Internal measurements of Alfvén eigenmodes with heavy ion beam probing in toroidal plasmas*

Abstract

Energetic ion driven Alfvén eigenmodes (AEs) are believed to be an important element disturbing the transport in a future fusion reactor. The studies of the AE properties in modern toroidal devices have made crucial contributions to the reactor relevant physics. AEs are conventionally studied by magnetic probes (MPs), which provide the poloidal m and toroidal n mode numbers and their spectral characteristics. Heavy ion beam probing (HIBP) has become a new tool to study AEs with high spatial and frequency resolution. HIBP in the TJ-II heliac observes locally (∼1 cm) resolved AEs over the whole radial interval. The set of low-m (m < 8) modes, detected with the high-frequency resolution (<5 kHz), present different types of AEs. AEs are pronounced in the local density, electric potential and poloidal magnetic field oscillations, detected simultaneously by HIBP in the frequency range 50 kHz < f AE < 300 kHz. Various AE modes are visible in the neutral beam injector (NBI)-heated plasma for co-NBI (<450 kW), counter- (<450 kW) and balanced NBI (<900 kW) from the plasma centre to the edge. A high coherence between MP and HIBP data was found for specific AEs. When the density rises, AE frequency decreases, , and the cross-phase between the plasma density, poloidal magnetic field and potential remains constant. The amplitude of the AE potential oscillations δφAE ∼ 10 V was estimated. Poloidally resolved density and potential measurements may provide information about the AE poloidal wavelength and the AE contribution to the poloidal electric field E pol and the turbulent particle flux 𝚪 E × B . The typical range of E pol oscillations for AEs is . Depending on the δn e and δE pol amplitudes and cross-phase, AEs may make a small or a significant contribution to the turbulent particle flux 𝚪 E × B for the observed wavenumbers k θ < 3 cm−1.

A Quasi-Coherent Electrostatic Mode in ECRH Plasmas on TJ-II

“…The density profile was obtained by calibrating I t (ρ) versus the interfer-ometer central chord. Two HIBP sample volumes are adjusted to be separated poloidally on a given magnetic surface in order to find the poloidal component of the electric field E pol by the difference in the local potential, E pol = (ϕ 1 − ϕ 2 )/Δx, Δx = ρΔθ ∼ 1 cm [6], limiting the observed k θ values to maximum 3 cm −1 . In this way we measure the radial turbulent particle flux Γ r (t) =ñ eṼr = 1/B tñe (t) E pol (t) = Γ E×B .…”

“…Instabilities driven by energetic ions have been observed in tokamaks [1,2] and stellarators [3][4][5][6][7] in regimes with NBI or ICRF heating. The induced modes, Alfvén Eigenmodes (AEs) or Energetic Particle Modes (EPMs) are driven by energy transfer from energetic ions to the waves.…”

Retrieving Models For Localised Measurements By Heavy Ion Beam Diagnostic

“…In this regard the Heavy Ion Beam Diagnostic (HIBD) is a powerful tool that can in principle provide the local measurement of the quantities of interest being these the electron density and temperature, the internal poloidal field and the plasma potential with spatial resolution of several mm and down to the micro second range temporal resolution. For instance, experiments in ISTTOK [2][3][4] [5], TJ-II [6], T-10 [6] and in the LHD device [7] have all together demonstrated the capabilities of the HIBD in plasma density, potential fluctuations and mean potential measurements, supporting MHD and turbulence analysis including Alfvèn Eigenmodes (TJ-II) [8] and Geodesic Acoustic modes (T-10) [9] studies.…”

Two Point Correlation Technique for the Measurements of Poloidal Plasma Rotation by Heavy Ion Beam Probe

“…3-6·10 19 m −3 ) in the bulk plasma by Heavy Ion Beam Probe (HIBP) [1] with two-slit energy analyzer [2]. Sample volumes (SV) determined by two slits were located on the same magnetic surface and separated poloidally by 1-2 cm, except the central area, ρ < 0.3.…”

“…HIBP secondary beam current I tot is proportional to local electron density multiplied by attenuation factor [3]: For the typical TJ-II discharges the path integral effect [4] is negligible since no global long wave modes with k r < 2π/a have been found so far. So, fluctuations of I tot are proportional to the local n e fluctuations [2]: Both I tot signals for each SV were analyzed with twopoint correlation technique, coherency and cross-phase θ 1,2 were determined and the phase velocity of fluctuations v phase = Δx · 2π f /θ 1,2 was calculated. This technique was first used to investigate the power of fluctuations as a function of frequency and wave-number in the direction of edge Langmuir probes measurements [5].…”