Loss Of Energetic Ions Research Articles

Absolute electron impact ionization cross-sections for CF4: Three dimensional recoil-ion imaging combined with the relative flow technique.

Here we present measurements of dissociative and non-dissociative cross-sections for the electron impact of the CF4 molecule. The present experiments are based on a Recoil Ion Momentum Spectrometer (RIMS), a standard gas mixing setup for CF4, and a reference gas. The measurements were carried out at several electron energies up to 1keV, covering the energy range of previous experiments. We apply the relative flow technique (RFT) to convert the relative cross-sections measured by the RIMS into absolute values. Using the combination of RIMS and RFT, ion collection and calibration errors were minimized. The results were compared with theoretical and experimental studies available in the literature. Previous electron impact experiments present relative cross-sections or use correction terms for the absolute cross-sections due to losses of energetic ions. We elucidate the differences between the new measurement method and the existing ones in the literature and explain why the present method can be considered reliable. Furthermore, we show how reducing correction terms affects the results.

Loss of energetic ions due to n = 1 internal kink instability in HL-2M

Effects of three-dimensional (3D) perturbations due to an unstable n = 1 (n is the toroidal mode number) internal kink (IK) on the energetic particles (EPs) are systematically investigated for the HL-2M tokamak, utilizing the MARS-F/K code [Liu et al., Phys. Plasmas 7, 3681–3690 (2000)] and a recently developed test particle tracing module. A high-beta sawteething HL-2M scenario, simulated by the TRANSP code [Breslau et al., Transp Computer Software (2018)], is chosen for this study. In general, the 3D perturbation associated with an unstable IK is found to affect the EP drift orbit, confinement, and loss in HL-2M. The instability-induced EP loss fraction is found to be typically less than 10%, without counting for the prompt orbit loss associated with the 2D equilibrium field for counter-current particles. The latter reaches about 16% in HL-2M. For co-current EPs, a 100 G 3D magnetic field (inside the plasma) due to the IK does not induce any EP loss assuming a static perturbation. A sawtooth-like time-varying perturbation field, with the peak amplitude reaching 1000 G, can however produce about 30% loss for the co-current EPs in HL-2M. The majority of lost EPs tend to strike the lower divertor region, with a small fraction of particles striking the low-field side mid-plane region of the limiting surface.

Investigation of energetic ion losses induced by long-lived saturated internal mode with energetic particle diagnostics in the HL-2A tokamak

Several sets of energetic particle diagnostics, including a set of neutron flux monitoring systems, a solid-state neutral particle analyzer and a fast ion loss probe (FILP), have been used to investigate the energetic ion losses induced by the long-lived saturated internal mode (LLM) in the HL-2A tokamak. Clear experimental evidence for different levels of energetic ion losses induced by LLM, sawtooth and minor disruption has been observed. A numerical calculation for the evolution of neutron emissions was carried out with the FBURN code, and it shows that the neutron emission drop rate linearly increases with the LLM amplitude and no threshold perturbation amplitude exists, illustrating that the loss mechanism for LLM induced energetic ion loss is dominantly convective. In addition, measurement results of the FILP demonstrate that LLM tends to expel energetic ions with relatively low energy (keV) and high pitch angle (), and can suppress the prompt loss of energetic ions with high energy and low pitch angle to a certain degree. Furthermore, the physical process for LLM induced energetic ion loss can be explained by orbit calculations, which show that LLM induced lost energetic ions will transport from center to peripheral region first, and then get lost out of plasma. The experimental observations are successfully reproduced by calculations using the ORBIT code combined with both the NUBEAM code and the MARS-K code. The paper clearly describes the whole physical process of LLM induced energetic ion loss for the first time in the HL-2A tokamak.

