Transcript A solenoid-free current start-up scenario utilizing outer poloidal field coils and center-post

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A solenoid-free current start-up
scenario utilizing outer poloidal field
coils and center-post
Wonho Choe, Jayhyun Kim
Korea Advanced Institute of Science and Technology
J. Menard, Masayuki Ono, and the NSTX team
Princeton Plasma Physics Laboratory
Yuichi Takase
Tokyo University
46th Annual Meeting of Division of Plasma Physics
American Physical Society
November 15 – 19, 2004
Savannah, Georgia
Columbia U
Comp-X
General Atomics
INEL
Johns Hopkins U
LANL
LLNL
Lodestar
MIT
Nova Photonics
NYU
ORNL
PPPL
PSI
SNL
UC Davis
UC Irvine
UCLA
UCSD
U Maryland
U New Mexico
U Rochester
U Washington
U Wisconsin
Culham Sci Ctr
Hiroshima U
HIST
Kyushu Tokai U
Niigata U
Tsukuba U
U Tokyo
JAERI
Ioffe Inst
TRINITI
KBSI
KAIST
ENEA, Frascati
CEA, Cadarache
IPP, Jülich
IPP, Garching
U Quebec
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
Conventional
ohmic solenoid
– 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
Plasma
R
a
of out-board and outer PF coils, and/or using
a conducting center-post.
Out-board
region
In-board
region
Null field generation using out-board induction coils
#1
Plasma
Plasma
#2
R
aa
Midplane
#3
• 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.
Major
axis
Z
BZ  2 BZ
,
R R 2
#1
matched
#2
Plasma
#3
R
Initial plasma
Null field region
Field null
region
BZ ,
BZ  2 BZ
,
0
R R 2
Sufficient mid-plane space for blanket access (NSST)
Coil #1 (+16 MA-t)
Coil #2 (-10 MA-t)
Coil # R (m)
Z (m)
I (kA-turn)
1
2.00
2.63
+16,000
Coil #3_1 (-3.7 MA-t)
2
3.80
1.90
-10,006
Coil #3_2 (+6.7 MA-t)
3_1
3.10
1.20
-3,689
3_2
3.40
1.00
+6,670
Net vertical field profile
Mid-plane vertical field profiles
Available
flux
BZ  2 BZ
,
R R 2 matched
Field null region
BZ ,
BZ  2 BZ
,
0
R R 2
Outer PF Start-Up could provide
multi-MA ‘seed’ current for future STs
Flux contours
Mod-B contours (Gauss)
Case 5 cont’d
NSST
Radial profile of flux
Plasma axis
• 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.
Breakdown contours (0.1 kV/m and 0.02 kV/m) (NSST)
NSST
Time-dependent calculation
592 ms
596 ms
600 ms
~1.2 m
0.02 kV/m
604 ms
608 ms
612 ms
0.1 kV/m
• 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)
Simulation for NSTX
+20 kA/turn
Flux contours
Breakdown contours
- 20 kA/t
+2.8 kA/t
0.02 kV/m
0.1 kV/m
Time-dependent calculation with vacuum vessel eddy
currents considered*
Mod-B contours (Gauss)
24 ms
23 ms
25 ms
• Lloyd’s condition, with strong pre-
ionization, ET·BT/BP ≥ 0.1 kV/m
satisfied in a significant volume.
27 ms
26 ms
28 ms
• Successful start-up with
ET·BT/BP ≥ 0.02 kV/m
ET
23 ms
BT
contours (kV/m)
BP
24 ms
25 ms
(Takase et al., IAEA EX/P4-34)
• Large 0.02 kV/m contour
~ 35 cm
26 ms
27 ms
28 ms
*J. Kim, W. Choe, M. Ono, Plasma Phys. Control.
Fusion 46 (2004) 1647
Loop voltage and poloidal flux
PF3
PF2
10
0
PF5
Null
-10
PF4
-20
0
5
10 15 20
Time (ms)
25
30
10
8
6
4
2
0
-2
20
Flux vs time (at 1.4 m)
Flux (Wb)
20
Loop voltage
Loop Voltage (V)
PF current (kA/turn)
Coil current
Null
22
24
26
Time (ms)
28
30
0.16
0.15
0.14
0.13
0.12
0.11
0.10
0.09
20
Null
22
24
26
Time (ms)
28
• 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.
30
Force balance and stability issues
• Force balance in the initial start-up phase needs to be checked carefully.
