Post on 04-Feb-2022
RFP Performance Enhancement ThroughMHD Mode Control
RFP Performance Enhancement ThroughMHD Mode Control
John Sarff & MST Team
MHD Mode Control Workshop — Columbia UniversityNovember 18-20, 2002
MSTStandard RFP confinement is limited by MHD tearing.
• Steady toroidal induction in low BT geometry drives current profile peaking:– m = 1,0 resistive MHD tearing unstable– Instability saturates by J(r) self-flattening through dynamo relaxation
• Performance characteristically high beta b < 20%, but relatively poorconfinement c > 50 m2/s.
r / a
Toro
idal
, f multiple tearing modesfl
global stochasticity
MSTTwo paths to improved RFP confinement.
• Optimized magnetic relaxation:a) Induce and control “single-helicity” dynamo (P. Martin’s talk)b) Possible favorable Lundquist number scaling for “multiple-helicity” relaxation
(all tearing modes decrease together at higher temperature)
• Current profile control for tearing stability: (this talk)– Minimize magnetic relaxation– All tearing modes vanish (in principle)
Goal for both paths is reduced magnetic stochasticity.
MSTOutline.
• MHD foundation for an RFP without dynamo
• Improved confinement in MST:– J(r) modification via inductive current drive– Tokamak-like confinement at high beta, low BT in MST
• Broadband mode reduction key to reduced transport in the RFP
• Control tools in development:– RF current drive (& heating)– Neutral beam heating
MST
MHD computation predicts current drive in outer regionreduces tearing and magnetic stochasticity.
ReducedMHD
tearing
Add current drive inouter region
(mostly poloidal)leads to …
MSTSteady toroidal induction Ef cannot support outer-region J||.
• Standard RFP requires dynamo current drive .
MST
Auxiliary current drive replaces dynamo sustainment of therequired poloidal current.
• Current drive “aligned” to current profile.
MSTMadison Symmetric Torus (MST)
a = 0.5 mR = 1.5 mI ≤ 0.5 MA
MST
Programmed inductive loop voltages provide current drivetargeted to edge region.
“PPCD” – Pulsed Poloidal (or Parallel) Current Drive
Time (ms)0 5 10 15 2520
0
10
20
05
10
Eq
EfE(a)(V/m)
˜ B pol ,rms
(G)
PPCD
ReducedMagnetic
Fluctuations
Time varyingLoop voltages
MSTEdge current drive simultaneously controls many modes.
• Broadband mode suppression during PPCD.
Pickup Loops(edge)
FIR Polarimetry(core)
not same shot
MSTJ/B larger in outer region, as intended.
• Toroidal equilibrium reconstruction using:– 11-chords FIR polarimetry (UCLA collab.)– B(0) from MSE with DNB (BINP collab.)– Conventional edge magnetics
MSTTemperature profile peaks, ce greatly reduced.
• Electrons are hotter with reduced Ohmic heating, gradient extends into core.• 100-fold increase in hard x-ray bremsstrahlung implies confined fast electrons.
0.2
0.4
0.6
0.8
1.0
Te
1
10
100
1000
ce(m2/s)
Standard
PPCD-Improved
0 0.2 0.4 0.6 0.8 1r/a
(KeV)
0 0.2 0.4 0.6 0.8 1r/a
Standard
PPCD-Improved
MST maximum Te!(0) = 1.3 keVfor I ="500 kA PPCD
I ="400 kA, n!=1019 m–3
MST
PPCD confinement comparable to same-current tokamak,but with 10X smaller B(a) in the RFP.
PPCD-Improved
ELMy H-Mode IPB98(y,2)
Standard RFP
• Compare tE"= 10 ms for 200 kA PPCD with tokamak tE empirical scaling:– use “engineering” formulas with MST’s I, n, P, size & shape, but tokamak BT!(a) = 1.0 T (corresponding to qa= 4).
Same-current RFP: • BT!(a) = 0.04 T fi 20X smaller • B!(a) = 0.09 T fi 10X smaller
Comparable tE does not implytokamak empirical scalingapplies to the RFP.
MSTPPCD doubles the already high beta value in RFP.
Standard
9%
5%
PPCD-Improved
15%
11%
Total Betabtotal ~ ·pÒ / B2!(a)
• Experimental b -limit unknown in the RFP (possibly pressure-tearing or g-mode??)
