Transport Of Energetic Ions Research Articles

Anomalous transport of energetic particles in ITER relevant scenarios

The anomalous transport of energetic ions in the presence of turbulent fields is investigated. Nonlinear simulations of a steady-state ITER [R. Aymar et al., Nucl. Fusion 41, 1301 (2001)] scenario have been carried out using the gyrokinetic turbulence code GENE [F. Jenko et al., Phys. Plasmas 7, 1904 (2000)], modeling the suprathermal particles as high temperature Maxwellian distributions in the passive tracer limit. Velocity space analysis shows that single particle diffusivities of ions above the critical energy are significantly larger than their neoclassical counterpart.

Particle simulation of energetic particle driven Alfvén modes in NBI heated DIII-D experiments

The mutual nonlinear interactions of shear Alfvén modes and alpha particles can enhance their transport in burning plasmas. Theoretical and numerical works have shown that rapid transport of energetic ions can take place because of fast growing Alfvén modes (e.g. energetic particle driven modes, EPMs). This kind of transport has been observed in experiments as well as in numerical simulations. Hybrid MHD-gyrokinetic codes can investigate linear and nonlinear dynamics of energetic particle (EP) driven modes, retaining the mutual interaction between waves and EPs self-consistently. Self-consistent nonlinear wave–particle interactions (both in configuration and velocity space) are crucial for a correct description of the mode dynamics in the case of strongly driven modes; thus, a non-perturbative approach is mandatory. The knowledge of the threshold characterizing the transition from weakly to strongly driven regimes is of primary importance for burning plasma operations (e.g. for ITER), in order to avoid EPM enhanced EP transport regimes. The hybrid MHD-gyrokinetic code (HMGC) has been applied to the interpretation of phenomena observed in present experiments with neutral beam (NB) heating. In reversed-shear beam-heated DIII-D discharges, a large discrepancy between the expected and measured EP radial density profiles has been observed in the presence of large Alfvénic activity. HMGC simulations with EP radial profiles expected from classical NB deposition as input give rise to strong EPM activity, resulting in relaxed EP radial profiles at saturation level close to experimental measurements. The frequency spectra obtained from several simulations with different toroidal mode numbers, as calculated during the saturated phase when the strong EPMs transform in weak reversed-shear Alfvén modes, are quite close to experimental observations both in absolute frequency and in radial localization. In this work, we discuss in particular the effects of nonlinear coupling between different toroidal mode numbers.

Electrostatic and magnetic transport of energetic ions in turbulent plasmas

Analytical and numerical work is used in tandem to address the problem of turbulent transport of energetic ions in magnetized plasmas. It is shown that orbit averaging is not valid under rather generic conditions, and that perpendicular decorrelation effects lead to a slow 1/E decay of the electrostatic particle diffusivity of beam ions, while the respective magnetic quantity is even independent of the particle energy E.

HETC-HEDS Code Validation Using Laboratory Beam Energy Loss Spectra Data

Recently, the Monte Carlo transport code HETC has been extended to include the interactions and transport of energetic heavy ions. Here, for the first time, we compare predictions of fragment production and energy loss with laboratory data measured at Brookhaven National Laboratory and at the National Institute of Radiological Sciences in Japan. Very good agreement is found for predicted and measured energy loss spectra for a variety of incident laboratory beams and targets.

Discovery of a transient radiation belt at Saturn

Radiation belts have been detected in situ at five planets. Only at Earth however has any variability in their intensity been heretofore observed, in indirect response to solar eruptions and high altitude nuclear explosions. The Cassini spacecraft's MIMI/LEMMS instrument has now detected systematic radiation belt variability elsewhere. We report three sudden increases in energetic ion intensity around Saturn, in the vicinity of the moons Dione and Tethys, each lasting for several weeks, in response to interplanetary events caused by solar eruptions. However, the intensifications, which could create temporary satellite atmospheres at the aforementioned moons, were sharply restricted outside the orbit of Tethys. Unlike Earth, Saturn has almost unchanging inner ion radiation belts: due to Saturn's near‐symmetrical magnetic field, Tethys and Dione inhibit inward radial transport of energetic ions, shielding the planet's main, inner radiation belt from solar wind influences.

Mechanisms and scalings of energetic ion transport via tokamak microturbulence

The turbulent E×B advection of energetic ions in three-dimensional tokamak geometry is investigated both analytically and numerically. It is shown that orbit averaging (leading to a significant reduction of the diffusivity) is only valid for low magnetic shear. At moderate or high magnetic shear, a rather slow decrease of the diffusivity is found, proportional to (E∕Te)−1 or (E∕Te)−1.5 for particles with a large or small parallel velocity component, respectively. The decorrelation mechanisms responsible for this behavior are studied and explained in detail. Moreover, it is found that resonances between the toroidal drift of the particles and the diamagnetic drift of the turbulence can lead to an enhancement of the fast ion transport.

