Fast Ion Distribution Research Articles

Evidence for Fast-Ion Transport by Microturbulence

Cross-field diffusion of energetic ions by microturbulence is measured during neutral-beam injection into the DIII-D tokamak. Fast-ion D(alpha), neutron, and motional Stark effect measurements diagnose the fast-ion distribution function. As expected for transport by plasma turbulence, anomalies relative to the classical prediction are greatest in high temperature plasmas, at low fast-ion energy, and at larger minor radius. Theoretical estimates of fast-ion diffusion are comparable to experimental levels.

Commissioning activities and first results from the collective Thomson scattering diagnostic on ASDEX Upgrade (invited)

The collective Thomson scattering (CTS) diagnostic installed on ASDEX Upgrade uses millimeter waves generated by the newly installed 1 MW dual frequency gyrotron as probing radiation at 105 GHz. It measures backscattered radiation with a heterodyne receiver having 50 channels (between 100 and 110 GHz) to resolve the one-dimensional velocity distribution of the confined fast ions. The steerable antennas will allow different scattering geometries to fully explore the anisotropic fast ion distributions at different spatial locations. This paper covers the capabilities and operational limits of the diagnostic. It then describes the commissioning activities carried out to date. These activities include gyrotron studies, transmission line alignment, and beam pattern measurements in the vacuum vessel. Overlap experiments in near perpendicular and near parallel have confirmed the successful alignment of the system. First results in near perpendicular of scattered spectra in a neutral beam injection (NBI) and ion cyclotron resonance heating (ICRH) plasma (minority hydrogen) on ASDEX Upgrade have shown evidence of ICRH heating phase of hydrogen.

Analysis of anisotropic suprathermal ion distributions using multidirectional measurements of escaping neutral atom fluxes

A feasible approach in obtaining experimental data on the angular dependence of the ion distribution function in a fusion plasma is to perform angle-resolved measurements of kinetic energy spectra of escaping neutral atoms. A general calculation scheme has been developed and realized as a FORTRAN code that has a predictive force to simulate the experimentally measurable anisotropic distributions and random samples of escaping neutral atom kinetic energies for any given angle-dependent ion distribution law, electron density, and temperature profiles, plasma composition, magnetic surface structure, and experiment geometry on any toroidal plasma device with magnetic confinement. As a particular application of the method to a specific experiment, measured signals for all 20 channels of the angle-resolved multisightline neutral particle analyzer on Large Helical Device have been numerically simulated for certain predefined model fast ion distribution functions.

Design of collective Thomson scattering system using 77 GHz gyrotron for bulk and tail ion diagnostics in the large helical device

Collective Thomson scattering (CTS) system is expected to be a strong diagnostic tool for measuring thermal and fast ion distribution function at a local point inside plasmas. The electron cyclotron resonance heating system using a gyrotron at the frequency range of 77 GHz has been installed at the large helical device (LHD). The feasibility of CTS system using the 77 GHz gyrotron is assessed in terms of scattering spectrum and a background noise of the electron cyclotron emission, which affect the signal to noise ratio, with the realistic plasma parameters and incident port locations of LHD. Based on the calculated scattering spectra for bulk and tail fast ion diagnostics, the scattering radiation receiver system with gyrotron frequency feedback circuit is proposed to avoid the frequency chirping.

Fast ion charge exchange spectroscopy measurement using a radially injected neutral beam on the large helical device

An experimental technique to investigate fast ion confinement based on charge exchange spectroscopy of H(alpha)-light was applied to evaluate the confinement property of perpendicular fast ions in large helical device (LHD). Sensitivities of the H(alpha) spectra to the pitch angles of injected neutral beams (NBs) and these to the angle between the sight line of the measurement and NB injection path are examined. The energy dependence of the charge exchange cross section significantly affects the observed spectra since the driving NB is injected perpendicular to the magnetic field lines in the geometry of LHD. The measured spectra are compared to the spectra of GNET simulation results and the simulated spectra agreed well with the experimental measurement when we take into account the contribution of halo neutrals. Although it is difficult to obtain the fast ion distribution functions directly, this technique provides useful experimental data in benchmarking simulation codes.

