Energetic Particle Physics Research Articles

15th Annual International Astrophysics Conference: “The Science of Ed Stone: Celebrating his 80th Birthday”

The 15th Annual International Astrophysics Conference was held at the Westin Cape Coral Resort at Marina Village, Cape Coral, Florida, during the week of 4-8 April 2016. This year the meeting was rather different from previous years in that it honoured the scientific achievements and recognized the extraordinary contribution of Ed Stone to space physics. Entitled “The Science of Ed Stone: Celebrating his 80th Birthday,” the meeting drew some 85 participants from all over the world. The number of participants and the range of topics covered paid testament to the enormous range of the science that Ed influenced profoundly. Indeed, Ed Stone has made foundational contributions to our understanding of all facets of the physics of energetic particles in the solar wind and the interstellar medium. Much of our current understanding of the interaction of the solar wind with the local interstellar medium is derived from the Voyager Interstellar Mission, which has operated for 39 years under Ed's leadership. His remarkable career has spanned the space age, and he has contributed not only to science but to the leadership of our community.It is with great sadness that we note that one of the people greatly influenced by Ed Stone, Harm Moraal, died in the past year. A brief obituary of Prof. Harm Moraal is presented below. He will be missed greatly in our community.Finally, we thank Adele Corona and ICNS for her continued excellent organization of the AIAC meetings and her help in providing the logistical support for this volume of papers.

Ion cyclotron emission studies: Retrospects and prospects

Ion cyclotron emission (ICE) studies emerged in part from the papers by A.B. Mikhailovskii published in the 1970s. Among the discussed subjects were electromagnetic compressional Alfvenic cyclotron instabilities with the linear growth rate ~ $\sqrt {n_\alpha /n_e } $ driven by fusion products, -particles which draw a lot of attention to energetic particle physics. The theory of ICE excited by energetic particles was significantly advanced at the end of the 20th century motivated by first DT experiments on TFTR and subsequent JET experimental studies which we highlight. More recently ICE theory was advanced by detailed theoretical and experimental studies on spherical torus (ST) fusion devices where the instability signals previously indistinguishable in high aspect ratio tokamaks due to high toroidal magnetic field became the subjects of experiments. We discuss further prospects of ICE theory applications for future burning plasma (BP) experiments such as those to be conducted in ITER device in France, where neutron and gamma rays escaping the plasma create extremely challenging conditions fusion alpha particle diagnostics.

Nonlinear dynamics of phase space zonal structures and energetic particle physics in fusion plasmas

A general theoretical framework for investigating the nonlinear dynamics of phase space zonal structures is presented in this work. It is then, more specifically, applied to the limit where the nonlinear evolution time scale is smaller or comparable to the wave–particle trapping period. In this limit, both theoretical and numerical simulation studies show that nonadiabatic frequency chirping and phase locking could lead to secular resonant particle transport on meso- or macro-scales. The interplay between mode structures and resonant particles then provides the crucial ingredient to properly understand and analyze the nonlinear dynamics of Alfvén wave instabilities excited by nonperturbative energetic particles in burning fusion plasmas. Analogies with autoresonance in nonlinear dynamics and with superradiance in free-electron lasers are also briefly discussed.

Diagnostics for energetic particle studies on the HL-2A tokamak.

About 13 kinds of diagnostics for energetic particle physics studied on the HuanLiuqi-2A (commonly referred to as HL-2A) tokamak are described in this paper. Their measurement ranges, resolutions, and arrangement are presented. Three under-construction diagnostics including imaging fast ion D-alpha, scintillator matrix (for hard X-ray detection), and bundle fission chamber are described in detail.

Vlasov versus reduced kinetic theories for helically symmetric equilibria

A new constant of motion for helically symmetric equilibria in the vicinity of the magnetic axis is obtained in the framework of Vlasov theory. In view of this constant of motion the Vlasov theory is compared with drift kinetic and gyrokinetic theories near axis. It turns out that as in the case of axisymmetric equilibria [H. Tasso and G. N. Throumoulopoulos, Phys. Plasmas 18, 064507 (2011)] the Vlasov current density thereon can differ appreciably from the drift kinetic and gyrokinetic current densities. This indicates some limitation on the implications of reduced kinetic theories, in particular, as concerns the physics of energetic particles in the central region of magnetically confined plasmas.

