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Null field generation using out-board induction coils Plasma #1 R Z #2 #3 Null field region Initial plasma Small radius PF coil #1 produces a ‘peaked’ B V profile. Large radius PF coil #2 produces a flat B V profile. Near-midplane ‘trim’ PF coil #3 produces a follow B V profile. matched Field null region a Plasma Midplane R a Plasma #1 #2 #3 Major axis
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46th Annual Meeting of Division of Plasma PhysicsAmerican Physical Society
November 15 – 19, 2004Savannah, Georgia
A solenoid-free current start-up scenario utilizing outer poloidal field
coils and center-post
Supported by
Columbia UComp-X
General AtomicsINEL
Johns Hopkins ULANLLLNL
LodestarMIT
Nova PhotonicsNYU
ORNLPPPL
PSISNL
UC DavisUC Irvine
UCLAUCSD
U MarylandU New Mexico
U RochesterU Washington
U WisconsinCulham Sci Ctr
Hiroshima UHIST
Kyushu Tokai UNiigata U
Tsukuba UU Tokyo
JAERIIoffe Inst
TRINITIKBSI
KAISTENEA, Frascati
CEA, CadaracheIPP, Jülich
IPP, GarchingU Quebec
Wonho Choe, Jayhyun Kim Korea Advanced Institute of Science and Technology
J. Menard, Masayuki Ono, and the NSTX team
Princeton Plasma Physics Laboratory
Yuichi TakaseTokyo University
Outer PF coil-only inductive plasma start-up
• Ohmic solenoid has been the work horse of fusion research for decades.
• Attractive fusion CTF and power plant design requires OH elimination– Compact CTF requires elimination of OH regardless of R/a.
– ARIES-AT and ARIES-ST design assumes no OH.
• PF coils have been used to start-up the plasma– MAST / START: Merging/compression using in-vessel PF coils
– JT-60U / TST-2: ~150 kA / 10 kA obtained using strong
(1 MW / 100 kW) ECH pre-ionization with ~0.02 kV/m
• Plasma start-up using appropriate combination
of out-board and outer PF coils, and/or using
a conducting center-post.
Plasma
Conventional ohmic solenoid
Ra
Out-boardregion
In-boardregion
Null field generation using out-board induction coils
Plasma
#1
R
Z
#2
#3
Null field regionInitial plasma
• Small radius PF coil #1 produces a ‘peaked’ BV profile.
• Large radius PF coil #2 produces a flat BV profile.
• Near-midplane ‘trim’ PF coil #3 produces a follow BV profile.
22,ZZBBRR∂∂∂∂
matched
22,,0ZZZBBBRR∂∂=∂∂Field null
region
a
Plasma
Midplane R a
Plasma
#1
#2
#3
Major axis
Available flux
22,ZZBBRR∂∂∂∂
matched
Mid-plane vertical field profiles
Coil #1 (+16 MA-t)Coil #2 (-10 MA-t)
Coil #3_1 (-3.7 MA-t)
Coil #3_2 (+6.7 MA-t)
22,,0ZZZBBBRR∂∂=∂∂Field null region
Net vertical field profile
Coil # R (m) Z (m) I (kA-turn)
1 2.00 2.63 +16,000
2 3.80 1.90 -10,006
3_1 3.10 1.20 -3,689
3_2 3.40 1.00 +6,670
Sufficient mid-plane space for blanket access (NSST)
Flux contours Mod-B contours (Gauss)
• Significant amount of volt-sec available for current ramp-up:~4.5 V-s at R0 = 1.75 m
• Generation of good quality multi-pole field null• Excellent out-board access (~1.8 m vertical spacing) Suitable for the interchangeable blanket modules for CTF.
Radial profile of flux
Plasma axis
Case 5 cont’dOuter PF Start-Up could provide multi-MA ‘seed’ current for future STs NSST
592 ms 596 ms 600 ms
604 ms 608 ms 612 ms
• Successful start-up with ET·BT/BP ≥ 0.02 kV/m
(Takase et al., EX/P4-34)
• Large 0.02 kV/m contour (~1.2 m diameter)
Breakdown contours (0.1 kV/m and 0.02 kV/m) (NSST)
0.02 kV/m
0.1 kV/m
~1.2 m
Time-dependent calculationNSST
Simulation for NSTX
Flux contours+20 kA/turn
- 20 kA/t
+2.8 kA/t
0.02 kV/m
0.1 kV/m
Breakdown contours
• Lloyd’s condition, with strong pre-
ionization, ET·BT/BP ≥ 0.1 kV/m
satisfied in a significant volume.
