rf start-up, heating and current drive studies on tst-2 and utst y. takase, tst-2 team, utst team...
TRANSCRIPT
RF Start-up, Heating and Current Drive Studies on
TST-2 and UTST
Y. Takase, TST-2 Team, UTST Team
The University of Tokyo
The 15th International Workshop on Spherical Tori 2009
22-24 October 2009Madison, Wisconsin, U.S.A.
TST-2
1UTST
R 0.38 m a 0.25 m B 0.3 TIp 0.2 MA
Outline
• Ip start-up experiments on TST-2
• High-harmonic fast wave (HHFW) experiments on TST-2 and UTST
• Plan for LHCD experiment on TST-2
2
Ip Start-up Experiments on TST-2
• In TST-2, Ip start-up, ST plasma formation and sustainment have been achieved by EC power (up to 5 kW at 2.45 GHz). – When Ip reaches a critical value, Ip increases abruptly (current jump) and
reaches a steady sustainment level Ipsus which is proportional to Bz.
– Before current jump the field configuration is open.
– After current jump an ST configuration with closed flux surfaces is sustained.
• Once initial plasma is formed, RF power (up to 30 kW at 21 MHz) injected using the HHFW loop antenna can induce a current jump and sustain the ST configuration with the same Ip
sus as the EC sustained case.
3
2-Strap HHFW Antenna(only 1 strap was used)
21MHz, up to 400 kW(up to 30 kW was used)
TST-2 Spherical Tokamak and Heating Systems
X-mode launch horn antenna for ECH2.45 GHz, up to 5 kW
R = 0.38 ma = 0.25 mA = 1.5
4
3 Phases of Ip Start-up by ECH
Open Field Lines Current Jump
Closed Flux Surfaces
z[m]00.4
-0.4
-0.40
0.40
0.4-0.4 x[m]y[m]
0 0.5
0
0.6
-0.6
0 0.5
0
0.6
-0.6
00.1-0.1
00.4
-0.4 0 0.4-0.4
z[m]
x[m]y[m]
Ip increases rapidly once Ip reaches a critical level determined by Bv.
5
particle orbit
RF (21MHz) power can induce a current jump.
Antenna excites a broad toroidal mode number spectrum, up to |n| ~ 20. But only |n| = 0, 1, 2 can propagate to the core.
Ion absorption is not expected due to high /ci (> 10). Ion (H/D, C, O) heating was not observed (< 10 eV).
Electron absorption is expected to be weak due to low e. Soft X-rays (up to 3 keV) were observed at high RF power (~ 30 kW).
Sustainment by RF Power Alone
0
2
4
6
8
10
0.5 1 1.5 2 2.5 3Energy [keV]
Log (N)
Exp(-E/42eV)
RF onlyRF
RF sust.EC sust.
Ip can be sustained by RF power alone.
6
Truncated Equilibrium
To treat finite p and j in the open field line region, “truncated equilibrium” is used. [A. Ejiri et. al., Nucl. Fusion 46, 709-713 (2006).]
. )4 ,2 ,1 ,5.0(over discretely variedare ,. channels) 80~( tsmeasuremen magnetic tofittingby
determined are ) , , and( , , Parameters
11 );(
11 );( where
);()1();(
as expressed isequation S-G
0
2
2
00
000
0
EddyPFpp
f
p
fppp
III
F
F
FR
rF
r
Rj
d
dff
Rd
dpRj
0.0
0.2
0.4
0.6
0.8
1.0
Fp
or F
()
(a)
=0.5 12
4
0.0
0.2
0.4
0.6
0.8
1.0
0 0.2 0.4 0.6 0.8 1
Pre
ssur
e
1/2
(b)
=0.5 1 2 4
Outboardlimiter
R
Top limiter
Bottom limiter
LCFS
Inboardlimiter
The following effects are not taken into account:• anisotropic pressure• parallel pressure gradient
Truncation boundaries
7
Equilibrium Reconstruction
Flux loops
Pickup coils
Saddle loops
Vacuum field and locations of magnetic measurements
x10
Red: measurementsBlack: fitDistribution of Ieddy is pre-calculated for given Ip(t).
Ieddy can become ~1/3 of Ip during current jump.