Energetic Particle Physics Experiments With High Power ECRH on HL-2A

The MHD instabilities driven by the energetic-particles are of particular importance for future burning plasma devices, where energetic particles will be abundantly produced by high power heating and fusion reaction. In this paper, some new phenomena induced by high power ECRH are presented.

A comparison of Vlasov with drift kinetic and gyrokinetic theories

A kinetic consideration of an axisymmetric equilibrium with vanishing electric field near the magnetic axis shows that ∇f should not vanish on axis within the framework of Vlasov theory while it can either vanish or not in the framework of both a drift kinetic and a gyrokinetic theories (f is either the pertinent particle or the guiding center distribution function). This different behavior, relating to the reduction of phase space which leads to the loss of a Vlasov constant of motion, may result in the construction of different currents in the reduced phase space than the Vlasov ones. This conclusion is indicative of some limitation on the implications of reduced kinetic theories, in particular, as concerns the physics of energetic particles in the central region of magnetically confined plasmas.

Identifying the impact of rotation, anisotropy, and energetic particle physics in tokamaks

In this paper we study the effects of poloidal and toroidal rotation, and anisotropy in tokamaks. To resolve these effects from uncertainties in the data, we introduce a Bayesian inference framework which calculates the magnetic configuration probabilistically using motional Stark effect and magnetic data. Drawing on these calculations, we compute the poloidal and toroidal Mach numbers in MAST for a discharge with good rotation data. Our calculations confirm that the poloidal Mach number Ms,θ = vθ/vi × B/Bθ is near zero (with vθ and vi the poloidal and thermal velocity, respectively), even on the outboard side where the scaling of poloidal field strength Bθ to total field B is large. In contrast, the toroidal rotation of this plasma reaches a Mach number of 0.5 on axis. The impact of the toroidal rotation on the equilibrium reconstructions of the plasma is however small: it acts to increase the radius of the magnetic axis by ≈1%, and lower the central safety factor by ≈5%. In comparison, corrections to make the pressure profile consistent with internal measurements such as charge exchange recombination spectroscopy and Thomson scattering have a much larger impact. In other work we compute the level of anisotropy from a TRANSP simulation of a neutral beam-heated MAST discharge. This shows a large level of anisotropy, with p⊥/p∥ ≈ 1.7, sufficient to boost the central safety factor by 15%. For this discharge, which is representative of many MAST discharges, the effect of anisotropy and consistent pressure profiles is more pronounced than the toroidal rotation of the plasma.

The physics of near-earth space plasma and energetic particles in the magnetosphere

The magnetosphere-ionosphere plasma system, like the Earth’s other envelopes, is a complex nonlinear open dissipative system with self-organization. Moreover, it is embedded in the extremely nonstationary environment formed by the solar wind. Explorations that were initiated by the pioneer studies of S.N. Vernov and his colleagues resulted in deep insight into the processes occurring in this system.

Chapter 5: Burning Plasma Studies at JET

Studies establishing key phenomena and developing diagnostics for energetic particle physics, which are essential for the next step burning plasma experiments such as the International Thermonuclear Experimental Reactor (ITER), have been performed at the Joint European Torus (JET). Experiments have demonstrated clear self-heating of deuterium-tritium (D-T) plasma by alpha particles as a maximum in electron temperature at an optimum mixture of 60 ± 20% tritium. The change in electron temperature produced by alpha heating, ΔTe(0) = 1.3 ± 0.23 keV, was as expected from classical heating, whereas the heating of thermal ions was higher than expected from reference deuterium discharges. Alfvén eigenmodes were stable in the highest fusion performance D-T plasmas, in agreement with the modeling. Systematic studies on the existence and properties of Alfvén eigenmodes with external antenna driving and detecting Alfvén eigenmodes are presented. The formation of fuel ion tails due to alpha-particle knock-on effects is described as derived from neutral particle analyzer and neutron emission spectrometry in D-T experiments. The gamma-ray diagnostics are shown to measure profiles and energy distribution functions of high-energy ions and alpha particles. Time- and space-resolved gamma-ray images demonstrated for the first time the possibility of measuring several types of energetic ions simultaneously. The novel technique of detecting unstable Alfvén eigenmodes with interferometry is found to be superior in detecting core-localized Alfvén eigenmodes.