Simulation Study of Energetic Ion Transport due to Alfvén Eigenmodes in LHD Plasma

The creation of holes and clumps in an energetic ion energy spectrum associated with Alfven eigenmodes wa se xamined using the neutral particle analyzer (NPA) on the LHD shot #47645. The difference in slowing- down times between the holes and clumps suggested that the energetic ions were transported over 10% of the plasma minor radius. The spatial profile and frequency of the Alfven eigenmodes were analyzed with the AE3D code. The phase space structures of the energetic ions on the NPA line-of-sight were investigated with Poincare plots, where an oscillating Alfven eigenmode was employed for each plot .T he phase space regions trapped by the Alfven eigenmodes appeared as islands in the Poincarep lots .T he radial width of the islands corresponded to the transport distance of the energetic ions. Since island width depends on Alfven eigenmode amplitude, it was found that Alfven eigenmodes with amplitude δBr/B ∼ 10 −3 transported energetic ions over 10% of the minor radius.

Collisionless fast particle transport in tokamak plasmas with rotating magnetic islands

A full orbit code is used to study the collisionless transport of energetic ions in spherical and conventional tokamak plasmas with rotating magnetic island chains. Such islands are associated with neoclassical tearing modes, which occur in the vicinity of mode rational surfaces in many tokamaks, and, in the case of the conventional tokamaks DIII-D and Asdex-Upgrade, have been observed to enhance fast particle transport. Magnetic island chains are represented as single-helicity perturbations to the equilibrium poloidal magnetic flux. Loss rates of neutral beam-injected fast ions are computed for a range of perturbation amplitudes and mode frequencies in plasma equilibria representative of those in the mega-ampère spherical tokamak and a large conventional tokamak. It is found that most of the island-induced losses are of ions that are born into passing orbits. For some injection scenarios the loss rate is shown to be a sensitive function of mode frequency, although the interaction of the mode with passing particles is non-resonant. The loss rate is generally lower for island chains rotating toroidally in the direction of beam injection than it is for island chains that are either static or rotating counter to the beam direction.

Confinement degradation and transport of energetic ions due to Alfvén eigenmodes in JT-60U weak shear plasmas

The confinement degradation and transport of energetic ions due to Alfvén eigenmodes (AEs) destabilized by negative-ion-based neutral beam injection are quantitatively evaluated for the first time. AEs were observed in JT-60U weak shear plasmas with frequencies sweeping up to 30–50 kHz on a time scale of a few hundred milliseconds as the safety factor at the plasma centre, q(0), decreases from 1.75 to 1.5. Then as q(0) decreases from 1.5 to 1.25 in about 600 ms, the AE frequencies are saturated and the AEs are identified as the TAEs. The measured total neutron emission rate (Sn) in the presence of these AEs is compared with that predicted by classical theory. As a result, the confinement degradation of energetic ions is confirmed and the reduction rate in Sn is largest when AEs are destabilized and are located at the large fast ion pressure gradient region in the plasma core. In order to investigate the energetic ion transport due to these AEs, line-integrated neutron emission profiles measured with a large neutron collimator array are compared with the classical confinement calculations. It is found that energetic ions are transported from the plasma core region to the outer region due to these AEs. Furthermore, changes in the energy distribution of charge exchange energetic neutral particle fluxes suggest that the radial energetic ion transport is due to the resonance interaction between the energetic ions and the AEs.

Observation of Magnetohydrodynamic Instabilities in Heliotron J Plasmas

Two kinds of magnetohydrodynamics instability are observed in electron cyclotron heating and neutral beam injection-heated Heliotron J plasmas. One is the pressure-driven interchange modes with m = 2/n = 1 and m = 5/n = 3 and low frequency (fexp < 20 kHz), which are observed in the plasma with a rational surface of ι/2π = 0.5 or 0.65. The other is the energetic ion-driven global Alfvén eigenmodes (GAEs) in the Alfvén frequency range. To identify the observed GAEs, the frequency of the observed mode with shear Alfvén spectra calculated by CAS3D3 is compared. The interchange mode with m = 5/n = 3 and bursting GAE with intense magnetic fluctuations may affect the bulk plasma confinement and energetic ion transport, respectively.

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

A bursting mode, whose time scale is a few milliseconds, is excited in low density neutral beam injected plasmas in the compact helical system. A scintillator-based probe equipped with a high speed video camera has been used to investigate the energetic ion losses induced by this fast evolving mode. This instrument reveals loss in a region of the gyroradius and the pitch angle space that occurs only during a burst. Namely, it was found that the bursting mode induces the transport of energetic ions to the region where the energetic ions cannot exist without such an enhanced transport. The dependence of the newly observed loss on the electron density and the neutral beam injected power was also investigated.