8
Evolution of Equilibrium
0.1 0.3 0.5 0.7-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.0
1.0
0.0
0.1
Pre
ssu
re [
Pa
]j
[kA
/m2]
Z [
m]
R [m]
#53783, t=25ms
jf
total
ff’
p’
(a)
0.1 0.3 0.5 0.7-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0
100.0
0.4
j f [k
A/m
2 ]Z
[m
]
R [m]
#53783, t=40ms
j
total
ff’
p’
LCFS
(b)
0.1 0.3 0.5 0.7-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0
80.0
0.4
j f [k
A/m
2 ]Z
[m
]
R [m]
#53783, t=50ms
j
total
ff’
p’
LCFS
-4
(c)
0.0
0.5
1.0 #53783
Plas
ma
curre
nt[k
A]
(a)Total
Inside LCFS
,
(b)
1
0.5
2
4
0.00
0.05
0.10
0
0.5
1
Stor
ed e
nerg
y[J
]
P max
[Pa]
(c)
0.4
0.6
Maj
or ra
dius
[m]
(d)
Rax
Rjmax
0.0
0.5
1.0
Plas
ma
volu
me
[m3 ] (e)
Total
Inside LCFS
0.0
1.0
2.0
20 30 40 50 60 70 80 90N
orm
aliz
ed
2
(f)
Time [ms]
(a) (b) (c)
appearance of closed flux surfaces
9
Comparison of Equilibria during Sustainment
Truncated boundary
LCFS
j
Inboard limiter LCFS Outboard
limiter
#53783 50ms
EC sustained, Ip = 0.6 kA
Truncated boundary
LCFS
j
Inboard limiter
LCFS Outboardlimiter
#53773 50ms
RF sustained, Ip = 0.6 kA
%80/ ,70~ ,4.1[ms] 01.0~[eV], 20~
0
0
pLCFS
pLCFS
E
IIq
T
Truncated boundary
LCFS
j
Inboard limiter LCFS Outboard
limiter
#53197 90ms
EC sustained, Ip = 1.3 kA
%70/ ,50~ ,0.1
[ms] 05.0~[eV], 180~
0
0
pLCFS
pLCFS
E
IIq
T
%93/ ,50~ ,1.1
[ms] 02.0~[eV], 45~
0
0
pLCFS
pLCFS
E
IIq
T
10
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2v||/v0
v^ /v0
1.0
0.8
0.6
0.4
0.2
0.0
(I)
(II)
(IV)
(III)
A
D
B
C
EF
(a)
0.0 0.2 0.4 0.6 0.8
Orbits-t1
R [m]
D
E
F
(c)
-0.8
-0.4
0.0
0.4
0.8
0.0 0.2 0.4 0.6 0.8
Orbits
Z [
m]
R [m]
AB
C
(b)
Limiter
Velocity Space Structure in Vacuum Field
0v v:condition Stagnation
v: velocitysticCharacteri
||D
0
t
p
p
B
BR
Velocity space for orbits starting from R = 0.38 m for PF2+PF5 configuration classes of particle orbits
A. Ejiri et. al., Nucl. Fusion 47, 403-416 (2007).11
• Banana orbits under the influence of self-generated E are analyzed.
– Angular momentum is conserved from axisymmetry
– Banana particles are frozen to flux surfaces, and move with flux surfaces towards the low field side.
– This movement causes kinetic energy and plasma current to decrease (inverse of Ware pinch).
• Passing particles have short energy decay times. They are accelerated in the direction to reduce Ip. Movement of orbit is small.
Ip stops increasing when closed flux surfaces are formed.
Effect of E on Particle Orbits in Start-up Plasma
12
const.~ || qRAmRvqRAmRvp
– 14 mV/m
Toroidal Field(t=35 – 40 ms)
0.2 0.60.4R [m]
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
–3 mV/m
Condition for Flux Conservation
-2 x 10-5
0
2 x 10-5
4 x 10-5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
#53783
[
Vs/
2]
R [m]
25 ms 30 ms 35 ms40 ms50 ms
45 ms
55 ms
0
0 time [ms] 10
P, qRA
mRV1
2
3 1 2 3
surface.flux with themove Particlesconst.~
If ||
qRAp
qRAmRv
Angular momentum conservation Flux conservation
13
of Orbits in Velocity Space
14
Trapped regionInverse of Ware pinch; = 0
< 0, R, Z ~ 0Counter moving
Acceleration < 0, R, Z ~ 0Co movingDeceleration
E-field dominated region
Velocity normalized by V0=Re pol
Transition regionCo Trapped:
~< 0, R ~> 0
Mixed transition regionCo Counter : < 0, R < 0Co Trapped: ~< 0, R ~> 0,
Discussion of Current Drive Mechanism
iele
e
double
doubleS
VVVVS
VVpZd
mkTSen
ekT
I
VR
IRRV
LEV
III
/2/
/2~
||||
0.1 0.3 0.5 0.7-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0
80.0
0.4
Pre
ssu
re [
Pa
]j f
[kA
/m2 ]
Z [
m]
R [m]
#53783, t=50ms
j
total
ff’
p’
LCFS
-4
4V/1.5 A=3
650-150=400A
Because of the V.V. current, the poloidal current is always in the diamagnetic direction.