• Self-consistent code including evolution of plasma current and parameters
10
2.0
2.0
0
1.6
1.6
-10
1.2
1.2
0.8
0.8
R (m)
Bz (Gauss)
Vertical field for force balance
-20
-30
Calculated Bz
Required Bz for Ip
-40
23
24
25
26 27 28
Time (ms)
0.4
29
30
0.0
23
0.4
Plasma Position
Field Index
24
0
25
26 27 28
Time (ms)
29
0.0
30
R Bz 3

for stability
Bz R 2
Field Index
Stable field structure against
axis-symmetric mode
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
Successful initiation thus far requires a large null region
Null Size Evolution During PF-Only Start-Up
(Null size: ETBT/BP > 0.1kV/m)
3
OH
Null Size (m^2)
2.5
XP433-I
2
XP433-II
1.5
1
XP448-I
XP448-II
XP431
0.5
0
0
2
4
6
8
10
Time (msec)
Successful initiation:
OH:112152, 4.5 kG
XP433-I: 113612, 3.5 kG
XP433-II:114405, 3 kG
Unsuccessful initiation:
XP431: H:11293, 4.5 kG
XP448-I: 113609, 3.5 kG
XP448-II:114484, 3 kG
Coil current waveforms used in XP-443
PF2
Similar to OH startup waveforms
PF4
114405
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
114405
8ms
• LRDFIT code used for
reconstructions
– IVessel  10  IP
• Careful control of BZ after
breakdown helped raise
IP from 10kA to 20kA
9ms
10ms
• More BZ evolution
optimization possible
Thomson measurements consistent with
plasma motion and peaked pe profiles
114405 t=10ms
C. Bush, ORNL
•
•
•
B. LeBlanc
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
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
Camera Gain = 95
3 ms
4 ms
6 ms
7 ms
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)
Bright emission (Hiroshima camera) in 2.5 – 8 ms
113611 (vac. shot)
B-dot loop signal
(near PF4)
Rogowski
B_L1DMPPPGL7
- RF noise
- IPF4 diff.
IPF4
B_L1DMPPPGU8
~50 G  Ip = ~ 15 kA
B_L1DMPPPGU7
B_L1DMPPPGL8
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 (bN)
4.1
7.0
Toroidal beta (bT, %)
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
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
89
89
Radial access test area (m2)
Center post weight (ton)
Capital cost ($B) with 40% contingency
.
R = 1.2 m, a = 0.8 m
1.47
Solid center-post envisioned for ST reactor could
provide additional poloidal flux
PF
Coils
• 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.
Plasma
• 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?
Center-post
• Possible as long as the poloidal flux is available at
the time of the plasma start-up.
TF column charging sequence
t=T1
PF1 current
T2
Vacuum vessel resistive skin time
t
PF2 and 3_1 current
Center-post resistive skin time
t
PF3_2 current
t
Charging
up
TF column
fully charged
Field null formation
and plasma initiation
Plasma current ramp up
Copper TF center-post charging sequence
At t = T2 ,
PF2 current reverses to create field
null before center-post flux decays
8
7
6
5
4
3
2
1
0
0.0
Copper TF Center-post
Bz (T)
8
7
6
5
4
3
2
1
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
R (m)
Copper TF Center-post
Bz (T)
At t = T1 ,
All PF currents in the same
polarity to maximize the charging
0.5
1.0
1.5
R (m)
2.0
2.5
3.0
Induced current waveforms and poloidal flux
PF1,2
20
CPshell at 0.16 m
CPshell at 0.32 m
CPshell at 0.48 m
Outermost shell
100
15
Flux (Wb)
300
200
At (1.0 m, 0)
CPshell at 0.08 m
CPshell at 0.24 m
CPshell at 0.40 m
CPshell at 0.56 m
400
0
14
12
10
8
6
4
2
0
-2
-0.5
0.0
0.5 1.0
Time (s)
1.5
2.0
Innermost VV
Outermost VV
1.5
2.0
Plasma
initiation time
Flux from CP
PF2
5
PF1
0
VV
0.0
0.5 1.0
Time (s)
1.5
2.0
Center-post
2.5
2.0
1.5
1.0
0.5
0.5 1.0
Time (s)
Net
PF1
PF2
VV
CP
Net
10
3.0
0.0
Loop
voltage
-0.5
Flux (Wb)
-100
-0.5
Induced eddy currents (kA)
Induced CP currents (kA)
Mid-plane
At 2 sec
0.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
R (m)
Evolution of Bz profile
Bz by center-post
200
150
PF1,2
Bz (kG)
PF coil current (kA/turn)
Driving current waveform
100
50
0
-20
PF1
PF2
-15
-10
-5
0
Time (s)
Total Bz profile
Bz (kG)
30
20
10
Due to vessel
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
R (m)
-5.0 s
+0.25 s
+1.00 s
Center-post
Bz profile of a typical solenoid
-5.0 s
+0.25 s
+1.00 s
20
15
Bz (kG)
40
-10.0 s
+0.00 s
+0.50 s
+2.00 s
0.0 0.5 1.0 1.5 2.0 2.5 3.0
R (m)
5
-10.0 s
+0.00 s
+0.50 s
+2.00 s
40
30
20
10
0
-10
-20
Solenoid
10
5
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
R (m)
Stray field due to the charged center-post
Centerpost
Central solenoid
Null
region
• 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.