Hybrid-kinetic simulation of synergy between fishbone/sawtooth and tearing mode-induced energetic-ion transport in a tokamak plasma

The resonant interaction between tearing mode (TM) and energetic ions redistributed by fishbone/sawtooth collapse has been observed recently in HL-2A neutral beam injection-heated high-density plasmas, where m is the poloidal mode number and n is the toroidal mode number. This event essentially results from a synergistic effect with a multiple-mode nature. Understanding such experimental phenomena, especially when using high-power auxiliary heating, is crucial for achieving high-performance plasmas with high confinement and high β N in the prospective thermonuclear fusion reactors. In this work, based on the typical experimental phenomenon of synergy between fishbone/sawtooth and TM on HL-2A, a self-consistent nonlinear hybrid-kinetic simulation is conducted utilizing the experimental parameters and profiles before the occurrence of this synergy as the code M3D-K input. In particular, we focus chiefly on the phase-space transport, loss and redistribution of energetic ions induced by two cases, one with only TM and one with synergy between fishbone/sawtooth and TM. It is found that the synergy of the fishbone/sawtooth and TM on energetic-ion transport, loss and redistribution is important and can dramatically enhance the loss rate of energetic ions compared with the case with only TM. It is due to the fact that such synergy can open up an efficient energetic-ion loss channel which connects the plasma center and edge. The phase space transport and redistribution of energetic ions induced by the two different cases are also respectively analyzed and compared in detail. Because of the overestimation of amplitudes of fishbone/sawtooth and TM in the simulations, the obtained energetic-ion transport level is only limited in the context of the numerical framework. These findings indicate that the synergy triggering the sawtooth collapse may lead to an enhancement of the energetic-ion transport and can help us understand the underlying mechanisms of synergy-induced energetic-ion transport.

Loss of Energetic Ions Comprising the Ring Current Populations of Jupiter's Middle and Inner Magnetosphere

AbstractThe low‐altitude, polar orbit of the Juno mission allows the Jupiter Energetic Particle Detector Instrument to view into, and resolve, the loss cone of energetic ions comprising the low‐altitude extension of Jupiter's ring current ions. For regions mapping from just inside Ganymede's orbit to well beyond Ganymede's orbit, energetic ions (>50 keV H+ and >130 keV Oxygen and Sulfur ions) are strongly scattered into the loss cone and lost to the magnetosphere at the “strong diffusion limit” at essentially all times. We conclude, by arguing against magnetic curvature scattering, that the cause is waves, perhaps associated with Alfvénic variations previously documented in this equatorial region. Scattering is generally weak or nonexistent near the orbits of the moons Europa and Io, except for the regions just downstream of the corotating plasmas. For Io, we sometimes observe moderate, but not saturated, scattering within roughly 60° downstream. Significantly, scattering is weak or nonexistent just upstream of Io's position, an asymmetry echoed in some previous wave observations. A preliminary accounting of the total (longitude‐averaged) scattering losses near Io's orbit yields loss rates of order 4%–5% of the strong diffusion limit for H+ and 5%–7% for heavy ions (O + S). We conclude that near Io's orbit, charge exchange losses likely dominate over scattering losses for heavy ions and for the lower energy H+ ions (roughly <200 keV).

Loss of energetic ions comprising the ring current populations of Jupiter's middle and inner magnetosphere

Loss of energetic ions comprising the ring current populations of Jupiter's middle and inner magnetosphere

A model for the fast evaluation of prompt losses of energetic ions in stellarators

A good understanding of the confinement of energetic ions in non-axisymmetric magnetic fields is key for the design of reactors based on the stellarator concept. In this work, we develop a model that, based on the radially-local bounce-averaged drift-kinetic equation, classifies orbits and succeeds in predicting configuration-dependent aspects of the prompt losses of energetic ions in stellarators. Such a model could in turn be employed in the optimization stage of the design of new devices.

Wolfgang Stodiek

Wolfgang Stodiek

Feasibility study of fast ion loss diagnostics for CFQS by beam ion loss calculation on vacuum vessel

A feasibility study for installing fast-ion loss diagnostics (FILDs) is initiated to understand energetic ion loss process in a quasi-axisymmetric stellarator CFQS. The calculations of beam ion orbit, the guiding center orbit code including the slowing down by bulk plasma in the Boozer coordinates and the collisionless Lorentz orbit code in the Cartesian coordinates, are performed to estimate the toroidal/poloidal distribution of beam ion losses on a vacuum vessel. We observed that loss points on the vacuum vessel are mainly located at the upper side of torus according to gradient magnetic field drift direction. Based on the loss distribution on the vacuum vessel, a possible location of FILD for CFQS is investigated. Although the energy of lost beam ions is ∼10 keV, relatively high-energy (> 20 keV) components can be measured on the upper side of the torus in the low-density case. Furthermore, the radial or vertical dependence of beam ion loss flux at candidate movable scintillator-type FILD positions is obtained. A considerable number of particles reach the scintillator-type FILD position. The energy and pitch angle distribution of beam ions reaching the FILD position shows that scintillator-type FILDs can measure co-going transit beam ions with energy of ∼25 keV and pitch angle of ∼140 degrees and transition beam ions with energy of 10–15 keV and pitch angle of ∼80–100 degrees.

