Status of Tokamak Research
Alcator CMo− d
Alcator C-Mod and the Path to ITER and Beyond
Forum on the Future of Fusion Fusion Power Associates Annual Meeting and Symposium November 20, 2003
Presented by E.S. Marmar on behalf of the C-Mod Team We are technically ready to begin Alcator CMo− d ITER construction
• Science and Technology issues to resolve, including – All metal plasma facing components: T retention, disruptions – Disruption Mitigation in high pressure plasma – Transport/Confinement with equilibrated electrons and ions – Pedestal physics: desire for small/no ELM regimes; scaling – NTM physics: direct stabilization; elimination of sawtooth seed – Rotation in the absence of direct momentum input: implications for RWM stabilization – Error fields and locked modes: size and field scaling – Alfven Eigenmode physics – AT physics toward steady state C-Mod Demonstrates Compatability of all Metal Alcator (Mo) PFCs with Clean, High Performance CMo− d Plasma
• Tritium retention may preclude carbon from ITER and Power Reactors Tungsten welding rod • C-Mod operates with reactor relevant edge power densities (P|| up to 0.5x109 W/m2)
–Zeff in range from 1.2 to 2 under most conditions • Planning for even higher power at lower ne (AT regimes) – Investigating ITER prototypical tungsten brush designs (collaboration with Sandia National Lab) – Install first modules in C-Mod, March 2004 1 cm • Disruptions can cause surface melting Disruption Mitigation needs Testing in Higher Alcator Electron Pressure Plasmas CMo− d
• Massive noble gas puff on DIII-D – Very encouraging results (D. Whyte, et al., PRL 2002) • C-Mod investigations (collaboration with U. Wisconsin):
– Higher Electron Pressure (Pe) plasmas (gas penetration) – Higher Energy Density plasmas (efficacy of radiation) • C-Mod/DIII-D/JET comparisons valuable to test size scaling
Device
• Giant ELMs could compromise Enhanced Dα (EDA) H-Mode ITER divertor in small number of discharges • Small/no ELM regimes with good energy confinement, particle regulation across barrier: – QH/QDB modes (DIII-D, also now on ASDEX-U, JET)
– EDA H-Mode (C-Mod, also now 920 seen on JFT2-M) • Particle transport in EDA driven by 910 separatrix location
mode just inside separatrix 900 – Features consistent with resistive ballooning mode seen 890 in modeling (Xu and Nevins) Major Radius (mm) 880 Amp. peaks ~1-3 mm inside separatrix 1.041.06 1.08 1.10 1.12 1.14 Time (sec) Off-Axis ICRF MCCD Experiments Alcator Potential for Sawtooth Stabilization with modest RF power CMo− d
Cntr-Current Drivectr-CDCo-Current Drive co-CD
PRF (MW) 2 CTR CD
] Co-CD 3 1
D-port D+J-port D-port D+J-port
Te0 (keV) [MW/m
3.5 abs S
3 rinv
0.7 0.75 0.8 0.7 0.75 0.8 Time [sec] Time [sec] ρ • Mode Conversion Current Drive: D(3He), 8 Tesla • Localized absorption on electrons (300 kW) just inside the sawtooth inversion radius τ τ – Counter-CD phase lengthens st, Co-CD phase shortens st • More power available – Stabilize sawteeth? (remove seed for NTM) – Direct NTM stabilization? Momentum Input Difficult in a Reactor Alcator Important to Understand Spontaneous Rotation CMo− d
ICRF and Ohmic H-modes • Spontaneous rotation in high 14 pressure (gradient?) 1.2 MA 12 plasmas 1.0 MA – Appears to be a transport effect; not due to RF or 10 0.8 MA
fast ions m/s) 0.6 MA
4 8 • Need to understand Ohmic underlying mechanism (10 6
• Also seen on Tore-Supra Tor and JET V 4 – Masked by beam torque in most other 2 experiments 0 • Scaling to ITER? 0 20 40 60 80 100 120 140 ∆W/Ip (kJ/MA) Non-axisymmetric error fields can cause locked Alcator modes: loss of confinement; disruption CMo− d
Locking ThresholdSize Scaling Scaling (LaHaye (LaHaye ’97) 97) Toroidal Field Scaling (Hender, LaHaye) 10–2 (q95≈ 3.5, OHMIC, DEUTERIUM, n πa2 / I ≈ 0.2 x 1020 m–3 x m2 / MA)
COMPASS-C
Combined Error Field Scaling 10–3 m,n = 1,1 2,1 & 3,1
FOR LOCKED MODE Original scaling T m,n= 2,1 / ) B m1 ⊥ 2 10–4 DIII–D *B m1 (W
C−Mod JET √ C-Mod
10–5 ITER?
