Energetic Particles in Plasmas

Transcription

1 Energetic Particles in Plasmas James W. Van Dam Institute for Fusion Studies The University of Texas at Austin May 1-2, 2006 GCEP: Energetic Particles in Plasmas 1

2 Introduction In addition to thermal ions and electrons, plasmas often contain a suprathermal species = energetic particles Highly energetic (T h >> T i ) and comparable pressure (n h T h! n i T i ) Energetic particles can be created from various sources: Ion/electron cyclotron heating or neutral beam injection > high energy tails Fusion reactions (e.g., for D-T, v "! v Alfvén, hence instabilities are possible) The plasma physics of energetic particles is of interest to: Laboratory fusion plasmas (alphas provide self-heating to sustain ignition) Space and astrophysical plasmas (e.g., proton ring in Earth s magnetosphere) High-energy-physics accelerators (collective effects) GCEP: Energetic Particles in Plasmas 2

3 Impacts Energetic particles per se: Excitation of various Alfvén-type instabilities (lead to anomalous transport) Redistribution and loss (reduces alpha particle heating efficiency; causes heat loading and damage to plasma-facing components) Integrated with overall plasma behavior: Macrostability (fishbones & monster sawteeth; ballooning modes; disruptions and runaway electrons) Transport (ripple loss; profile modification; rotation generation) Heating and current drive (dominant nonlinear self-heating) Edge physics (resistive wall mode stability) Burn dynamics (thermal burn stability; fuel dilution by helium ash) GCEP: Energetic Particles in Plasmas 3

4 GCEP Questions Scientific issues: What are the scientific and technical barriers to the realization of fusion power that are being addressed in energetic particle physics? What breakthroughs are still required for overcoming them? Suggestions: Summarize the research priorities, and why. Are they covered by or complimentary to current programs? Where could GCEP contribute and have best impact? GCEP: Energetic Particles in Plasmas 4

5 Barriers and Breakthroughs Sci/Tech Barriers Instability thresholds Nonlinear wave dynamics Energetic particle losses Diagnostics Needed Breakthroughs Growth/damping rates Fluid & kinetic resonances Near-marginality; hard vs soft behavior Convective vs diffusive n=0 response (Geodesic Acoustic Mode) Multi-mode experiments (avalanche) Indirect (infer from wave properties) Direct (measure core plasma fluctuations and energetic particle distribution function) GCEP: Energetic Particles in Plasmas 5

6 Instability Thresholds Fast particles can destabilize a large taxonomy of Alfvén modes (*AE) e.g., Toroidal Alfvén Eigenmode (TAE) Mode identification is robust: Frequency, mode structure, polarization Threshold is determined by balance of: Growth rate (reliably calculate) Damping rate (calculation is very sensitive to parameters, profiles, length scales but can measure with active/passive antennas) GCEP: Energetic Particles in Plasmas 6

7 ITER Stability ITER will operate with a large population of super-alfvénic energetic particles New small-wavelength (# * ) regime implies presence of many modes NSTX (low-b, low-shear) is an excellent laboratory for fast particle studies # * -1 = Fredrickson Pitchfork bifurcations (JET) GCEP: Energetic Particles in Plasmas 7

8 Nonlinear Theory: Comparisons Excellent agreement between theory (single mode) and simulation Excellent agreement with experiments Amplitude (a.u.) Experiment Simulation Solid curve = Berk-Breizman theory (with sources and sinks) Circles = White-Chen $f code Central line Upshifted sideband Downshifted sideband t (sec) GCEP: Energetic Particles in Plasmas Amplitude (a.u.) Pitchfork bifurcations (JET)

9 TAE Intermittent Losses Toroidal Alfvén Eigenmode exp ts Loss of fast heating ions (seen from reduced neutron rate) K.L. Wong (1990) Simulations of rapid losses Recently added Geodesic Acoustic Mode Counterinjected beam ions Notable incident of hole punched in TFTR vacuum vessel by lost fast ions Coinjected beam ions GCEP: Energetic Particles in Plasmas Todo et al. (2003) 9

10 Using Wave Properties -1 Determine internal fields from frequency sweeping Determine internal fields from 2nd harmonic Alfvén Cascade perturbed density Theory & simulation (Petviashvili et al.) TAEs in MAST (Gryaznevich) MHD spectroscopy GCEP: Energetic Particles in Plasmas 10

11 Using Wave Properties -2 Temperature inferred from lowfrequency suppression of Cascade modes Monitor q min (for creating an internal transport barrier) with Grand Cascade onset GCEP: Energetic Particles in Plasmas 11 Joffrin et al.

