Experiments on excitation of Alfvén eigenmodes by alpha-particles with bump-on-tail distribution in JET DTE2 plasmas

Dedicated experiments were performed in JET DTE2 plasmas for obtaining an α-particle bump-on-tail (BOT) distribution aiming at exciting Alfvén Eigenmodes (AEs). NBI-only heating with modulated power was used so that fusion-born α-particles were the only ions present in the MeV energy range in these DT plasmas. The beam power modulation on a time scale shorter than the α-particle slowing down time was chosen for modulating the α-particle source and thus sustaining a BOT in the α-particle distribution. High-frequency modes in the TAE frequency range and multiple short-lived modes in a wider frequency range have been detected in these DT discharges with interferometry, soft X-ray cameras, and reflectometry. The modes observed were localised close to the magnetic axis, and were not seen in the Mirnov coils. Analysis with the TRANSP and Fokker-Planck FIDIT codes confirms that α-particle distributions with bump-on-tail in energy were achieved during some time intervals in these discharges though no clear correlation was found between the times of the high-frequency mode excitation and the BOT time intervals. The combined MHD and kinetic modelling studies show that the high-frequency mode in the TAE frequency range is best fitted with a TAE of toroidal mode number n= 9. This mode is driven mostly by the on-axis beam ions while the smaller drive due to the pressure gradient of α-particles allows overcoming the marginal stability and exciting the mode [H.J.C. Oliver et al. Toroidal Alfvén eigenmodes observed in low power JET deuterium-tritium plasmas, to be submitted to Nuclear Fusion (2023)]. The observed multiple short-lived modes in a wider frequency range are identified as the on-axis kinetic Alfvén eigenmodes predicted in [M.N. Rosenbluth, P.H. Rutherford, Phys. Rev. Lett. 34 (1975) 1428].

本研究在JET装置的DTE2等离子体中开展专属实验，旨在获得α粒子凸起尾（bump-on-tail, BOT）分布，以激发阿尔文本征模（Alfvén Eigenmodes, AEs）。实验采用带调制功率的纯中性束注入（Neutral Beam Injection, NBI）加热方式，使得该氘氚等离子体兆电子伏特能量区间内的离子仅为聚变产生的α粒子。我们选取比α粒子慢化时间更短的时间尺度进行束功率调制，以此调控α粒子源，进而维持α粒子分布的凸起尾结构。借助干涉诊断、软X射线相机与反射计，我们在该批次氘氚放电实验中探测到了环向阿尔文本征模（Toroidal Alfvén Eigenmodes, TAE）频段内的高频模，以及更宽频段内的多个短时模。观测到的模结构均局域于磁轴附近，且未在米尔诺夫线圈（Mirnov coils）诊断中被捕捉到。借助TRANSP代码与福克-普朗克FIDIT代码开展的分析证实，本批次放电实验的部分时段内确实实现了能量维度带凸起尾结构的α粒子分布，但高频模激发时刻与凸起尾分布时段之间并未发现明确关联。磁流体动力学（Magnetohydrodynamics, MHD）与动理学建模的联合研究表明，TAE频段内的高频模可最优拟合为环向模数n=9的TAE。该模主要由轴上束离子驱动，而α粒子压强梯度带来的弱驱动则可克服边际稳定性条件，进而激发该模[H.J.C. Oliver 等. 低功率JET氘氚等离子体中观测到的环向阿尔文本征模, 拟投稿至Nuclear Fusion (2023)]。观测到的更宽频段内的多个短时模，被认定为[M.N. Rosenbluth、P.H. Rutherford, Phys. Rev. Lett. 34 (1975) 1428]中预言的轴上动理学阿尔文本征模。