Importance of edge physics in optimizing ICRF performance

Transcription

1 Importance of edge physics in optimizing ICRF performance D. A. D'Ippolito and J. R. Myra Research Corp., Boulder, CO Acknowledgements D. A. Russell, M. D. Carter, RF SciDAC Team Presented at the ECC Workshop Myrtle Beach, SC, April 3, 2006 dasd - ECC

2 Introduction - 1 Edge and SOL physics issues are very important to the success of a longpulse experiment or fusion reactor. Analogy to laser fusion: coupling high energy to plasma surface highly nonlinear loss mechanisms. This talk will show that the edge turbulence and rf fields from ICRF antennas are coupled in a highly nonlinear way. blob transport density and particle flux in far SOL rf wave coupling and antenna-plasma (sheath) interactions rf convection interacts with turbulence and blobs other nonlinear effects [(i) rf sheath power dissipation on PFC, (ii) PDI and possibly (iii) wave scattering off turbulence] can reduce heating efficiency. the self-consistent SOL density profile requires coupled solution to both problems and divertor and antenna efficiencies dasd - ECC

3 Introduction - 2 required ITER safety margins prevent trial and error optimization need predictive tools. example: nonlinear parasitic absorption of power in edge / SOL already a problem in present experiments much more important for long-pulse or steady-state machines Now is a good time to develop coupled edge / rf codes recent progress in physics understanding of edge and rf physics growth of computing power need for ITER modeling dasd - ECC

4 Outline of talk Experimental evidence for nonlinear rf effects Physics issues for rf optimization Code integration for quantitative SOL / rf model Preliminary simulations of coupled SOL turbulence + rf dasd - ECC

5 ICRF Big Picture Ion Cyclotron Range of Frequencies " ~! i edge absorption e l e c t r o n core electron absorption (broad heat, CD) core ion absorption (localized: ω ~ nω i ) E ~ x, E ~ y k x SOL ~ J y Fast Wave (FW) antenna wall edge absorption dasd - ECC

6 Experimental evidence - 1 rf specific effects impurities (RF-enhanced sputtering) density rise arcs and antenna damage missing edge power rf convection across SOL rf sheaths + other nonlinear effects JET: Bures et al., 1991 dasd - ECC

7 Experimental evidence - 2 hot spots and damage to antennas and limiters & reduced heating efficiency due to sheath power dissipation implications for long-pulse operation (Tore Supra, LHD, ITER) IR camera image of the antenna on Tore Supra after 15 s operation at 8 MW ICRF power. (There is no LH power at the time of the picture.) courtesy of L. Colas, dasd - ECC

8 Experimental evidence - 3 rf sheath formation on antennas Φ sh (x,y) rf convection modifies SOL density profile in front of antenna increases plasma flux to the antenna TFTR: D Ippolito et al., 1998 (reflectometer data from Wilgen) data / model = dotted / solid curves dasd - ECC

9 Physics issues for rf optimization - 1 Linear physics of rf coupling to plasma (k dependence) high k Fast Wave (FW) evanescent at low n need larger n near antenna (n = density) good (single pass) absorption in core, less edge absorption good electron heating, but no current drive decreases nonlinear interactions (sheaths) low and intermediate k FW propagating near antenna (low density at antenna is allowed) lower single pass absorption, more edge absorption (lower heating efficiency) directional low-k waves needed for current drive increases nonlinear interactions (sheaths) Fast wave current drive sheaths dasd - ECC

10 ICRF antenna phasing k poloidal toroidal 0 π dipole (0,π) high k low k stronger rf sheaths (more magnetic flux enclosed) monopole (0,0) low k nb: current drive: (0, π/2, π, 3π /2, ) intermediate k dasd - ECC

11 Physics issues for rf optimization - 2 Nonlinear physics - rf sheaths B-field lines contacting surfaces are biased to large sheath potential Φ sh confines energetic electrons (accelerated by rf fields) restores quasi-neutrality Φ sh depends on k (~ phasing of the current straps in the antenna) Near field (antenna) sheaths typically, E rf has a component B (misalignment with antenna) Φ sh larger for small-k Φ sh E L (10 V/cm) (1 m) = 1 kv e Φ sh /T e >> 1 Far field sheaths occur when B has a component normal to wall (wall not a flux surface) larger for scenarios with poor single pass absorption (small-k ) Φ sh?? [Work in progress] dasd - ECC

12 Near field sheath Φ on 2-strap antenna, (a) k = 0 and (b) k = π Far field sheaths edge rf fields on walls/limiters poor single pass or edge modes flux surfaces and conduction boundaries not aligned wave polarization SW E B 0 incoming fast wave dasd - ECC

13 Physics issues - 3 rf sheath physics models have been tested by detailed modeling of JET, TFTR, Tore Supra, and qualitative comparisons with data from many other experiments. rf sheath power dissipation anomalous power loss P sh = n i c s eφ sh = ion flux ion acceleration in sheath antenna-edge interactions particle flux ( density) dasd - ECC

14 Physics issues - 4 Blob physics turbulence-generated transport of density, heat and momentum to wall density in far SOL is a critical issue for ITER antennas antennas must be built into the first wall for protection (far from the core plasma) high-k FW is cut off at low density need density in far SOL BUT density antenna sheath interactions Self-consistent SOL density profile blob density transport sheath effects: rf convection and shear-flow generation interaction with walls (recycling, outgassing, ionization) with strong rf dasd - ECC

15 Optimization of rf scenarios Requires coupling of multiple physics codes: antenna code (k spectrum of antenna) full-wave code (wave propagation, central power absorption) sheath BC sheath, ponderomotive, PDI (nonlinear interactions, edge power dissipation) surface interaction codes (recycling, outgassing, atomic physics) turbulence / blob transport codes [far SOL n(x)] dasd - ECC

16 Preliminary simulations of blob-antenna interactions with coupled codes 2D SOLT turbulence code n(x, y) () 2D MORRFIC antenna code Φ sh (x,y) (ORNL) Goal: iterate by hand to obtain self-consistent n and Φ sh t = PL BT FS Here, SOLT evolves hand-made blob, which interacts with antenna convective cells 2000 simulations: D. Russell, 2005 dasd - ECC

17 2D turbulence code with Φ sh n(r) profile modification near antenna Time-avg. turbulent density fluctuations (SOLT code, D. Russell, 2005) e! T e sh FS turbulence blobs rf convective cells Φ sh (r - r ant ) calculated from 2D MORRFIC antenna code (M. Carter, 2005) dasd - ECC

18 Summary and conclusions the edge plasma with rf is a coupled and complicated system turbulence, transport, rf and atomic physics highly nonlinear successful operation of an rf-heated tokamak requires understanding the edge and predictive codes much is known about edge turbulence, linear and nonlinear rf physics, but the pieces need to be fit together and optimized integrating codes on the slow (transport, turbulence) and fast (rf) timescales will be a challenge rf SciDAC project is working on adding nonlinear rf physics to antenna and wave codes; code integration with edge codes from other grand challenge projects important for ITER. dasd - ECC