Validating TRANSP simulations using neutron emission spectroscopy with dual sight lines

A method to generate modeled neutron spectra from bulk and fast ion distributions simulated by TRANSP has been developed. In this paper, modeled data generated from fuel ion distributions modeled with TRANSP is compared to measured data from two neutron spectrometers with different lines of sight; TOFOR with a radial one and the MPRu with a tangential one. The information obtained from the analysis of the measured neutron spectra such as the relative intensity of the emission from different ion populations places additional constraints on the simulation and can be used to adjust the parameters of the simulation.

Effect of radial electric field and ripple on edge neutral beam ion distribution in ASDEX Upgrade

The neutral beam injected fast ion distribution at the ASDEX Upgrade edge region is studied focusing on the difference between co- and counter-injected neutral beams. The slowing-down distribution of beam ions is simulated using the orbit-following Monte Carlo code ASCOT. The edge fast ion density and its gradient are higher for counter-injection than for co-injection. Also the distribution in the velocity space is different: for co-injection, there exists a population of untrapped particles which for counter-injection is found only when the effect of a non-constant, experimentally obtained quiescent H-mode radial electric field is included in the simulation. Toroidal ripple removes ions having small particle pitch, thereby reducing the density and density gradient, whereas the radial electric field has the opposite effect. Including simultaneously the effects of both ripple and the radial electric field restores the distribution close to the ideal case where both of them are neglected. The radial electric field is found to squeeze the orbit of a counter-injected neutral beam ion but to widen the orbit of a co-injected ion, and to cause transitions in the orbit topologies which are reflected in the fast ion distribution.

Temporal evolution of confined fast-ion velocity distributions measured by collective Thomson scattering in TEXTOR

Fast ions created in the fusion processes will provide up to 70% of the heating in ITER. To optimize heating and current drive in magnetically confined plasmas insight into fast-ion dynamics is important. First measurements of such dynamics by collective Thomson scattering (CTS) were recently reported [Bindslev, Phys. Rev. Lett. 97, 205005 2006]. Here we extend the discussion of these results which were obtained at the TEXTOR tokamak. The fast ions are generated by neutral-beam injection and ion-cyclotron resonance heating. The CTS system uses 100-150kW of 110-GHz gyrotron probing radiation which scatters off the collective plasma fluctuations driven by the fast-ion motion. The technique measures the projected one-dimensional velocity distribution of confined fast ions in the scattering volume where the probe and receiver beams cross. By shifting the scattering volume a number of scattering locations and different resolved velocity components can be measured. The temporal resolution is 4ms while the spatial resolution is approximately 10cm depending on the scattering geometry. Fast-ion velocity distributions in a variety of scenarios are measured, including the evolution of the velocity distribution after turnoff of the ion heating. These results are in close agreement with numerical simulations.

Ion Distribution Function Evaluation Using Escaping Neutral Atom Kinetic Energy Samples

A reliable method to evaluate the probability density function of escaping atom kinetic energies is required for analyzing neutral particle diagnostic data used to study the fast ion distribution function in fusion plasmas. In this paper, digital processing of solid state detector signals is proposed as an improvement of the simple histogram approach. Probability density function of kinetic energies of neutral particles escaping from plasma has been derived in a general form, taking into consideration the plasma ion energy distribution, electron capture and loss rates, superposition along the diagnostic sight line, and the magnetic surface geometry. A pseudorandom number generator has been realized to simulate a sample of escaping neutral particle energies for given plasma parameters and experimental conditions. Empirical probability density estimation code has been developed and tested to reconstruct the probability density function from simulated samples assuming Maxwellian ion energy distribution shapes for different temperatures and classical slowing down distributions with different slowing down times. The application of the developed probability density estimation code to the analysis of experimental data obtained by the novel Angular-Resolved Multi-Sightline Neutral Particle Analyzer has been studied to obtain the suprathermal particle distributions. The optimum bandwidth parameter selection algorithm has also been realized.

Fast-ion Dα measurements and simulations in quiet plasmas

The Dα light emitted by neutralized deuterium fast ions is measured in magnetohydrodynamics (MHD)-quiescent, magnetically confined plasmas during neutral beam injection. A weighted Monte Carlo simulation code models the fast-ion Dα spectra based on the fast-ion distribution function calculated classically by TRANSP [R. V. Budny, Nucl. Fusion 34, 1247 (1994)]. The spectral shape is in excellent agreement and the magnitude also has reasonable agreement. The fast-ion Dα signal has the expected dependencies on various parameters including injection energy, injection angle, viewing angle, beam power, electron temperature, and electron density. The neutral particle diagnostic and measured neutron rate corroborate the fast-ion Dα measurements. The relative spatial profile agrees with TRANSP and is corroborated by the fast-ion pressure profile inferred from the equilibrium.

