Bertelli, N; Valeo, E.J.; Green, D.L.; Gorelenkova, M.; Phillips, C.K.; Podesta, M.; Lee, J.P.; Wright, J.C.; Jaeger, E.
At the power levels required for significant heating and current drive
in magnetically-confined toroidal plasma, modification of the particle distribution
function from a Maxwellian shape is likely [T. H. Stix, Nucl. Fusion, 15 737
(1975)], with consequent changes in wave propagation and in the location and
amount of absorption. In order to study these effects computationally, both the
finite-Larmor-radius and the high-harmonic fast wave (HHFW), versions of the
full-wave, hot-plasma toroidal simulation code TORIC [M. Brambilla, Plasma Phys.
Control. Fusion 41, 1 (1999) and M. Brambilla, Plasma Phys. Control. Fusion
44, 2423 (2002)], have been extended to allow the prescription of arbitrary velocity
distributions of the form f(v||, v_perp, psi , theta). For hydrogen (H) minority heating of a
deuterium (D) plasma with anisotropic Maxwellian H distributions, the fractional
H absorption varies significantly with changes in parallel temperature but is
essentially independent of perpendicular temperature. On the other hand, for
HHFW regime with anisotropic Maxwellian fast ion distribution, the fractional
beam ion absorption varies mainly with changes in the perpendicular temperature.
The evaluation of the wave-field and power absorption, through the full wave
solver, with the ion distribution function provided by either aMonte-Carlo particle
and Fokker-Planck codes is also examined for Alcator C-Mod and NSTX plasmas.
Non-Maxwellian effects generally tends to increase the absorption with respect to
the equivalent Maxwellian distribution.
Gas puff imaging (GPI) is a diagnostic of plasma turbulence which uses
a puff of neutral gas at the plasma edge to increase the local visible
light emission for improved space-time resolution of plasma
fluctuations. This paper reviews gas puff imaging diagnostics of edge
plasma turbulence in magnetic fusion research, with a focus on the
instrumentation, diagnostic cross-checks, and interpretation
issues. The gas puff imaging hardware, optics, and detectors are
described for about 10 GPI systems implemented over the past ~15
years. Comparison of GPI results with other edge turbulence diagnostic
results are described and many common features are observed. Several
issues in the interpretation of GPI measurements are discussed, and
potential improvements in hardware and modeling are suggested.
The ability of an injected lithium granule to promptly trigger an edge localized mode (ELM) has been established in multiple experiments. By horizontally injecting granules ranging in diameter from 200 microns to 1mm in diameter into the low field side of EAST H-mode discharges we have determined that granules with diameter > 600 microns are successful in triggering ELMs more than 95% of the time. It was also demonstrated that below 600 microns the triggering efficiency decreased roughly with granule size. Granules were radially injected from the outer midplane with velocities ~ 80 m/s into EAST upper single null discharges with an ITER like tungsten monoblock divertor. These granules were individually tracked throughout their injection cycle in order to determine their efficacy at triggering an ELM. For those granules of sufficient size, ELM triggering was a prompt response to granule injection. By simulating the granule injection with an experimentally benchmarked neutral gas shielding (NGS) model, the ablatant mass deposition required to promptly trigger an ELM is calculated and the fractional mass deposition is determined.
Stotler, D.P.; Battaglia, D.J.; Hager, R.; Kim, K.; Koskela, T.; Park, G.; Reinke, M.L.
Modifications of the drift-kinetic transport code XGC0 to include the
transport, ionization, and recombination of individual charge states,
as well as the associated radiation, are described. The code is first
applied to a simulation of an NSTX H-mode discharge with carbon
impurity to demonstrate the approach to coronal equilibrium. The
effects of neoclassical phenomena on the radiated power profile are
examined sequentially through the activation of individual physics
modules in the code. Orbit squeezing and the neoclassical inward
pinch result in increased radiation for temperatures above a few
hundred eV and changes to the ratios of charge state emissions at a
given electron temperature. Analogous simulations with a neon
impurity yield qualitatively similar results.
Non-axisymmetric control coils and the so-called snowflake divertor configuration are two potential solutions proposed to solve two separate outstanding issues on the path towards self-sustained burning plasma operations, namely the transient energy bursts caused by edge localized modes and the steady state heat exhaust problem. In a reactor, these two proposed solutions would have to operate simultaneously and it is, therefore, important to investigate their compatibility and to identify possible conflicts that could prevent them from operating simultaneously. In this work, single- and two-fluid resistive magnetohydrodynamic calculations are used to investigate the effect of externally applied magnetic perturbations on the snowflake divertor configuration. The calculations are based on simulated NSTX-U plasmas and the results show that additional and longer magnetic lobes are created in the null-point region of the snowflake configuration, compared to those in the conventional single-null. The intersection of these longer and additional lobes with the divertor plates are expected to cause more striations in the particle and heat flux target profiles. In addition, the results indicate that the size of the magnetic lobes, in both single-null and snowflake configurations, are more sensitive to resonant magnetic perturbations than to non-resonant magnetic perturbations. The results also suggest that lower values of current in non-axisymmetric control coils would be required to suppress edge localized modes in plasmas with the snowflake configuration.
