H. Duong

Studies of energetic confined alphas using the pellet charge exchange diagnostic on TFTR

Results from recent DT experiments on TFTR to measure the energy distribution and radial density profile of fast confined alphas with the use of Li pellets and neutral particle analysis are presented. When a pellet is injected into the plasma, a toroidally extended ablation cloud is formed. A small fraction of the fusion alphas incident on the cloud is converted to helium neutrals as a result of electron capture processes. The escaping energetic helium neutrals are analysed and detected by the neutral particle analyser. Radially resolved energy spectra of trapped confined alphas in 0.5-2 MeV range and radial alpha density profiles are presented in this paper. The experimental data are compared with modelling results obtained with the TRANSP Monte Carlo code and with a specially developed Fokker-Planck post-processor (FPP) that uses the alpha source distribution produced by TRANSP. Comparison of the experimental data with TRANSP and FPP shows that the alphas in the plasma core of sawtooth free discharges in TFTR are well confined and slow down classically. The energy and radial profiles distributions outside the plasma core show the influence of stochastic ripple losses on alphas. Measurements for sawtoothing plasmas show a significant outward radial transport of trapped alphas

Design and operation of the pellet charge exchange diagnostic for measurement of energetic confined α particles and tritons on the Tokamak Fusion Test Reactor

Radially resolved energy and density distributions of the confined α particles in D–T experiments on the Tokamak Fusion Test Reactor (TFTR) are being measured with the pellet charge exchange (PCX) diagnostic. Other energetic ion species can be detected as well, such as tritons produced in D–D plasmas and H, He3, or tritium rf‐driven minority ion tails. The ablation cloud formed by injected low‐Z impurity pellets provides the neutralization target for this active charge exchange technique. Because the cloud neutralization efficiency is uncertain, the PCX diagnostic is not absolutely calibrated so only relative density profiles are obtained. A mass and energy resolving E∥B neutral particle analyzer (NPA) is used which has eight energy channels covering the energy range of 0.3–3.7 MeV for α particles with energy resolution ranging from 5.8% to 11.3% and a spatial resolution of ∼5 cm. The PCX diagnostic views deeply trapped ions in a narrow pitch angle range around a mean value of v∥/v=−0.048±10−3. For D–T op...

Measurements of confined alphas and tritons in the MHD quiescent core of TFTR plasmas using the pellet charge exchange diagnostic

The energy distributions and radial density profiles of the fast confined trapped alpha particles in DT experiments on TFTR are being measured in the energy range 0.5 - 3.5 MeV using the pellet charge exchange (PCX) diagnostic. A brief description of the measurement technique which involves active neutral particle analysis using the ablation cloud surrounding an injected impurity pellet as the neutralizer is presented. This paper focuses on alpha and triton measurements in the core of MHD quiescent TFTR discharges where the expected classical slowing-down and pitch angle scattering effects are not complicated by stochastic ripple diffusion and sawtooth activity. In particular, the first measurement of the alpha slowing-down distribution up to the birth energy, obtained using boron pellet injection, is presented. The measurements are compared with predictions using either the TRANSP Monte Carlo code and/or a Fokker - Planck Post-TRANSP processor code, which assumes that the alphas and tritons are well confined and slow down classically. Both the shape of the measured alpha and triton energy distributions and their density ratios are in good agreement with the code calculations. We can conclude that the PCX measurements are consistent with classical thermalization of the fusion-generated alphas and tritons.