Measurements of fast-ion acceleration at cyclotron harmonics using Balmer-alpha spectroscopy

Combined neutral beam injection and fast wave heating at the fourth and fifth cyclotron harmonics accelerate fast ions in the DIII-D tokamak. Measurements with a nine-channel fast-ion D-alpha (FIDA) diagnostic indicate the formation of a fast-ion tail above the injection energy. Tail formation correlates with enhancement of the d–d neutron rate above the value that is expected in the absence of fast-wave acceleration. FIDA spatial profiles and fast-ion pressure profiles inferred from the equilibrium both indicate that the acceleration is near the magnetic axis for a centrally located resonance layer. The enhancement is largest 8–10 cm beyond the radius where the wave frequency equals the cyclotron harmonic, probably due to a combination of Doppler-shift and orbital effects. The fast-ion distribution function calculated by the CQL3D Fokker–Planck code is fairly consistent with the data.

Collisional bulk ion transport and poloidal rotation driven by neutral beam injection

Neutral beam injection (NBI) is known to significantly affect radial transport in a tokamak plasma. Furthermore, recent observations have shown poloidal velocities, in the presence of NBI, significantly in excess of the standard neoclassical value. Motivated by this, the additional collisional radial bulk ion fluxes of particles, heat and toroidal angular momentum, and the poloidal velocity, driven by fast ions from NBI have been evaluated for a low-collisionality, pure plasma, with strong toroidal rotation and arbitrary aspect ratio. Higher order velocity space structure of the fast ion distribution function can be significant, whilst the effects of toroidal acceleration caused by strong NBI dominate at large aspect ratio. The driven poloidal velocity depends strongly on system parameters, becoming larger at higher beam density and lower beam energy.

Particle simulation of bursting Alfvén modes in JT-60U

The results of particle-in-cell simulations of a negative neutral beam heated Alfvén-mode experiment in the Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U) [H. Ninomiya et al., Fusion Sci. Technol. 42, 7 (2002); A. Kitsunezaki et al., Fusion Sci. Technol. 42, 179 (2002)] are presented. They seem to match quite well the dynamics of the abrupt large-amplitude event (ALE) experimentally observed in the reference JT-60U discharge. The time scale and frequency spread of the ALE are well reproduced too. The issue of the weaker Alfvén fluctuation phase following the ALEs, characterized by fast frequency sweeping modes, is also investigated and an interpretation of the full JT-60U bursting-mode phenomenology is presented. Finally, the simulation tool is exploited by ad hoc synthetic diagnostics on the fast ion distribution function to get a deeper insight into the ALE nonlinear dynamics. The underlying fast-growing energetic particle mode saturates as resonant energetic ions are scattered out of the resonance region and displaced outwards. The radially displaced ions resonate with outer Alfvén modes and enhance their local drive, consistently with the “avalanche” paradigm for mode nonlinear dynamics and energetic ion transports.

Measurement of the Dα spectrum produced by fast ions in DIII-D

Fast ions are produced by neutral beam injection and ion cyclotron heating in toroidal magnetic fusion devices. As deuterium fast ions orbit around the device and pass through a neutral beam, some deuterons neutralize and emit D(alpha) light. For a favorable viewing geometry, the emission is Doppler shifted away from other bright interfering signals. In the 2005 campaign, we built a two channel charge-coupled device based diagnostic to measure the fast-ion velocity distribution and spatial profile under a wide variety of operating conditions. Fast-ion data are acquired with a time resolution of approximately 1 ms, spatial resolution of approximately 5 cm, and energy resolution of approximately 10 keV. Background subtraction and fitting techniques eliminate various contaminants in the spectrum. Neutral particle and neutron diagnostics corroborate the D(alpha) measurement. Examples of fast-ion slowing down and pitch angle scattering in quiescent plasma and fast-ion acceleration by high harmonic ion cyclotron heating are presented.

Energy and charge distributions of fast nitrogen ions reflected from metal surface at grazing incidence

Energy and charge distributions of ions scattered under grazing incidence conditions from metallic surfaces are studied theoretically. The energy distribution of scattered ions with fixed charge is calculated by the energy distribution of reflected ions with equilibrium charge and charge fraction of these ions. The charge fractions of scattered ions are estimated by charge exchange cross sections. We calculated the energy and charge distributions of 1MeV nitrogen ion scattered from platinum surface. The calculation results are in qualitative agreement with experimental data.

