Comparison of experimental and theoretical fast ion slowing-down times in DIII-D

Abstract

ABSTRACT. Short deuterium beam pulses are injected into the D III-D tokamak to study the variation of beam slowingdown time with temperature and density. The slowing-down time is inferred from the rate of decay of the d (d,n) 3 He neutron emission. To within ~30%, the results are consistent with Sivukhin's classical theory. The short beam pulses,are also useful for measurements of the central deuterium density.Analysis of tokamak plasmas routinely assumes classical Coulomb coupling between different plasma species. For example, the conclusion that ion thermal conduction is anomalous in beam heated tokamaks [1] hinges on the assumption that the beam power is deposited in the ion channel classically. Only a few experiments have tested the assumption of classical coupling between species. Beam ion spectra measured by charge exchange agreed with spectra computed from classical theory [2,3], although discrepancies of a factor of two were observed in the fits. A higher impurity temperature than hydrogen temperature in low density, * Permanent address: University of California, Irvine, CA 92717, USA.beam heated PLT plasmas was explained by using classical coupling arguments [4]. Measurements of the time evolution of the 15 MeV proton emission indicated that 0.8 MeV 3 He ions [5] and fast wave heated 3 He minority ions [6] slowed down at roughly the expected rate. On T-10, the time evolution and magnitude of the drop in ion temperature during electron cyclotron heating was consistent with the expected reduction in power flow from the electrons [7]. Although each of these experiments was consistent with classical theory, their accuracy was such that deviations as large as 50%, or more, from theory may have escaped detection.Perhaps the best quantitative check of beam energy loss is from studies of the rate of decay of the neutron emission following deuterium beam injection [8,9]. This emission comes from the d (d,n) 3 He reaction between the fast ions and the plasma deuterons. Since the neutron measurement is volume averaged, ±10% uncertainties in the profiles of electron temperature and density have a relatively small effect on the expected rate of decay; hence, the interpretation of the measurements is straightforward. The major weakness of the previous studies [8,9] is that the decay in neutron emission was measured immediately after beam injection, when the classical slowing-down time T S [10] was rapidly changing due to changes in T e and n e . In a preliminary study, Kim et al. attempted to improve the accuracy of the neutron technique by injecting short beam pulses into the D-III tokamak [11].In our experiment, we have studied the deceleration of beam ions by measuring the decay in neutron 1897LETTERS emission following the injection of ~2 ms pulses of deuterium beams into steady state deuterium plasmas in the D III-D tokamak. The total energy injected during the beam pulse was only ~3 % of the plasma stored energy, and, hence, density and temperature of the discharge were not perturbed (T e changed by <5%), ...

Accurate measurements of the pitch-angle scattering of beam ions

“…The jump in neutron rate during the beam pulse provides an independent check on the deuterium density. 32 Following the pulse, the neutron rate decays approximately exponentially as the injected beam ions thermalize. Effectively, this volume-integrated measurement constitutes a cross section weighted average of the entire beam population, 33 so the decay of the neutron signal measures the average deceleration of beam ions.…”

“…Effectively, this volume-integrated measurement constitutes a cross section weighted average of the entire beam population, 33 so the decay of the neutron signal measures the average deceleration of beam ions. 32 Neutral particle diagnostics 34 measure local densities in phase space. In addition to providing beam ions, some of the beam sources supply neutrals for active charge exchange measurements.…”

Measurements of Prompt and MHD-Induced Fast Ion Loss from National Spherical Torus Experiment Plasmas

“…This method [5] provides two measurements of possible losses of neutral beam ions. First, the absolute height of the resulting neutron pulse can be compared to the rate expected from the beam injection power and the plasma density to obtain a measure of the loss fraction of the beam ions arising from short time scale (<3 ms) processes, typically prompt loss.…”

Fast-ion transport induced by Alfvén eigenmodes in the ASDEX Upgrade tokamak

“…Apart from the recently installed ECEI systems, the upgraded fluctuation diagnostics, used in these studies, include 1D ECE [17], Soft X-Ray (SXR) [18] and reflectometry [19]. The main systems utilized to diagnose the fast-ion internal Fast-Ion Transport Induced by Alfven Eigenmodes in the ASDEX Upgrade Tokamak 3 redistribution and eventual loss are the neutron rates [20], the Fast-Ion D-Alpha (FIDA) spectroscopy [21] and the scintillator based Fast-Ion Loss Detectors (FILDs) [22]. An overview of the AUG diagnostics utilized in the present work is shown in Fig.1.…”