Confined trapped alpha behaviour in TFTR deuterium-tritium plasmas

Confined trapped alpha energy spectra and differential radial density profiles in TFTR D-T plasmas were obtained with the pellet charge exchange (PCX) diagnostic, which measures high energy (Eα = 0.5-3.5 MeV) trapped alphas (v||/v = -0.048) at a single time slice (Δt ≈ 1 ms) with a spatial resolution of Δr ≈ 5 cm. Tritons produced in D-D plasmas and RF driven ion tails (H, 3He or T) were also observed and energetic tritium ion tail measurements are discussed. PCX alpha and triton energy spectra extending up to their birth energies were measured in the core of MHD quiescent discharges where the expected classical slowing down and pitch angle scattering effects are not complicated by stochastic ripple diffusion and sawtooth activity. Both the shape of the measured alpha and triton energy distributions and their density ratios are in good agreement with TRANSP predictions, indicating that the PCX measurements are consistent with classical thermalization of the fusion generated alphas and tritons. From calculations, these results set an upper limit on possible anomalous radial diffusion for trapped alphas of Dα ≤ 0.01 m2s-1. Outside the core, where the trapped alphas are influenced by stochastic ripple diffusion effects, the PCX measurements are consistent with the functional dependence of the Goldston-White-Boozer stochastic ripple threshold on the alpha energy and the q profile. In the presence of strong sawtooth activity, the PCX diagnostic observes significant redistribution of the alpha signal radial profile wherein alphas are depleted in the core and redistributed to well outside the q = 1 radius, but apparently not beyond the energy dependent stochastic ripple loss boundary. The helical electric field produced during the sawtooth crash plays an essential role in modelling the sawtooth redistribution data. In sawtooth free discharge scenarios with reversed shear operation, the PCX diagnostic also observes radial profiles of the alpha signal that are significantly broader than those for supershots. ORBIT modelling of reversed shear and monotonic shear discharges is in agreement with the q dependent alpha profiles observed. Redistribution of trapped alpha particles in the presence of core localized toroidal Alfven eigenmode (TAE) activity was observed and modelling of the PCX measurements based on a synergism involving the α-TAE resonance and the effect of stochastic ripple diffusion is in progress.

Alpha diagnostics using pellet charge exchange: Results on the Tokamak Fusion Test Reactor and prospects for ITER

Confinement of alpha particles is essential for fusion ignition and alpha physics studies are a major goal of the TFTR, JET, and ITER DT experiments, but alpha measurements remain one of the most challenging plasma diagnostic tasks. The pellet charge exchange (PCX) diagnostic has successfully measured the radial density profile and energy distribution of fast (0.5–3.5 MeV) confined alpha particles in TFTR. This article describes the diagnostic capabilities of PCX demonstrated on TFTR and discusses the prospects for applying this technique to ITER. Major issues on ITER include the pellet’s perturbation to the plasma and obtaining satisfactory pellet penetration into the plasma.

The effect of toroidal field ripple on confined alphas in TFTR DT plasmas

The pellet charge exchange (PCX) diagnostic on the Tokamak Fusion Test Reactor (TFTR) presently measures trapped alpha distribution functions with very small pitch angles ( nu ||/ nu ~0.05) at the midplane. The measured PCX alpha signal exhibits a depletion region near the outboard region. Results of the alpha energy spectra and radial profile suggest that stochastic ripple diffusion is the cause of the depletion. Comparison of the ripple stochastization boundary with Goldston-White-Boozer theory also shows the correct functional dependence on alpha energy and q profile

Radio frequency-driven energetic tritium ion tail measurements in the Tokamak Fusion Test Reactor using the pellet charge exchange diagnostic

Charge exchange interactions of ions with the ablation cloud of an injected low-Z impurity pellet can be used to measure the energy spectrum and radial profile of confined fast ions in a fusion plasma. On the Tokamak Fusion Test Reactor we use this technique to directly measure energetic alphas from D–T reactions, tritons from D–D reactions, and radio frequency (rf)-driven minority tail ions (e.g., H, 3He, T). We describe the status of the pellet charge exchange (PCX) diagnostic including a brief description of the measurement technique and discussion of operational experience. PCX measurements of the energy spectrum, radial density distribution, and heating deposition profile of rf-driven tritium ions during 2ΩT heating of L-mode plasmas are presented.