CRITICAL 0.1 1.0 Major Radius, Ro (m) 10.0
• Original size scaling pessimistic for ITER – C-Mod data implies much more optimistic extrapolation • Toroidal field scalings very different on JET and Compass
– Coordinated C-Mod/JET experiments planned to investigate BT scaling (Identical dimensionless parameters) Alfven Eigenmodes could be Important in Alcator Burning Plasmas CMo− d
• Magnetics and Phase Contrast Imaging measurements reveal Alfven cascades – Fast ions from ICRF minority heating (E ~ 200 keV)
– Can also be used as qmin diagnostic with negative shear (as on JET)
• Active spectroscopy used to drive and study stable modes – Antenna optimized to drive moderate n~4, ITER relevant modes C-Mod accesses relevant non-dimensional and Alcator dimensional parameters for ITER CMo− d
8 • Standard operation at β = 1.8 5.3 Tesla N B = 5.3 T • Matches β and 6 absolute pressure PC-Mod(MW) P(MW) • Gyrosize: 4 ρ ≤ ρ ρ ITER ≤ * 4 ( */ * ) 6.5 νe/iτ B • Different ν/ω E collisionalities for 2 ν neo * * νe/iτ gB different physics E Ratio to ITER;
0 0 2 4 6 8 10 n (1020 m-3) C-Mod should reach ITER pressure at same β Alcator Power reactors require higher pressure CMo− d
ARIES-RS ASSTR2 10.00 ARIES-AT ARIES-ST FIRE(AT) FIRE(H) IGNITOR MHH C-Mod(H) ITER C-Mod(AT) C-Mod(H) C-Mod(H) Alc-C 1.00 DIII-D JET TFTR
=5% Projected =1% = 34% β t β t
• Steady State – High bootstrap fraction + Efficient current drive • C-Mod uniquely positioned to study fully relaxed current profiles τ C-Mod (5s) 1.4a 2κT 3/2 = e CR Z eff
JET (50 s) AUG (10s) ITER (1000 s)
Pulse Length/Relaxation Time DIII-D (10s) JT60-U (10s) (Major Radius)-2 (m-2) Lower Hybrid will be used for far Off-Axis Alcator Current Drive CMo− d
Ip = 0.86 MA Ilh = 0.24 MA fbs = 0.7 • Far off-axis LHCD (4.6 GHz, 4 MW) q β = 3 • Target plasma is fully non-inductive, N
q(r) B = 5 TeslaTot at no-wall limit, 70% bootstrap , fraction BS LH • First experiments: Spring 2004 j (MA/m2) Klystrons installed in experimental cell seed
r/a Unique C-Mod Capabilities Alcator ⇒ Address Key Questions: Next Step Burning Plasmas CMo− d
• Unique dimensional parameters (B/R, Power/Area, Plasma Pressure) – Key points on scaling curves – Tests non-similar processes through Dimensionless Identity experiments (neutrals, radiation, …) – Pedestal structure and regulation • Equilibrated electrons and ions • Very high SOL power density, all-metal Plasma Facing Components – Reactor relevant regime – Unique recycling properties, D/T retention • Reactor-like normalized neutral mean free path (depends only on B) • Prototypical disruption forces – ITER level plasma pressure, energy density (disruption mitigation) • Exclusively RF driven – Heating decoupled from particle, momentum and current sources – Efficient off-axis current drive with Lower Hybrid • Long pulse relative to skin and L/R times