Theory of Alfvén waves and energetic particle physics in burning plasmas

We present an overview on one issue of practical interest for burning plasmas, i.e. whether fast ions and charged fusion products are sufficiently well confined such that they transfer their energy and/or momentum to the thermal plasma without appreciable degradation due to collective modes. In the present work, we address this issue by analysing theoretically the dynamics of shear Alfvén waves collectively excited by energetic particles in tokamak plasmas. Both linear physics, such as spectral and stability properties, and key non-linear wave and particle dynamics are identified and considered. We also discuss the investigations of such processes via computer simulations as well as the importance of benchmarking with existing or future experimental observations.

Energetic Particle Physics Studies on Alcator C-Mod

Energetic particle physics is studied in Alcator C-Mod in reactor relevant regimes with high density and equilibrated electron and ion temperatures. Stable Alfvén eigenmodes are excited with low-power active magnetohydrodynamic antennas in the absence of a significant energetic particle tail to directly measure the damping rate of the modes. Stable toroidal Alfvén eigenmode (TAE) damping rates between 0.5% < γ/Ω < 4.5% have been observed in diverted and limited plasmas. Alfvén eigenmodes are destabilized with high-power hydrogen minority ion cyclotron radio frequency (ICRF) heating (PICRF < 6 MW) in lower-density plasmas in the current rise and in relatively high-density ([bar]ne < 2.5 × 1020 m-3) H-mode plasmas, which creates an energetic hydrogen ion tail with calculated energies up to 400 keV. Low toroidal mode number (n < 4) unstable modes are observed in the current rise with magnetic pickup coils at the wall and phase contrast imaging density fluctuation measurements in the core. Observations of energetic particle modes or TAEs that decrease in frequency and mode number with time up to a large sawtooth collapse indicate that fast particles play a role in stabilizing sawteeth. Alfvén eigenmodes can also be used as diagnostics to precisely constrain the q profile and provide a qualitative measure of the fast particle distribution time evolution.

‘Burning plasma’ diagnostics for the physics of JET and ITER

JET's recent experimental programme proved that ‘burning plasma diagnostics’, i.e. neutron, alpha particle, He ash, and fuel mixture measurements, can provide very useful information about crucial physical aspects of great reactor relevance. First of all, several of these diagnostics can improve the diagnostic capability of the ion fluid significantly. During TTE spatially resolved neutron measurements at JET were essential in obtaining the isotopic composition and the transport of the hydrogen isotopes, allowing a direct comparison between the measured transport coefficients and the neoclassical theory. The neutron emission profiles can also give crucial indications for assessing the merits of various heating schemes and their current drive capability. Neutron spectroscopy in its turn provides a clear and direct measurement of the temperature and the velocity distribution of the fuel ions. For example, the dependence of the toroidal velocity from the ion cyclotron radiofrequency heating phasing was clearly seen during TTE. The requirements of accurate neutron measurements are also promoting considerable research in detector technology, in particular in the fields of compact spectrometers and solid state detectors. ‘Burning plasma’ diagnostics can also strongly contribute to the physics of energetic particles and their interaction with the main plasma. γ-Ray spectroscopy is now an established method to determine the spatial localization, to visualize the trajectories of the alpha particles and the fast deuterons and to obtain estimates of their slowing down. A completely new method to detect the energetic particles, exploiting the line intensity ratio of extreme ultraviolet radiation emitted by suitable extrinsic impurities, is also being pursued. This technique allows investigating the energy range below 600 keV, extremely interesting for the study of wave–particle interactions.

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.

Report on the 7th European Fusion PhysicsWorkshop, Kloster Seeon, Germany, 8-10 December 1999

The 7th EFPW took place in December 1999 at Kloster Seeon in Germany. It washosted by the Association IPP-Euratom, Garching and sponsored by the EuropeanCommission. Within an overall theme of plasma heating and current drivesystems, four topics of importance to the future development of magneticallyconfined fusion were discussed in detail: the injection of heating, currentand momentum; the interplay between heating systems and transport barriers;MHD control issues for heating and current drive systems; and energeticparticle physics. In addition, a review of recent JET experiments, theorganization of the JET scientific programme under EFDA and the new ITERdesign were presented. The main issues discussed in each session and the areasidentified as requiring further study are summarized here.

