Stochastic orbit loss of neutral beam ions from NSTX due to toroidal Alfvén eigenmode avalanches

Abstract

Abstract. Short toroidal Alfvén eigenmode (TAE) avalanche bursts in the National Spherical Torus Experiment (NSTX) cause a drop in the neutron rate and sometimes a loss of neutral beam ions at or near the full injection energy over an extended range of pitch angles. The simultaneous loss of wide ranges of pitch angle suggests stochastic transport of the beam ions occurs. When beam ion orbits are followed with a guiding center code that incorporates plasma's magnetic equilibrium plus the measured modes, the predicted ranges of lost pitch angle are similar to those seen in the experiment, with distinct populations of trapped and passing orbits lost. These correspond to domains where the stochasticity extends in the orbit phase space from the region of beam ion deposition to the loss boundary. IntroductionIn National Spherical Torus Experiment (NSTX)[1-3] plasmas, short (≤2 ms) bursts of toroidal Alfvén eigenmodes [4,5] are sometimes seen with multiple toroidal mode numbers, n, present simultaneously. These are termed TAE avalanches.[6] These avalanches are always accompanied by drops in the neutron rate of the order of 5 to 25 percent. Neutron production in NSTX results predominantly from reactions of the injected 90 keV deuterons with bulk plasma deuterons, which typically have a temperature in the range of 1-2 keV. Because the avalanches do not usually change the bulk plasma profiles appreciably, the drops in neutron rate have typically been attributed to neutral beam ion redistribution or loss. NSTX is equipped with a gyro radius and pitch angle dispersing scintillator-type fast ion loss diagnostic. [7] Interestingly, this diagnostic shows fast ion loss for some but not all avalanches. In addition, when losses are seen, the pitch angle range of the lost particles can vary substantially. In this work, we compare avalanches in two similar discharges in NSTX, one that evidences loss on the scintillator detector and one that does not. In concert with this, we apply recently a developed phase space mapping technique [8] that can identify which portions of the particle orbit phase space are stochastic given an equilibrium, mode structures, and mode amplitudes. This technique predicts little loss to the detector in the avalanche where no losses

Recent progress on spherical torus research

“…103(c). 321 As can be seen, the fast ion loss comes in a rapid short burst coinciding with the avalanche. It should be noted that the neutron drop is usually observed during a TAE avalanche but the fast ion loss is not always observed, indicating that fast ion transport during an avalanche does not necessarily lead to fast ion loss.…”

“…103(a), a TAE avalanche is shown where the magnetic pickup coil frequency spectrogram with toroidal mode numbers 1 through 4 appear together in an avalanche on an expanded time scale. 321 A total neutron rate drop of $13% is shown in Fig. 103(b).…”

“…The TAE avalanche is the most commonly observed avalanche process, involving a simultaneous excitation of multiple TAE modes in NSTX. 313,321 In Fig. 103(a), a TAE avalanche is shown where the magnetic pickup coil frequency spectrogram with toroidal mode numbers 1 through 4 appear together in an avalanche on an expanded time scale.…”

“…A major advance in recent years is in improved diagnostics (as described above) and simulation/modeling capabilities for explaining their observations. 313,320,321 The simulations usually involve multiple steps. First, the mode amplitude profiles are needed.…”

Properties of Alfven Eigenmodes in the TAE range on the National Spherical Torus Experiment-Upgrade

“…From previous studies, it is well known that TAEs can efficiently interact with NB ions [4][5], leading to loss or redistribution of fast particles [6] [7][8] [9]. The resulting NB-driven current profile can deviate considerably from what predicted if fast ions behaved classically [10] [11].…”