Surface loads and edge fast ion distribution for co- and counter-injection in ASDEX Upgrade

The fast particle flux onto the material surfaces and the fast ion edge distribution are compared between ASDEX Upgrade H-mode and quiescent H-mode (QH-mode) in the presence of toroidal ripple and radial electric field Er by using the orbit-following Monte Carlo code ASCOT. So far, the QH-mode has been obtained only with counter-injection of the neutral beams. The wall load caused by co-injected beams in H-mode is small and without ripple it is negligible. With counter-injection (QH-mode case), the wall load is substantial even without the ripple. The ripple always increases the wall load, but the divertor load is either decreased or is unchanged. The effect of Er alone is small, but it boosts the ripple-trapping of beam particles near the location of its maximum absolute value on the horizontal midplane. The fast ion density and its gradient in the pedestal region are higher for counter-injected than for co-injected particles. The ripple decreases the density gradient in both the cases. To make a connection with the experiment, the flux of high-energy tritons from beam–plasma interactions onto the surfaces is evaluated. The simulated flux of tritons impinging on the material surfaces is in qualitative agreement with the measured tritium distribution on the wall and divertor.

Fast-Ion Dynamics in the TEXTOR Tokamak Measured by Collective Thomson Scattering

Here we present the first measurements by collective Thomson scattering of the evolution of fast-ion populations in a magnetically confined fusion plasma. 150 kW and 110 Ghz radiation from a gyrotron were scattered in the TEXTOR tokamak plasma with energetic ions generated by neutral beam injection and ion cyclotron resonance heating. The temporal behavior of the spatially resolved fast-ion velocity distribution is inferred from the received scattered radiation. The fast-ion dynamics at sawteeth and the slowdown after switch off of auxiliary heating is resolved in time. The latter is shown to be in close agreement with modeling results.

Initial angle resolved measurements of fast neutrals using a multichannel linear AXUV detector system on LHD

A new multichannel diagnostic for fast ion distribution studies has been developed and successfully tested on the Large Helical Device (LHD) in different plasma heating conditions. The diagnostic is based on a linear array AXUV detector consisting of 20 segments, charge sensitive preamplifiers, and a set of pulse height analysis channels. The main advantage of this system is the possibility to make time, energy, and angle-resolved measurements of charge exchange neutral particles in a single plasma discharge. This feature makes the new diagnostic a very helpful and powerful tool intended to contribute to the understanding of fast ion behavior in a complex helical plasma geometry like the one of LHD.

Chord integrated neutral particle diagnostic data analysis for neutral beam injection and ion cyclotron radio frequency heated plasma in a complex Large Helical Device geometry

Energy and angle-resolved measurements of charge exchange neutral particle fluxes from the plasma provide information about Ti, as well as non-Maxwellian substantially anisotropic ion distribution tails due to neutral beam injection (NBI) and ion cyclotron radio frequency (ICRF) heating. The measured chord integral neutral flux calculation scheme for the Large Helical Device magnetic surface geometry is given. Calculation results are shown for measurable atomic energy spectra corresponding to heating-induced fast ion distributions from simplified Fokker-Planck models. The behavior of calculated and experimental suprathermal particle distributions from NBI and ICRF heated plasma is discussed in the context of the experimental data interpretation.

Fast ion loss in a ‘sea-of-TAE’

Super-thermal fast ions provide a source of free energy to excite instabilities, which in turn can enhance the loss of fast ions. It has been proposed that when multiple modes with resonances closely spaced in phase space reach sufficient amplitude so that the fast ion trajectories overlap, very rapid non-linear growth can occur. The modification of the fast ion distribution by this loss may in turn excite additional, otherwise stable, modes, leading to an ‘avalanche’ effect greatly enhancing the transport of fast ions (Berk H.L., Breizman B.N., Fitzpatrick J. and Wong H.V. 1995 Nucl. Fusion 35 1661). It has been proposed that in ITER (ITER Physics Basis Editors et al 1999 Nucl. Fusion 39 2137), the transport of fast ions will be through a similar interaction of many modes. In NSTX (Ono M. et al 2000 Nucl. Fusion 40 557) bursts of multiple TAE-like instabilities are correlated with fast ion losses in a manner which qualitatively resembles avalanche behaviour.