Escaping ion measurement with high time resolution on CHS

A scintillator-based lost ion probe can measure the temporal evolution of both the gyroradius and the pitch angle of energetic ions escaping a magnetically confined plasma. For the probe on the Compact Helical System, the time resolution of this detailed two-dimensional measurement is determined by a framing rate of the video camera that records the luminous images produced by the ions striking the scintillator plate. The framing rate of the old camera was 30Hz, thus the time resolution was about 33ms. Our interest is to understand the energetic ion transport in fast events such as a bursting energetic ion driven mode. The typical time scale of these events is less than 1ms, meaning that the old camera was too slow. By replacing it with an image-intensified high-speed video camera system, the temporal resolution was improved from 33to0.07ms. We have successfully installed the fast camera and captured some fast events caused by magnetohydrodynamics, which were unobservable using the original camera.

Observation of confinement degradation of energetic ions due to Alfvén eigenmodes in JT-60U weak shear plasmas

Confinement degradation of energetic ions due to Alfvén eigenmodes (AEs) has been investigated with negative ion based neutral beam injection at ∼370 keV into JT-60U weak shear plasmas. AEs with a rapid frequency sweeping and then saturation as the minimum value of the safety factor decreases have been observed. This frequency behaviour can be explained by the reversed-shear induced AE (RSAE) and the transition from RSAEs to the toroidicity-induced AEs (TAEs). The associated change in the charge exchange neutral particle flux suggests energetic ion transport due to these AEs. The total neutron emission reduction rate in the presence of these AEs is evaluated by calculation using the orbit following Monte-Carlo code taking into account the change in bulk plasmas. The evaluation indicates confinement of energetic ions is degraded due to these AEs. In particular, it is found that the confinement degradation of energetic ions is maximum during the transition from RSAEs to TAEs and the maximum reduction rate (ΔSn/Sn)MAX is estimated to be ∼45%.

Recent enhancements in MCNPX: Heavy-ion transport and the LAQGSM physics model

Calculations involving the transport of energetic heavy ions have recently received more attention from projects such as the Rare Isotope Accelerator (RIA) and from areas such as space radiation shielding. In these areas, the transport and reactions must be calculated for heavy ions such as 56Fe or 238U traveling at energies of ⩾1 GeV/nucleon. To serve these needs, recent upgrades to the particle transport code MCNPX have expanded the previously useful ion transport capability from a small suite of light ions (deuterons, tritons, 3He, and alpha particles) to a nearly complete list of those heavy and light ions that span the Table of Isotopes. To enable nuclear spallation from energetic collisions of these ions and targets, the LAQGSM physics model has been integrated into the MCNPX code. This physics model supplements the existing physics models already contained in the code, only one of which, ISABEL, could handle heavy-ion collisions (and then only over a limited range of masses and energies). The implementation of these new features now greatly expands the usefulness of MCNPX in energetic ion transport. The heavy-ion transport feature also allows the transport of residuals from all nuclear reactions that occur in the physics model regime, even when initiated by non-heavy ions. The implementation and use of heavy ions in MCNPX is explained. Also, computations with MCNPX are compared with benchmark experiments to show agreement with results.

HETC radiation transport code development for cosmic ray shielding applications in space

In order to facilitate three-dimensional analyses of space radiation shielding scenarios for future space missions, the Monte Carlo radiation transport code HETC is being extended to include transport of energetic heavy ions, such as are found in the galactic cosmic ray spectrum in space. Recently, an event generator capable of providing nuclear interaction data for use in HETC was developed and incorporated into the code. The event generator predicts the interaction product yields and production angles and energies using nuclear models and Monte Carlo techniques. Testing and validation of the extended transport code has begun. In this work, the current status of code modifications, which enable energetic heavy ions and their nuclear reaction products to be transported through thick shielding, are described. Also, initial results of code testing against available laboratory beam data for energetic heavy ions interacting in thick targets are presented.