200 kA
400 kA
¥ 1.7
¥ 2.2
Toroidal beta decreases during PPCD to bT ~ 80% (from >>100%)
MST
Bulk electron heat transport in standard plasmas agreeswith stochastic diffusion expectations.• Field line tracing used to directly evaluate .
†
Dm = ·(Dr)2 /DLÒ
StandardStandard RFP RFP
3D resistive MHD computationS = 106, R/a = 3
expt. measured h(r) profile
MST
Heat conductivity reduction greatest where stochasticity isusually most intense.
StandardStandard RFP RFP
30-fold reduction in core
MST
Sustained reduction of mid-radius resonant modes leads tohighest Te (0) , therefore largest tE .
0 1 2 3 4 5 (G)0.20.40.60.81.01.2
0 5 10 15 (G)0.20.40.60.81.01.2
Te(0)(keV)
Te(0)(keV)
Variable fluctuation reduction reveals
mode dependence.
†
bq rms = bqn2 (a)
n=8
15ÂTime Average
Standard
PPCD-Improved
mid-radius modesm =1, n ≥ 8
MSTTe (0) weakly correlated with dominant core mode.
0 1 2 3 4 5 (G)0.20.40.60.81.01.2
0 5 10 15 (G)0.20.40.60.81.01.2
Te(0)(keV)
Te(0)(keV)
†
bq rms = bqn2 (a)
n=8
15ÂTime Average
Standard
PPCD-Improved
Time Average
m"="1, n ="6
†
bq 1,6
Broadband mode suppression keyto improved RFP confinement.
MSTThe ion temperature doesn’t change during PPCD.
• Standard: Pe-i ≤ PCX ; Ti / Te > 0.5 (>1 at sawtooth crash) fi anomalous ion heating• PPCD: Pe-i ≥ PCX ; Ti / Te < 0.5
0.2
0.4
0.6
0.8
1.0
Ti, Te(KeV)
0 0.2 0.4 0.6 0.8 1r/a
†
ciR-R =
Ti
Te
me
mi
ceR-R
Standard~ 10 m2/s core~ 1 m2/s q = 0
PPCD~ 0.3 m2/s
†
Ï
Ì Ô Ô
Ó Ô Ô
Stochastic Transport
†
ciclassical ~ ri
2 /ti ~ 0.1 m2 /sClassical
CHERS
MSTTools to extend plasma control in development.
• RF current drive for precise, localized J(r)-control and auxiliary heating:– Lower-hybrid (800 MHz, n|| = 8, stripline antenna)– Electron Bernstein wave (~ 3.5 GHz, waveguide antenna)– Required power ~ 1-2 MW both waves (LH theoretically more efficient)
• Oscillating Field Current Drive for DC current sustainment (and possibly profilecontrol) from purely AC inductive loop voltages (AC helicity injection).
• Neutral beam heating, e.g., investigate beta limit (PW < 1 MW during PPCD).
MSTSecond generation lower-hybrid antenna installed.
• Interdigital line antenna, 800 MHz, n|| = 8
0
20
40
15 20 25 30 3510
15
20
25
30
Time (ms)
InputPower(kW)
Phot
on E
nerg
y ( k
eV)
Hard x-rays
MSTTwin-waveguide installed to drive EBW in overdense RFP
• 3.2-3.7 GHz (variable) from two 60 kW traveling wave tube sources.
MSTGood fast ion confinement encouraging for NB heating.
BINP collab.
1 MW short pulse next stepFast neutral-decay following DNB
Beam in gas target
DNB
inje
ctio
n
Neut
ral f
lux
MSTSummary.
• Two paths for enhanced RFP energy confinement, with one common goal —reduced transport from magnetic stochasticity
• Tokamak-like energy confinement demonstrated at high beta and 10X smaller B(a)in MST with (transient) edge inductive current drive:– ce ~ 5 m2/s and btotal ≤ 15%– Fast electrons confined, with velocity-independent diffusivity
• Broadband mode reduction key to improved confinement, but one remaining largemode is tolerable (analogous to tokamak sawtoothing)– Bodes well for “single-helicity” dynamo ... maybe even for multiple-helicity
dynamo if high-n scaling is strong
• Extending plasma control– RF, neutral beam, and OFCD tools in development (low power)– PPCD: find the optimum E(a,t), evaluate pulsed-reactor scenario