Escaping Ion Measurement with High Time Resolution during Bursting Modes Induced by Neutral Beam Injection on CHS

Shinohara, K.; Isobe, M.; Darrow, D. S.; Shimizu, A.; Nagaoka, K.; Okamura, S.

doi:10.1585/pfr.2.042

Cited by 6 publications

(4 citation statements)

References 13 publications

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“…(See figure 8 for the interpretation grid of pitch angle and gyroradius.) This transient loss of a wide range of pitch angles is similar to beam ion loss features seen in Alfvénic modes in the CHS device [8].…”

Section: Resultssupporting

confidence: 63%

MHD induced neutral beam ion loss from NSTX plasmas

et al. 2008

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Bursts of ~60 kHz activity on Mirnov coils occur frequently in NSTX plasmas and these are accompanied by bursts of neutral beam ion loss over a range in pitch angles. These losses have been measured with a scintillator type loss probe imaged with a high speed (>10,000 frames/s) video camera, giving the evolution of the energy and pitch angle distributions of the lost neutral beam ions over the course of the events. The instability occurs below the TAE frequency in NSTX (~100 kHz) in high beta plasmas and may be a beta driven Alfvén acoustic (BAAE) mode. The data from one burst that causes a 13% reduction in the neutron rate has been studied extensively and has several interesting features. First, the burst begins with the mode having a purely n=1 character, sweeping downward in frequency. However, there is no change to the underlying loss signal during this time interval. This indicates there is no phase space transport of fast (80 keV D) ions into the loss cone seen by the probe from the frequency sweeping. As the burst evolves further in time, a concurrent n=2 mode arises, followed quickly by a concurrent n=3 mode. During this period when multiple modes are present, loss over a wide range of pitch angles is seen simultaneously, suggesting stochastization of the beam ion phase space. There is no evidence of any sweeping in pitch angle of the loss in either phase of the burst, at least not on the 100 microsecond time scale.

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Section: Resultssupporting

confidence: 63%

MHD induced neutral beam ion loss from NSTX plasmas

et al. 2008

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“…Detailed guiding center orbit modeling of these events has been performed and indicates that the beam ion losses can be attributed to mode-driven stochastization [4] of the beam ion orbit phase space. Similar interactions between ICRF heated tail ions and MHD modes in the ASDEX-Upgrade device have also been observed, with similar conclusions about the underlying mechanisms of fast ion transport [5], and signatures of stochastic loss of beam ions have also been seen in Compact Helical System plasmas [6].…”

Section: Introductionsupporting

confidence: 64%

Comparison of Measurement and Modeling of Current Profile Changes due to Neutral Beam Ion Redistribution during TAE Avalanches in NSTX

Darrow

Crocker

Gorelenkov

et al. 2013

Plasma and Fusion Research

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Brief 'avalanches' of toroidal Alfvén eigenmodes (TAEs) are observed in NSTX plasmas with several different n numbers simultaneously present. These affect the neutral beam ion distribution as evidenced by a concurrent drop in the neutron rate and, sometimes, beam ion loss. Guiding center orbit modeling has shown that the modes can transiently render portions of the beam ion phase space stochastic. The resulting redistribution of beam ions can also create a broader beam-driven current profile and produce other changes in the beam ion distribution function.

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“…A similar rapid loss of ICRF heated tail ions in the ASDEX-Upgrade tokamak occurs when multiple Alfvénic modes are present [9], and these have similarly been understood by consideration of resonance overlaps in phase space [10]. Loss of neutral beam ions over an extended range of pitch angles due to bursting MHD has also been reported from the Compact Helical System (CHS) [11].…”

Section: Introductionmentioning

confidence: 83%

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

et al. 2012

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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

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Escaping Ion Measurement with High Time Resolution during Bursting Modes Induced by Neutral Beam Injection on CHS

Cited by 6 publications

References 13 publications

MHD induced neutral beam ion loss from NSTX plasmas

MHD induced neutral beam ion loss from NSTX plasmas

Comparison of Measurement and Modeling of Current Profile Changes due to Neutral Beam Ion Redistribution during TAE Avalanches in NSTX

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

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