*J. Kim, W. Choe, M. Ono, Plasma Phys. Control. Fusion 46 (2004) 1647
26 ms 27 ms 28 ms
Time-dependent calculation with vacuum vessel eddy currents considered*
Mod-B contours (Gauss)23 ms 24 ms 25 ms
contours (kV/m)
TTPBEB
~ 35 cm
23 ms 24 ms 25 ms
27 ms26 ms 28 ms
• Successful start-up with
ET·BT/BP ≥ 0.02 kV/m
(Takase et al., IAEA EX/P4-34)
• Large 0.02 kV/m contour
Loop voltage and poloidal flux
Coil current Flux vs time (at 1.4 m)
• Significant V-s is available for current ramp-up.
• Full ramp-up scenario will require bi-polar PF5. But, initial breakdown
experiment to ~100 kA should be possible with the existing power supplies.
Loop voltage
20 22 24 26 28 30-202468
10
Loop Voltage (V)
Time (ms)
Null
20 22 24 26 28 300.090.100.110.120.130.140.150.16
Flux (Wb)
Time (ms)
Null
0 5 10 15 20 25 30-20
-10
0
10
20
PF current (kA/turn)
Time (ms)
Null
PF2
PF3
PF4
PF5
23 24 25 26 27 28 29 30
-40
-30
-20
-10
0
10
Bz (Gauss)
Time (ms)
Calculated Bz Required Bz for Ip
23 24 25 26 27 28 29 300.0
0.4
0.8
1.2
1.6
2.0
0.0
0.4
0.8
1.2
1.6
2.0
Field Index R (m)
Time (ms)
Plasma Position Field Index
• Force balance in the initial start-up phase needs to be checked carefully.
• Self-consistent code including evolution of plasma current and parameters
Force balance and stability issues
Vertical field for force balance Stable field structure against
axis-symmetric mode
23 0 <
∂∂
−<RB
BR z
zfor stability
Preliminary PF-only Start-up Experiments in NSTX
• Pre-ionize plasma near RF antenna with ECH + 400 kW HHFW, nD2
= (1 - 2)10-5 Torr
• Create high-quality field-null with (5 - 15) loop-volts at antenna
So far, require EfBf /BP > 0.1 kV/m over substantial plasma volume
• Have created 20 kA plasmas that terminate near center-stack
Shot 114405
J. Menard
Null Size Evolution During PF-Only Start-Up
0
0.5
1
1.5
2
2.5
3
0 2 4 6 8 10Time (msec)
Null Size (m^2)
(Null size: ETBT/BP > 0.1kV/m)OH
XP433-I
XP448-IXP431
XP433-II
XP448-II
Successful initiation:OH:112152, 4.5 kGXP433-I: 113612, 3.5 kGXP433-II:114405, 3 kG
Unsuccessful initiation:XP431: H:11293, 4.5 kGXP448-I: 113609, 3.5 kGXP448-II:114484, 3 kG
Successful initiation thus far requires a large null region
Coil current waveforms used in XP-443
Similar to OH startup waveforms
114405
PF2
PF4
Field and loop voltage
114405
At plasma breakdown
Near maximum plasma current
Camera images and reconstructions show plasmas are born on LFS and have an inward radial trajectory
• LRDFIT code used for reconstructions– IVessel 10 IP
• Careful control of BZ after breakdown helped raise IP from 10kA to 20kA
• More BZ evolution optimization possible
8ms
9ms
10ms
114405
Thomson measurements consistent with plasma motion and peaked pe profiles
Thomson Te < 35eV and camera images consistent with lack of burn-through
need more plasma heating power:• More HHFW power during breakdown• Higher VLOOP – keep plasma outboard• EBW power could be very helpful
114405 t=10ms
B. LeBlanc
C. Bush, ORNL
Solenoid-free current-start-up scenario including PF4
• Try to store more poloidal flux at null region for IP ramp
• Start PF2 & 3 coils with large positive bias– Balanced by negative PF4– Store 50-100mWb at null– Null size 1/3 of XP448
• Null formation very sensitive to coil current time-history and vessel current model
113609
LRDIAG simulations predict vertical merging of X-points
Shot 113608 - C. Bush, ORNL
3 ms 4 ms
6 ms 7 ms
Camera Gain = 95
s113609 (N. Nishino - Hiroshima fast camera)
2.442 ms 2.664 ms 2.886 ms 3.108 ms 3.330 ms 3.552 ms
3.774 ms 3.996 ms 4.218 ms 4.440 ms 4.662 ms 4.884 ms
5.106 ms 5.328 ms 5.550 ms 5.772 ms 5.994 ms 6.216 ms
6.438 ms 6.660 ms 6.882 ms 7.104 ms 7.326 ms 7.548 ms
Magnetics bound IP to < 15kA (probably only few kA)
B_L1DMPPPGU7
B_L1DMPPPGL7
B_L1DMPPPGU8
B_L1DMPPPGL8
IPF4
Rogowski
Bright emission (Hiroshima camera) in 2.5 – 8 ms
~50 G
- RF noise- IPF4 diff.