In this paper we present initial simulations of pedestal control by Lithium Granule Injection (LGI) in NSTX. A model for small granule ablation has been implemented in the M3D-C1 code , allowing the simulation of realistic Lithium granule injections. 2D simulations in NSTX L-mode and H-mode plasmas are done and the effect of granule size, injection angle and velocity on the pedestal gradient increase are studied. For H-mode cases, the amplitude of the local pressure perturbation caused by the granules is highly dependent on the solid granule size. In our simulations, reducing the granule injection velocity allows one to inject more particles at the pedestal top.
Lunsford, R.; Bortolon, A.; Roquemore, A.L.; Mansfield, D.K.; Jaworski, M.A.; Kaita, R.; Maingi, R.; Nagy, A.
By employing a neutral gas shielding (NGS) model to characterize impurity granule
injection the pedestal atomic deposition for three different species of granule:
lithium, boron, and carbon are determined. Utilizing the duration of ablation
events recorded on experiments performed at DIII-D to calibrate the NGS model we
are able to quantify the ablation rate and mass deposition location with respect
to the plasma density profile. The species specific granule shielding constant
is then used to model granule ablation within NSTX-U discharges. Simulations of
300, 500 and 700 micron diameter granules injected at 50 m/sec are presented for
NSTX-U L-mode type plasmas as well as H-mode discharges with low natural ELM
frequencies. Additionally, ablation calculations of 500 micron granules of each
species are presented at velocities ranging from 50 � 150 m/sec. In H-mode type
discharges these simulations show that the majority of the injected granule is
ablated within or just past the steep gradient region of the discharge. At this
radial position, the perturbation to the background plasma generated by the ablating
granule can lead to conditions advantageous for the rapid triggering of an ELM crash
Stotler, D.P.; Lang, J.; Chang, C.S.; Churchill, R.M.; Ku, S.-H.
The effects of recycled neutral atoms on tokamak ion temperature
gradient (ITG) driven turbulence have been investigated in a steep
edge pedestal, magnetic separatrix configuration, with the full-f
edge gryokinetic code XGC1. Ion temperature gradient turbulence is
the most fundamental and robust edge plasma instability, having a long
radial correlation length and an ability to impact other forms of
pedestal turbulence. The neutral atoms enhance the ITG turbulence,
first, by increasing the ion temperature gradient in the pedestal via
the cooling effects of charge exchange and, second, by a relative
reduction in the ExB shearing rate.
Results of 3D nonlinear simulations of neutral-beam-driven compressional Alfven eigenmodes (CAEs) in the National Spherical Torus Experiment (NSTX) are presented. Hybrid MHD-particle simulations for the H-mode NSTX discharge (shot 141398) using the HYM code show unstable CAE modes for a range of toroidal mode numbers, n=4-9, and frequencies below the ion cyclotron frequency. It is found that the essential feature of CAEs is their coupling to kinetic Alfven wave (KAW) that occurs on the high-field side at the Alfven resonance location. High-frequency Alfven eigenmodes are frequently observed in beam-heated NSTX plasmas, and have been linked to flattening of the electron temperature profiles at high beam power. Coupling between CAE and KAW suggests an energy channeling mechanism to explain these observations, in which beam-driven CAEs dissipate their energy at the resonance location,
therefore significantly modifying the energy deposition profile. Nonlinear simulations demonstrate that CAEs can channel the energy of the beam ions from the injection region near the magnetic axis to the location of the resonant mode conversion at the edge of the beam density profile. A set of nonlinear simulations show that the CAE instability saturates due to nonlinear particle trapping, and a large fraction of beam energy can be transferred to several unstable CAEs of relatively large amplitudes and absorbed at the resonant location. Absorption rate shows a strong scaling with the beam power.
A scintillator type fast ion loss detector measures the gyroradius and pitch angle distribution of superthermal ions escaping from a magnetically confined fusion plasma at a single location. Described here is a technique for optimizing the angular orientation of such a detector in an axisymmetric tokamak geometry in order to intercept losses over a useful and interesting ranges of pitch angle. The method consists of evaluating the detector acceptance as a function of the fast ion constants of motion, i.e. energy, canonical toroidal momentum, and magnetic moment. The detector acceptance can then be plotted in a plane of constant energy and compared with the relevant orbit class boundaries and fast ion source distributions. Knowledge of expected or interesting mechanisms of loss can further guide selection of the detector orientation. The example of a fast ion loss detector for the National Spherical Torus Experiment-Upgrade (NSTX-U) is considered.