The upgraded JET toroidal Alfvén eigenmode diagnostic system

Abstract

The main characteristics of toroidal Alfvén eigenmodes (TAEs) have been successfully investigated in JET (Joint European Torus) using the scheme of sweeping-frequency external excitation with tracking of the synchronously-detected resonances. However, due to technical limitations, only modes with low values of the toroidal mode number n ⩽ 7 could be effectively excited and unambiguously identified by the Alfvén Eigenmode Active Diagnostic (AEAD) system. This represents a serious restriction because theoretical models indicate that medium-n Alfvén eigenmodes (AEs) are the most prone to be destabilized by energetic particles in ignited plasmas and, therefore, reliable measurement of their damping rates remains a relevant issue to properly access their effect in ignited plasmas. For this reason, a major upgrade of the AEAD system has been carried out aiming at providing a state-of-the-art excitation and real-time detection system for the planned DT campaign in JET. This required the development of a new type of radio frequency amplifier and filter, not commercially available, and also a control system. In this paper, details of the concepts that are relevant to understand the operation of the new system in the next experimental campaigns are presented, as are the results of numerical simulations to model its performance.

Scenario development for the observation of alpha-driven instabilities in JET DT plasmas

“…It shows that the corresponding TAEs are characterised by n = 4, 5 and 6. Also visible on this figure is the signal from the active antenna used for TAE excitation in JET [34].…”

Experimental studies of plasma-antenna coupling with the JET Alfvén Eigenmode Active Diagnostic

“…Six amplifiers power six of the eight antennas with currents typically of the order 5 − 10 A; thus, the system is only slightly perturbative with |δB/B| ≤ 10 −3 at the plasma edge [22,31]. Independent phasing of the antennas allows power to be injected into a spectrum of toroidal modes with n ≤ 20 [31,34]. Three frequency filters allow the AE antenna to scan the ranges ∆f = 25 − 50 kHz, 75 − 150 kHz, and 125 − 250 kHz.…”

Results from the Alfvén Eigenmode Active Diagnostic during the 2019-2020 JET deuterium campaign

“…One explanation for this is that the (fixed) antenna currents have a lower perturbative effect as I p increases. The driven magnetic perturbation by the antenna at the plasma edge is of order δB ≈ 0.1 − 1 G [4,25]; therefore, a threefold increase in the poloidal field strength could reduce the antenna perturbation and/or plasma response |δB/B| to below measureable levels. At the same time, there could be a variety of other conflating factors in these high Table 3: Ranges of plasma parameters for the resonance database: plasma current, on-axis toroidal magnetic field, central electron density and temperature, NBI and RF heating powers, plasma-antenna separation, ELM frequency, central and edge safety factors, edge magnetic shear, elongation, upper and lower triangularities, normalized beta and internal inductance.…”

“…In this paper, we provide an overview of the operation and measurements of the AE antenna during the 2019-2020 JET deuterium campaign. We note that many past works have analyzed or reported data from previous campaigns; these include studies with the original, low-n saddle coils [3,[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24], the intermediate-to-high-n, eight antenna system [25][26][27][28][29][30][31][32][33], and the most recent upgrade [4,34,35], among others. Novel to this work are the following: (i) The recently upgraded independent phasing of the eight AE antennas allows probing of high toroidal mode numbers |n| ≤ 20.…”