12 New Diagnostics-1 JET data A number of new/upgraded diagnostics can now measure internal fluctuations Interferometry Reflectrometry Far Infrared Scattering Phase Contrast Imaging Beam Emission Spectroscopy Electron Cyclotron Emission GCEP: Energetic Particles in Plasmas Sharapov, PRL 93 (2004)

13 New Diagnostics-2 Gamma-ray tomography (Kiptily) D-alpha (Heidbrink) Recent new fast ion profile diagnostics Collective Thomson scattering Solid-state Neutral Particle Analysis Neutron Collimators GCEP: Energetic Particles in Plasmas 13

14 Research Priorities Priorities Develop new fast particle diagnostics Exploit wave properties for indirect diagnostics Understand nonlinear dynamics Quantify fast ion transport Assess instability thresholds (e.g., ITER) Current Programs Good work being done on existing experiments Some work being done on existing experiments Modest effort Modest effort Some work being done GCEP Contribute New diagnostics for burning plasma context Int l collaboration (EU) Joint postdoc: fishbone; marginal stability profiles Int l collaboration (JA) User-friendly codes GCEP: Energetic Particles in Plasmas 14

15 Opportunities/Alternatives Energetic particle physics area: Alpha channeling Rotation generation and current drive generation by alpha particles to maintain Advanced Tokamak operation Alfvén waves in linear device (LAPD) High-energy particles in space physics Other areas: Liquid metal walls Advanced divertors Educational proposal GCEP: Energetic Particles in Plasmas 15

16 Alpha Channeling Idea for transferring energy of fusion alphas directly to plasma ions through waves Avoids inefficient intermediate step of slowing down on thermal electrons TFTR experiments showed that the reverse process energy transfer to beam ions by RF wave heating can occur The corresponding interaction with alpha particles has not yet been observed Fisch & Rax GCEP: Energetic Particles in Plasmas 16

17 Rotation Generation Idea for creating sheared rotation and negative magnetic shear (conducive to formation of internal transport barrier ) by having Alfvén instabilities redistribute fast ions radially outward Recent experimental indications (DIII-D) Suggests phase-space engineering in burning plasma to optimize performance by using trapped energetic particles to generate flow and control non-inductive current profile (sustain Advanced Tokamak operation as a natural steady state?) GCEP: Energetic Particles in Plasmas 17 K. Wong

18 Basic Wave Studies LArge Plasma Device (LAPD), aka Basic Plasma Science Facility (BaPSF) Long (20 m), large-diameter (1 m), well-diagnosed linear plasma facility with uniform guiding magnetic field Useful for basic studies of propagation and nonlinear properties of waves Recent idea to apply quasi-periodic multi-mirror field to study Alfvén gap modes and trapped particle effects GCEP: Energetic Particles in Plasmas 18

19 Space Physics MIT & Columbia Use fast particle methodology for analysis of dipole stability of veryhigh-pressure plasma Also explains substorms in Earth s magnetosphere GCEP: Energetic Particles in Plasmas 19

20 Liquid Metal Walls Innovation: confine plasma with liquid (instead of solid) metal walls Removes high heat flux Stabilizes plasma Immune to neutrons Enhance tritium breeding No thermal stress u u u J a % B Radial B J a Toroidal Poloidal Invention of soaker hose concept: Coat the walls with slow-streaming liquid metal GCEP: Energetic Particles in Plasmas 20

21 Advanced Divertors Fusion reactor heating power is 5-10 times higher than in ITER (P " ~ 100 MW) ITER is at the limit for standard divertor; hence does not extrapolate to a reactor New Inboard X-Divertor Coils New X-divertor coils create an extra x-point on each divertor leg New Outboard X-Divertor Coils Kotschenreuther et al. GCEP: Energetic Particles in Plasmas 21

22 ITER Summer School Need to train the next generation of young people to work on ITER Propose a GCEP Summer School on ITER: Teach the integrated physics and technology of burning plasmas (including energetic particles) Include lectures on global climate and world energy Publish the lectures (book; online videos) Hold it on university campuses; rotate the location around the country Scholarships to cover student costs Accessible to postdocs, graduate students, and advanced undergraduates GCEP: Energetic Particles in Plasmas 22

23 References U.S. Burning Plasma Workshop (Oak Ridge, TN, 2005): Energetic particle physics plenary talk, break-out group presentations, and summary 9th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems (Takayama, Japan, 2005): Joint Transport Task Force/US-Japan JIFT Workshop on Energetic Particles (Napa, CA, 2005): 8th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems (La Jolla, CA, 2003): ITER Physics Basis Document, Chap. 5 Energetic Particles, Nuclear Fusion 29, 3471 (1999). GCEP: Energetic Particles in Plasmas 23