Theory of stochastic diffusion in Wendelstein-line stellarators in the presence of radial electric field

In optimized stellarators of the Wendelstein line prompt losses of fast ions in superbanana orbits are reduced through a modification of the magnetic configuration by high β (ratio of the plasma pressure to the magnetic field pressure), so that most contours of the longitudinal adiabatic invariant of locally trapped particles are closed inside the plasma volume. However, magnetic drift leads to the transformation of some of these particles into the locally passing ones and vice versa. Orbit transformation upon crossing the separatrix between locally trapped and locally passing states is accompanied by jumps in that lead to collisionless stochastic diffusion (SD), which was shown to be an important mechanism of the loss of energetic ions in the optimized Wendelstein-line stellarators (C D Beidler et al 2001 Phys. Plasmas 8 2731). The theory of SD that took into account the phase space asymmetry between locally passing particles with opposite signs of was developed in (A V Tykhyy 2018 Ukr. J. Phys. 63(6) 495). This work extends that theory to consider the effect of radial electric field E r on SD. It is found that negative E r of such magnitudes as those observed in W7-X and predicted in Helias reactor can mitigate SD losses from the plasma core of NBI ions in W7-X and α-particles in Helias. This loss mitigation occurs because added drift increases the range of pitch angles for which separatrices are closed inside the plasma volume, even while it increases the SD coefficient. This means that E r can be used in addition to magnetic configuration changes for both loss mitigation and energy deposition profile optimization.

Fast-ion physics in SPARC

Potential loss of energetic ions including alphas and radio-frequency tail ions due to classical orbit effects and magnetohydrodynamic instabilities (MHD) are central physics issues in the design and experimental physics programme of the SPARC tokamak. The expected loss of fusion alpha power due to ripple-induced transport is computed for the SPARC tokamak design by the ASCOT and SPIRAL orbit-simulation codes, to assess the expected surface heating of plasma-facing components. We find good agreement between the ASCOT and SPIRAL simulation results not only in integrated quantities (fraction of alpha power loss) but also in the spatial, temporal and pitch-angle dependence of the losses. If the toroidal field (TF) coils are well-aligned, the SPARC edge ripple is small (0.15–0.30 %), the computed ripple-induced alpha power loss is small ( ${\sim } 0.25\,\%$ ) and the corresponding peak surface power density is acceptable ( $244\ \textrm{kW}\ \textrm {m}^{-2}$ ). However, the ripple and ripple-induced losses increase strongly if the TF coils are assumed to suffer increasing magnitudes of misalignment. Surface heat loads may become problematic if the TF coil misalignment approaches the centimetre level. Ripple-induced losses of the energetic ion tail driven by ion cyclotron range of frequency (ICRF) heating are not expected to generate significant wall or limiter heating in the nominal SPARC plasma scenario. Because the expected classical fast-ion losses are small, SPARC will be able to observe and study fast-ion redistribution due to MHD including sawteeth and Alfvén eigenmodes (AEs). SPARC's parameter space for AE physics even at moderate $Q$ is shown to reasonably overlap that of the demonstration power plant ARC (Sorbom et al., Fusion Engng Des., vol. 100, 2015, p. 378), and thus measurements of AE mode amplitude, spectrum and associated fast-ion transport in SPARC would provide relevant guidance about AE behaviour expected in ARC.

Effect of varying N2 pressure on DC arc plasma properties and microstructure of TiAlN coatings

Detailed knowledge of correlations between direct current (DC) cathodic arc deposition process parameters, plasma properties, and the microstructure of deposited coatings are essential for a comprehensive understanding of the DC cathodic arc deposition process. In this study we have probed the plasma, generated by DC arc on a Ti-50 at.% Al cathode in a N2 ambience, at the growth front of the TiAlN coating. Several consequences of an increasing N2 pressure are observed, including a decreased electron temperature, an increased electron density, and a loss of energetic ions. As a result, the preferred growth texture switches from 220 to 111. It is also observed that neutrals in the plasma can significantly contribute to the growth of TiAlN coatings.