Ip = ~ 15 kA
113611 (vac. shot) B-dot loop signal (near PF4)
Results
• HHFW pre-ionization necessary– Need sufficient neutral density, 0-0 phasing
– Increase from 0.5 MW to > 1 MW w/ more straps
– EBW could be very helpful (was on TST-2)
• Large null required for IP initiation thus far
– Need more work on finding optimal balance of stored flux vs. null size vs. initial plasma shape
• Good plasma position evolution following breakdown crucial
to high IP
– DINA modeling should be helpful here
Inductive plasma start-up scheme using a conducting center-post
Single turn TF leads to an attractive ST CTF
Wall Loading at Test Modules (MW/m2) 1.0 3.0
HH (ITER98pby2) 1.4 1.8
Applied toroidal field (T) 2.4 2.2
Plasma current (MA) 12.6 11.4
Normalized beta (N) 4.1 7.0
Toroidal beta (T, %) 26.8 45.1
n/nGW (%) 17 52
Q (using NBI H&CD) 2.4 5.8
Fusion power (MW) 72 214
Number of radial access ports 7 7
Radial access test area (m2) 12.8 12.8
PHeat/R (MW/m) 37 67
Tritium burn rate (kg/full-power-year) 4 12
Total facility electrical power (MW) 286 272
Fraction of neutron capture (%) 81.6 81.6
Local T.B.R. for self-sufficiency 1.23 1.23
Toroidal field coil current (MA) 14.6 13.2
Center post weight (ton) 89 89
Capital cost ($B) with 40% contingency 1.47. R = 1.2 m, a = 0.8 m
Plasma
PF Coils
Cen
ter-
post
• Possible as long as the poloidal flux is available at the time of the plasma start-up.
• Assuming the center-post radius is 50 cm for NSST (or CTF-like device), Bz ~ 0.7 T would mean the center-post may provide ~0.55 Wb.
• However, if we use the same coils energized in the same current direction, Bz ~ 8 T can be obtained in the center-post.
~ 6 Wb is stored in the center-post.
• Then, can one reverse PF #2 and #3_1 before the center-post eddy current decays away?
