the presence of REs constitutes a high risk for a fusion reactor, nonlinear interaction of REs and AEs may be of high interest for topics of RE mitigation and study of their losses.…”

“…2, 5, and 6), and similar chirping modes were also observed on the DIII-D machine. 3 In COMPASS (Ref. 7), the frequency of the detected modes is in the range 0.5 to 2 MHz, scaling with the plasma density and its profile, safety factor, magnetic field, and shape of the plasma.…”

“…A change of the RE distribution function during the frequency chirp was directly measured in DIII-D (Ref. 3).…”

Обнаружение Высокочастотных Альфвеновских Колебаний В Омических Разрядах Сферического Токамака Глобус-М2

“…На рис.1, b штриховой (синей в электронной версии статьи) линией показана эволюция частоты согласно степенному закону. Степень зависимости порядка 0.5 получена путем аппроксимации методом наименьших квадратов и соответствует предсказанию модели δω ∼ √ t. Эффект снижения (роста) частоты, как правило, наблюдается [18] при дестабилизации различных (в том числе альфвеновских) неустойчивостей быстрыми ионами, однако сообщается [19] и о возбуж-дении hole-clump-неустойчивости на частотах порядка нескольких MHz ускоренными электронами.…”