Energetic particle physics in JET

Results achieved on JET during the 1997-1999 experimental campaigns in the physics of energetic ions and runaway electrons are reviewed. Heating of deuterium-tritium (DT) plasmas by fusion born alpha particles is found to be similar to that achieved by comparable ICRF heating of deuterium plasmas. The stability of alpha particle driven Alfvén eigenmodes (AEs) in the highest fusion power ELM-free H mode discharges is shown to be consistent with the existing theoretical analysis for AEs. Direct measurements of the trapped alpha particle and knock-on deuteron distribution functions by a neutral particle analyser (NPA) are described. New ICRF heating scenarios tested in JET DT plasmas are presented in view of the possible use of the ICRF heating of ITER-like DT plasmas on route to ignition. The energetic ion pinch in the presence of toroidally asymmetric ICRF waves is studied experimentally on JET. Recent experimentally observed effects of ICRF accelerated ions on sawteeth in JET are reviewed. Detailed time and space resolved X ray images of the electron runaway beam in flight spontaneously generated by disruptions on JET are described.

Chapter 1: Overview and summary

The ITER Physics Basis presents and evaluates the physics rules andmethodologies for plasma performance projections, which provide the basis for thedesign of a tokamak burning plasma device whose goal is to demonstrate the scientificand technological feasibility of fusion energy for peaceful purposes. This Chaptersummarizes the physics basis for burning plasma projections, which is developed indetail by the ITER Physics Expert Groups in subsequent chapters. To set context, thedesign guidelines and requirements established in the report of ITER Special WorkingGroup 1 are presented, as are the specifics of the tokamak design developed in theFinal Design Report of the ITER Engineering Design Activities, which exemplifiesburning tokamak plasma experiments. The behaviour of a tokamak plasma is determined bythe interaction of many diverse physics processes, all of which bear on projectionsfor both a burning plasma experiment and an eventual tokamak reactor. Key processessummarized here are energy and particle confinement and the H-mode power threshold;MHD stability, including pressure and density limits, neoclassical islands, errorfields, disruptions, sawteeth, and ELMs; power and particle exhaust, involvingdivertor power dispersal, helium exhaust, fuelling and density control, H-mode edgetransition region, erosion of plasma facing components, tritium retention; energeticparticle physics; auxiliary power physics; and the physics of plasma diagnostics.Summaries of projection methodologies, together with estimates of their attendantuncertainties, are presented in each of these areas. Since each physics element hasits own scaling properties, an integrated experimental demonstration of the balancebetween the combined processes which obtains in a reactor plasma is inaccessible tocontemporary experimental facilities: it requires a reactor scale device. It isargued, moreover, that a burning plasma experiment can be sufficiently flexible topermit operation in a steady state mode, with non-inductive plasma current drive, aswell as in a pulsed mode where current is inductively driven. Overall, the ITERPhysics Basis can support a range of candidate designs for a tokamak burning plasmafacility. For each design, there will remain a significant uncertainty in theprojected performance, but the projection methodologies outlined here do suffice tospecify the major parameters of such a facility and form the basis for assuring thatits phased operation will return sufficient information to design a prototypecommercial fusion power reactor, thus fulfilling the goal of the ITER project.

Physics of energetic particles in ITER

The paper summarizes issues and presents results of theITER physics work in the area of energetic particles such as fusionalpha particles, energetic ions produced by plasma auxiliary heating,and runaway electrons which can be generated during plasmadisruption.

Fusion plasma experiments on TFTR: A 20 year retrospective

The Tokamak Fusion Test Reactor (TFTR) (R. J. Hawryluk, to be published in Rev. Mod. Phys.) experiments on high-temperature plasmas, that culminated in the study of deuterium–tritium D–T plasmas containing significant populations of energetic alpha particles, spanned over two decades from conception to completion. During the design of TFTR, the key physics issues were magnetohydrodynamic (MHD) equilibrium and stability, plasma energy transport, impurity effects, and plasma reactivity. Energetic particle physics was given less attention during this phase because, in part, of the necessity to address the issues that would create the conditions for the study of energetic particles and also the lack of diagnostics to study the energetic particles in detail. The worldwide tokamak program including the contributions from TFTR made substantial progress during the past two decades in addressing the fundamental issues affecting the performance of high-temperature plasmas and the behavior of energetic particles. The progress has been the result of the construction of new facilities, which enabled the production of high-temperature well-confined plasmas, development of sophisticated diagnostic techniques to study both the background plasma and the resulting energetic fusion products, and computational techniques to both interpret the experimental results and to predict the outcome of experiments.