Energetic ion transport by abrupt large-amplitude event induced by negative-ion-based neutral beam injection in the JT-60U

To investigate energetic ion transport induced by bursting modes in the frequency range of Alfvén eigenmodes, which is called abrupt large-amplitude events (ALEs) driven by negative-ion-based neutral beam (N-NB) injection, neutron emission profile measurement and charge exchange (CX) neutral particle (flux) measurement using a natural diamond detector have been performed simultaneously in JT-60U. It is found from the CX neutral particle (flux) measurement that energetic neutral particles in a limited energy range (100–370 keV) are enhanced due to ALEs, and the neutron radial profile is flattened. The change in the energetic ion density profile inferred from these measurements indicates that ALEs expel energetic ions from the core region of the plasma and induce both redistribution and loss of energetic ions. It has been shown that the energy range of transported energetic ions is consistent with a resonance condition between energetic ions and ALEs, and the energetic ion transport results from the resonant interaction between energetic ions and ALEs. Further, a fraction of the energetic ion loss has been quantitatively estimated to be ∼4%.

Transition from weak to strong energetic ion transport in burning plasmas

The change in nonlinear energetic particle mode (EPM) dynamics that accompanies the transition from weak to strong energetic ion transport is discussed in this work. It is demonstrated that the nonlinear threshold in fast ion energy density for the onset of strong convective transport occurring in avalanches is close to the linear EPM excitation threshold. This phenomenology is strictly related to the resonant character of the modes, which tend to be radially localized where the drive is strongest. After the convective loss phase, during which nonlinear EPM mode structure is displaced outwards, fast ion transport continues owing to diffusive processes. Theoretical analyses, presented here, are the basis for consistency analyses of operation scenarios in proposed burning plasma experiments. Comparisons between theoretical predictions and both simulation and experimental results are also briefly discussed.

Energetic ion transport and concomitant change of the fusion reactivity during reconnection events in spherical tori

Effects of MHD (magnetohydrodynamic) reconnection events on the beam-plasma fusion reactivity and transport of the beam ions are studied. Based on the analysis of fusion reactivity changes induced by MHD events, the conclusion is drawn that the strong drops of the neutron yield during sawtooth crashes observed in the national spherical torus experiment (NSTX) [M. Ono et al., Nucl. Fusion 40, 557 (2000)] are associated with both a particle redistribution inside the plasma and a loss of the beam ions. Mechanisms of the energetic ion transport during sawtooth crashes are analyzed, in particular, with the use of the resonance adiabatic invariant derived in this paper. A numerical simulation of the particle motion during a sawtooth crash in NSTX is done with the code OFSEF (orbit following in the sawtooth electromagnetic field) [Ya. I. Kolesnichenko et al., Nucl. Fusion 40, 1325 (2000)] extended for a better description of the particle precession. It is shown that the motion of toroidally passing particles in NSTX can become stochastic under the influence of a crash. This stochasticity, as well as the motion along the resonance island, leads to the escape of some particles from the plasma.

Energetic ion driven Alfvén eigenmodes in Large Helical Device plasmas with three-dimensional magnetic structure and their impact on energetic ion transport

In the Large Helical Device (LHD), energetic ion driven Alfvén eigenmodes (AEs) and their impact on energetic ion transport have been studied. The magnetic configuration of the LHD is three-dimensional and has negative magnetic shear over a whole plasma radius in the low beta regime. These features introduce the characteristic structures of the shear Alfvén spectrum. In particular, a core-localized type of toroidicity-induced AE (TAE) is most likely because the TAE gap frequency rapidly increases towards the plasma edge. Moreover, helicity-induced AEs (HAEs) can be generated through a toroidal mode coupling as well as poloidal one in the three-dimensional configuration. The following experimental results have been obtained in LHD plasmas heated by tangential neutral beam injection: (1) observation of core-localized TAEs having odd as well as even parity, (2) eigenmode transition of the core-localized TAE to global AEs (GAEs), which phenomenon is very similar to that in a reversed shear tokamak, (3) observation of HAEs of which the frequency is about eight times higher than the TAE gap frequency, (4) enhanced radial transport/loss of energetic ions caused by bursting TAEs in a relatively high beta regime, and (5) seed formation of internal transport barriers induced by TAE-induced energetic ion transport. These results will be important and interesting information for AE physics in toroidal plasmas.

Energetic particle physics in JT-60U and JFT-2M

Recent energetic particle physics research in JT-60U and JFT-2M is reported. Alfvén eigenmodes (AEs) are investigated in reversed-shear (RS) plasmas in JT-60U where frequency sweeping (FS) modes are observed to follow the q-profile evolution. The RS-induced AE model can explain the FS of the modes within the context of an evolving q-profile. Enhanced energetic ion transport is also investigated with the appearance of modes in the toroidicity-induced AE range of frequency in JT-60U using a multi-channel neutron profile monitor and in JFT-2M using a lost ion probe. Additionally, the ripple loss in the complex toroidal field ripple due to ferritic steel inserts in JFT-2M is shown and compared with model analysis. The simulation code developed to predict ripple loss in JFT-2M will be of use in estimating the heat flux in the complex ripple field of a future device such as ITER.