Effects of resonant magnetic perturbations on the loss of energetic ions in tokamak pedestal

Resonant magnetic perturbations (RMPs) are extensively applied to mitigate or suppress the edge localized mode in tokamak plasmas, but will break the axisymmetric magnetic field configuration and increase the loss of energetic ions. The mechanism of RMPs induced energetic ion loss has been extensively studied, and is mainly attributed to resonant effects. In this paper, in the perturbed non-axisymmetric tokamak pedestal, we analytically derive the equations of guiding center motion for energetic ions including the bounce/transit averaged radial drift velocity and the toroidal precession frequency modified by strong radial electric field. The loss time of energetic ions is numerically solved and its parametric dependence is analyzed in detail. We find that passing energetic ions cannot escape from the plasma, while deeply trapped energetic ions can escape from the plasma. The strong radial electric field plays an important role in modifying the toroidal precession frequency and resulting in the drift loss of trapped energetic ions. The loss time of trapped energetic ions is much smaller than the corresponding slowdown time in DIII-D pedestal. This indicates that the loss of trapped energetic ions in the perturbed non-axisymmetric pedestal is important, especially for the trapped energetic ions generated by perpendicular neutral beam injection.

Symplectic Computations of Fast Ion Trajectory and Radial Diffusion Coefficient in MHD Perturbed Tokamak

The magneto-hydrodynamics (MHD) modes existing in conducting plasmas can greatly affect charged particle’s dynamics. Low frequency MHD perturbations, called the neo-classical tearing (NTM) modes, give rise to magnetic islands which can deteriorate fast ion confinement, especially in tokamak plasmas. Here, a Hamiltonian guiding-center code, based on symplectic algorithm which allows efficient computations of energetic ion trajectories and transport coefficients, has been applied to magnetically perturbed tokamak plasmas. The effects of NTM perturbations on the energetic ion trajectories and diffusion coefficient have been presented. It is demonstrated that the inclusion of such steady state magnetic perturbations in the guiding center drift orbit computations escalates the loss of energetic ions via radial diffusion. Moreover, the broader NTMs lead to a significant enhancement of the radial diffusion coefficient.

Resonant effects on the loss of energetic trapped ions induced by low-n resonant magnetic perturbations

The fast ion dynamics and the associated loss channels under resonant magnetic perturbation (RMP) fields are investigated numerically with newly upgraded Monte-Carlo code ORBIT-RF. Special attention is paid to the effect of low-n () perturbations on the confinement of energetic trapped ions in an equilibrium from EAST experiments. Here, n donates the toroidal mode number. It is found that the dominant loss mechanism under n = 2 RMP is essentially different from that under n = 1 RMP. For n = 2 RMP, the enhanced losses are mainly due to the single resonance with . Here, and are, respectively, the precession and bounce frequencies. For n = 1 RMP, the losses are mainly due to orbital stochasticity near the trapped-passing boundary, which is mainly promoted by the nonlinear resonances. It is found that the loss fraction has a sine-like dependence on the phase difference between the upper and lower coil currents under both n = 1 and n = 2 RMPs, which suggests the possibility of active control of fast ion profile by designing the spectrum. The loss rate under n = 2 RMP shows a linear dependence on the perturbation amplitude, while the loss rate under n = 1 RMP shows a quadratic dependence. The results here highlight the importance of nonlinear resonances in fast ion confinement under RMP, in addition to that of linear resonances frequently emphasized before.

Energetic ion losses ‘channeling’ mechanism and strategy for mitigation

Results from two different sets of JET experiments are presented. In experiments in which toroidicity-induced Alfvén eigenmodes (TAEs) localized at different radial locations had the same frequencies and toroidal mode numbers, the occurrence of enhanced losses after the excitation of TAEs in the core of the plasma was observed. On the contrary, enhanced losses were not observed if the TAEs localized at different radial locations had different frequencies and toroidal mode numbers. Numerical modeling indicates that, in the first set of experiments, the enhanced losses were caused by a combined effect of the TAEs localized at different radial locations. The TAEs localized in the plasma core transported energetic ions from the core to outer regions of the plasma. Then, the TAEs localized in outer regions of the plasma interacted with these ions just transported by the core-localized TAEs causing a further radial displacement of the ions to the plasma edge. This process eventually ends up causing the loss of the resonant ions. In the second set of experiments, it was found that TAEs localized in the plasma core and in outer regions did not interact with the same ions and so no enhanced losses were measured. Sheared profiles of the safety factor combined with flat mass density profiles lead to larger differences on the frequencies of the TAEs localized at different radial locations, eventually avoiding loss of energetic ions through the described mechanism.