Solid center-post envisioned for ST reactor could provide additional poloidal flux
TF column charging sequence
Field null formation and plasma initiation
Charging up
TF column fully charged
Plasma current ramp up
t
PF3_2 current
t
PF2 and 3_1 current Center-post resistive skin time
t=T1 T2
t
PF1 currentVacuum vessel resistive skin time
Copper TF center-post charging sequence
At t = T1 , All PF currents in the same polarity to maximize the charging
At t = T2 , PF2 current reverses to create field null before center-post flux decays
Cop
per T
F C
ente
r-po
st
0.0 0.5 1.0 1.5 2.0 2.5 3.0012345678
Bz (T)
R (m)C
oppe
r TF
Cen
ter-
post
0.0 0.5 1.0 1.5 2.0 2.5 3.0012345678
Bz (T)
R (m)
-0.5 0.0 0.5 1.0 1.5 2.0-100
0
100
200
300
400
Induced CP currents (kA)
Time (s)
CPshell at 0.08 m CPshell at 0.16 m CPshell at 0.24 m CPshell at 0.32 m CPshell at 0.40 m CPshell at 0.48 m CPshell at 0.56 m
-0.5 0.0 0.5 1.0 1.5 2.0-202468
101214
Induced eddy currents (kA)
Time (s)
Innermost VV Outermost VV
PF1,2Outermost shell
At 2 sec
-0.5 0.0 0.5 1.0 1.5 2.0
0
5
10
15
20
Flux (Wb)
Time (s)
Net PF1 PF2 VV CP
Flux from CP
Loopvoltage
Net
PF2PF1
VV
Plasma initiation time
At (1.0 m, 0)
0.0 0.5 1.0 1.5 2.0 2.5 3.00.00.51.01.52.02.53.0
Flux (Wb)
R (m)
Center-post
Induced current waveforms and poloidal flux
Mid-plane
-20 -15 -10 -5 0 50
50
100
150
200
PF coil current (kA/turn)
Time (s)
PF1 PF2
Evolution of Bz profile
Driving current waveform
0.0 0.5 1.0 1.5 2.0 2.5 3.0
0
10
20
30
40
Bz (kG)
R (m)
-10.0 s -5.0 s +0.00 s +0.25 s +0.50 s +1.00 s +2.00 s
Total Bz profile
0.0 0.5 1.0 1.5 2.0 2.5 3.0
-20-10
010203040
Bz (kG)
R (m)
-10.0 s -5.0 s +0.00 s +0.25 s +0.50 s +1.00 s +2.00 s
Bz by center-post
0.0 0.5 1.0 1.5 2.0 2.5 3.00
5
10
15
20
Bz (kG)
R (m)
Bz profile of a typical solenoid
PF1,2
Due to vessel
Solenoid
Center-post
Centerpost Central solenoid
Null region
Stray field due to the charged center-post
• Like OH, the stray field is relatively small and uniform in the breakdown region.
• The stray field can be easily nulled out by compensation coils (on-going).
• The present method is quite compatible with the PF-only start-up concept utilizing the same hardware.
Some thoughts
• The center-post flux storage concept can essentially double the available flux by using the same hardware of the outer-PF-only start-up concept.
• ~ 10 Wb could potentially solve the CTF start-up and ramp-up issue. NSTX gets 1 MA with 0.35 Wb. So, it should be possible to reach 15 MA with twice the major radius (R = 1.7 m) with 10 Wb ( R2).
• The concept works better with high R/a. If we go with R/a = 2 CTF, then one can envision R = 75 cm core, which can essentially double the flux compared to R = 50 cm core.
• The induced loop voltage is determined by the stored flux and the flux decay rate. It can be adjusted by controlling the center-post resistivity (material, temperature) or resistance (thickness).
• If the inner torus radius is 4 m for example for R = 6 m advanced tokamak reactor, one can conceive a metal ring structure (a combination of vacuum vessel, shield, and support structure) of mean-radius of 3.5 m. In that case, one can store up to 230 Wb of flux assuming 7 T charging field ( x 3.52 x 7)! Even a fluctuation of that would be sufficient for start-up.
• Since pure metal is much more radiation resistance and simple compared to highly stressed OH solenoid, it could also be a more attractive reactor option for tokamak as well.
Summary
• Two complementary inductively-based concepts to aid the solenoid-free start-up for future ST and tokamak reactors are presented.
• A combination of out-board PF coils placed outside the vacuum vessel is shown to create a good quality field null region while retaining significant volt-second capability for current ramp-up.
- For NSST, 4 - 5 Wb possible for ramping the current to a few MAs.- For NSTX, ~0.12 Wb possible for Ip ~ a few hundred kA.
• Inboard-side conducting material to store the magnetic flux which is initially charged up by the outboard-side outer PF coils. For ST, it is conceivable to utilize the central TF conducting post as the flux storage.
- The NSST (CTF) size device can provide additional 2 - 4 Wb with this method.
• Like the OH solenoid, the stray field generated by the center-post flux is relatively small which would make it suitable for the plasma start-up utilization.