Plasma-neutrals transport modeling of the ORNL plasma-materials test stand target cell J.M. Canik, L.W. Owen, Y.K.M. Peng, J. Rapp, R.H. Goulding Oak Ridge National Laboratory
ORNL is developing a helicon-based plasma-materials test stand PMI Test Stand - RF based source Provides ITER-like divertor parameters Steady state operation at high temp. Material Analysis & Preparation Stations Compatible with high dpa targets Focus on PMI and PFC proto-type tests Developed as a user facility Prototype built to reduce risk PMTS could be realized in 5 years Material Science Test Station Test Target Transfer Cask Helicon and magnetic mirror (1T) RF heating: 200 kw each ICRH, ECRH 20 MW/m 2 on target, 10 23-24 /m 2 s under high recycling conditions Helicon & RF Heating Source Proto-types are currently being developed at ORNL with internal funds 2 Managed by UT-Battelle Magnetic Mirror with RF Heating Helicon RF-based
Outline Scoping studies for the ORNL PMTS using 2-pt model 2D plasma-neutrals modeling of PMTS with SOLPS 2D modeling of the existing helicon Future modeling plans 3 Managed by UT-Battelle
Goals of PMTS target plasma are guided by expected parameters in ITER B2-Eirene simulation for ITER, including impurities (A.S. Kukushkin) flux [1/m 2 s], density [1/m 3 ] temperature [ev] 1E25 1E23 ion flux atom flux electron density electron temperature 20 15 1E21 10 1E19 5 1E17 0-0.2 0.0 0.2 0.4 0.6 distance along outer plate [m] 4 Managed by UT-Battelle T e ~1-15 ev Particle flux ~ 10 24 /m 2 /s n e ~10 20-10 21 /m 3
Scoping of PMTS requirements with 2- pt modeling Goal is a target plasma with T e ~a few ev, n e ~10 20-10 21 /m 3 Reaching the very high desired n e is beyond capabilities of source Instead, use high recycling to achieve desired T e, n e at the target Allows higher T e, lower n e at source, rely on parallel conduction to reduce T e to ~1 ev Defines requirements on source heat flux and electron density Also requires that source provided dominantly heat, not particles (avoid convection) Initial estimates of required source n e, q are made with 2-pt model Pressure balance: 2 n t T t = n u T u Energy balance: T u 7/2 = T t 7/2 + (7L/2k e )f c q Target heat flux boundary condition: (1 f p )q = γkt t n t c st 5 Managed by UT-Battelle
Source densities of ~3-6x10 19 /m 3 are required for T e,t ~1eV L=3.25 m system considered Pure conduction assumed, but with 30% power loss 6 Managed by UT-Battelle
Target densities are lower than ITER, but should be sufficient Absolute density is low ~a few 10 20 m -3 at expected q, n u But dimensionless parameters are similar In-sheath ionization: Ratio of ionization MFP to sheath width ~n/b PMTS (3x10 20 /1T) ~ ITER (1.5x10 21 /5T) Particle motion vs. system size Gyroradius 5x larger than ITER Plasma width ~10 cm is 5x larger than SOL heat flux width Need to consider dust 7 Managed by UT-Battelle
2-D modeling of PMTS with SOLPS 2-pt model useful for rough estimates, but many factors are unknown and are expected to be important More realistic 2-D modeling is being performed using SOLPS 5.0 (B2/EIRENE) Solves conservation equations for density of each charge state parallel momentum of each charge state electron energy ion energy charge Includes models for plasma transport Parallel: classical along field lines with particle and heat fluxes limited to simulate kinetic effects Radial: D, χ e, χi adjusted to fit measured plasma density and temperature profiles. In the work reported here D, χ are assumed. ExB and grad B drift effects available but not yet included. Neutral transport: Kinetic, using the Eirene Monte Carlo code, or fluid 8 Managed by UT-Battelle
Z (m) 2-D grid generated for 3 m, 1 T PMTS Grid radius ~7 cm, strongly refined in axial direction near target Power, particle input specified as boundary condition at Z=3.0 m Source region not modeled directly Uniform radial profile of heat/particle fluxes over central 5 cm Pumping done at outer wall in EIRENE Source Axial grid index Heat flux R p =0.99 Target R (m) 9 Managed by UT-Battelle R (m)
Particle and heat fluxes at source and target: P tot =100 kw, Γ tot =2x10 21 #/s Input power is 100 kw, full PMTS capacity ~400 kw Heat flux at target reduced by ~60% Strong volumetric losses Heat flux onto target (5 MW/m 2 ) is less than PMTS design goals Particle flux at target > 10 24 /m 2 /s Flux amplification due to high recycling, meets design goal 10 Managed by UT-Battelle