Active control of Alfvén eigenmodes in magnetically confined toroidal plasmas

Alfvén waves are electromagnetic perturbations inherent to magnetized plasmas that can be driven unstable by a free energy associated with gradients in the energetic particles’ distribution function. The energetic particles with velocities comparable to the Alfvén velocity may excite Alfvén instabilities via resonant wave–particle energy and momentum exchange. Burning plasmas with large population of fusion born super-Alfvénic alpha particles in magnetically confined fusion devices are prone to excite weakly-damped Alfvén eigenmodes (AEs) that, if allowed to grow unabated, can cause a degradation of fusion performance and loss of energetic ions through a secular radial transport. In order to control the fast-ion distribution and associated Alfvénic activity, the fusion community is currently searching for external actuators that can control AEs and energetic ions in the harsh environment of a fusion reactor. Most promising control techniques are based on (i) variable fast-ion sources to modify gradients in the energetic particles’ distribution, (ii) localized electron cyclotron resonance heating to affect the fast-ion slowing-down distribution, (iii) localized electron cyclotron current drive to modify the equilibrium magnetic helicity and thus the AE existence criteria and damping mechanisms, and (iv) externally applied 3D perturbative fields to manipulate the fast-ion distribution and thus the wave drive. Advanced simulations help to identify the key physics mechanisms underlying the observed AE mitigation and suppression and thus to develop robust control techniques towards future burning plasmas.

Trigger mechanism for the abrupt loss of energetic ions in magnetically confined plasmas

Interaction between a quasi-stable stationary MHD mode and a tongue-shaped deformation is observed in the toroidal plasma with energetic particle driven MHD bursts. The quasi-stable stationary 1/1 MHD mode with interchange parity appears near the resonant rational surface of q = 1 between MHD bursts. The tongue-shaped deformation rapidly appears at the non-resonant non-rational surface as a localized large plasma displacement and then collapses (tongue event). It curbs the stationary 1/1 MHD mode and then triggers the collapse of energetic particle and magnetic field reconnection. The rotating 1/1 MHD mode with tearing parity at the q = 1 resonant surface, namely, the MHD burst, is excited after the tongue event.

Loss and redistribution of energetic passing ions with resonant magnetic perturbations

The effects of resonant magnetic perturbations (RMPs) and the magnetic drift on the loss and redistribution of passing ions are investigated numerically by the upgrade version of the orbit following code GYCAVA. The drift island structures of passing ion orbits induced by RMPs and the magnetic drift are consistent with the given toroidal and poloidal mode numbers of RMPs in the orbit simulations. The redistribution and loss of energetic and thermal passing ions with and without RMPs are numerically studied and compared with each other. The redistribution near the edge is due to the loss induced by RMPs and the magnetic drift. The extra loss of passing ions induced by RMPs is related to the drift island structure induced by RMPs and the magnetic drift and the stochasticity induced by overlap of magnetic islands. The loss of passing ions with n = 1 RMPs is larger than that with n = 4 RMPs for the same perturbation amplitude parameter, which is due to the fact that the magnetic islands produced by n = 1 RMPs are larger than those produced by n = 4 RMPs. The loss fraction of passing ions with n = 1 RMPs increases with the amplitude of RMPs and the radial profile parameter of the ion density. The pitch and energy scan for the loss of energetic passing ions with n = 1 RMPs is simulated. For counter-passing ions, the loss fraction of low-pitch ions is larger than that of high-pitch ions. The dependence of the loss of energetic ions with RMPs on energy is related to the pitch angle.

Development towards a fast ion loss detector for the reversed field pinch.

A fast ion loss detector has been constructed and implemented on the Madison Symmetric Torus (MST) to investigate energetic ion losses and transport due to energetic particle and MHD instabilities. The detector discriminates particle orbits solely on pitch and consists of two thin-foil, particle collecting plates that are symmetric with respect to the device aperture. One plate collects fast ion signal, while the second aids in the minimization of background and noise effects. Initial measurements are reported along with suggestions for the next design phase of the detector.