T e and n e at source and target: P tot =100 KW, Γ tot =2x10 21 #/s Density at target is ~10 20 /m 3 Somewhat below design goals, likely need to raise power Temperature at target ~2 ev In the right range, ultimately would like < 1eV capability Indicates source density ~5x10 19 needed for this system length, input power 11 Managed by UT-Battelle
Volumetric power losses ~50% of input power is lost in volumetric processes 45 kw hydrogen radiation 10 kw CX Neutral losses very concentrated near target Plasma sufficiently ionizing to confine recycling neutrals near target Some CX loss persists upstream, where T i (~T e ) is high Electron cooling rate (W/m 3 ) Ion power loss (W/m 3 ) Atomic hydrogen density (/m 3 ) 12 Managed by UT-Battelle
Density scan: n e and T e Density is scanned by varying gas input at source Same pumping conditions (R=0.99 on outer wall) in all cases Total particle inputs: 0.2,0.3,0.5,1.0,2.0 x10 21 /s Gives source densities in the range 1-5x10 19 /m 3 13 Managed by UT-Battelle
Density scan: Heat and particle flux Density is scanned by varying gas input at source Same pumping conditions (R=0.99 on outer wall) in all cases Total particle inputs: 0.2,0.3,0.5,1.0,2.0 x10 21 /s Gives source densities in the range 1-5x10 19 /m 3 14 Managed by UT-Battelle
On-axis parameters vs. source density Quickly get into high-recycling regime for source n e >10 19 /m 3 Upstream density above ~4x10 19 sufficient to reach T e < ~5 ev, particle flux > 10 24 Volumetric losses strong at high densities (~50%) Impurities not included Need to increase input power to reach goal heat flux Substantial pressure drop at high density Momentum loss high More difficult to reach high target densities 15 Managed by UT-Battelle
Future directions for 2D modeling Modeling so far indicates that higher power levels are needed to raise density at target Well within planned capabilities of PMTS Higher power also needed to make up for volumetric losses Relatively high density at source needed for low target T e At upper end of what is feasible System length needs to be optimized Easier to reduce target temperature Should reduce density demands on source Higher densities to be considered in future Explore T e < 1eV, detached regime Self-consistent modeling of source region 16 Managed by UT-Battelle
Radius from axis (cm) Whole device modeling of existing helicon experiment EMS-2D Helicon RF plasma heating Accounts for B, n, profiles, geometry, antenna, heating Benchmarks well with various Helicon measurements Determines design and stable Helicon operating parameters SOLPS plasma-neutrals-wall transport and interactions Accounts for B, wall, and pump geometries; plasma n, Ti, Te; atoms, molecules, ionization, recombination, wall reflection, desorption, pumping, etc. Benchmarked and applied to tokamak edge-divertor plasmas Adapted to model linear Helicon, PhIX, PMTS with RF heating Determines fueling-pumping configuration producing optimal plasma/neutral radial/axial density profiles for RF heating GENRAY whistler and EBW ray tracing and plasma heating Accounts for B, n, profiles, geometry, launcher, elec. Heating Benchmarked and applied to toroidal experiments Modified to use Cartesian framework for linear devices Determines launcher configuration and heating efficiency 17 Managed by UT-Battelle Plasma Resistance ( ) vs. B (T) 0.75 T 0.9T 1.5T Plasma Peak Density (10 19 /m 3 ) 8.0 Helicon RF Heating on SOLPS Grid 0 0 Distance along B (cm) 140.0 Whistler Launche r Wave Trajectories B field Plasma
2-D Helicon RF heating distribution has been used in SOLPS modeling Instead of using boundary conditions to mimic source conditions, model volumetric sources within heating region directly Existing helicon experiment modeled in order to validate this approach 18 Managed by UT-Battelle
Electron and ion temperatures are predicted to be ~1-6 ev Values are within reasonable range Awaiting measurements to validate these calculations 19 Managed by UT-Battelle
About 1/3 of the input rf power needed to reproduce the observed electron density In experiment, ~30 kw is input ~10 kw is required in modeling to reproduce measured density 20 Managed by UT-Battelle
Modeling is aiding the development of PMTS 2-pt modeling indicates source densities of ~5x10 19 /m 3 are needed 2-D modeling with SOLPS highlights importance of volumetric processes Power losses ~ 50% at high density Momentum loss makes raising target density more challenging System length and heating power to be optimized Moving towards whole device modeling Heating power calculations coupled to SOLPS transport Present effort is focused on existing helicon experiment 21 Managed by UT-Battelle