In addition to the precessional current of trapped particles, Pfirsch-Schülter current can give net toroidal current in the open field line region.
The vertical drift current (Id) returns partially through the plasma (IpZ) and partially throught the vacuum vessel (IVV).
jd
Id IpZIVV
RS/2
RS/2
RVV
Top limiter
Bottom limiter
Plasma
V.V.
RpZV
j||
Z
jvv
j
15
HHFW Experiments on TST-2 and UTST
• Up to 300 kW of RF power at 21 MHz has been injected into TST-2 plasmas. Two-strap antenna excites HHFW with a wavenumber spectrum peaked at k ~15 m-1 at the antenna.
– When parametric decay instability (PDI) is observed, the Te increment becomes smaller, the edge density shows a rapid increase, and the impurity Ti increment increases.
– Wave measurements by microwave reflectometry, electrostatic and electromagnetic probes are consistent with the HHFW pump wave decaying into the ion Bernstein wave (IBW) or the HHFW lower sideband, and the low frequency ion-cyclotron quasi-mode (ICQM).
– The lower sideband power varies approximately quadratically with the local pump wave power, which becomes smaller as absorption of the pump wave by the plasma increases.
• In UTST, direct wave measurements inside the plasma were made with a 2-D array of magnetic probes. – The measured wave field profile was roughly consistent with the result
of TORIC full-wave calculation.16
φ = -60°
φ = -30°φ = -55°φ = -65°
φ = -115°φ = -120°
φ = 155°φ = 150°
φ = 65°φ = 60°φ = 55°
φ = 30°
φ = 0°
2cm
Direction of B field to be measured
Core (insulator)
1 turn loop
S. S enclosureSlit
Semi-rigid Cable
φ = -125°
RF Diagnostics
Bφ Bz Reflectometer
φ = 145°TOP VIEW
center stackprobes
strap
Parametric Decay Observed by Reflectometer
There is a threshold in pump wave power.
→ Parametric Decay Instability (PDI)
Ion Cyclotron Quasi-Mode
reflectometer reflectometer reflectometer
fci
RF probe
pumpQM
LS
pump
LS
Correlation Between PDI and Electron/Ion Heating
Stronger PDI Less electron heating More ion heating
inboard-shifted
outboard-limited
outboard-limited
inboard-shifted
Spectral Broadening of the Pump Wave
Spectral broadening can occur by• scattering by density fluctuations• parametric decay instability
Spectral broadening becomes larger farther away from the antenna.
Downshifted and broadened pump wave was observed at the inboard wall.
UTST Experiment (Univ. Tokyo and AIST)
• High- ST formation by double-null merging (DNM) • High- ST sustainment by additional heating: NBI and RF
Objectives:PF Pair Coils
PF Pair Coils
0.7m
HHFW antenna
2m
Magnetic probe arraylocated 45 away toroidallyfrom the antenna
RF B2 Profile Comparison
• HHFW field is stronger in the periphery for single-strap excitation.
• RF magnetic field strength is lower for double-strap excitation
Single-strap excitation Double-strap excitation
Plan for LHCD Experiment on TST-2
• Preparation is underway for lower hybrid (LH) current drive experiments on TST-2. – Up to 400 kW of power at 200 MHz will be used to
ramp-up Ip from a very low current (~ 1 kA), very low density (< 1018 m-3) ST plasma.
• Wave propagation and absorption were investigated using the TORIC-LH full wave code. – Core absorption is expected initially, but absorption is
predicted to move radially outward with the increase in Ip and density.
26
Preparation for LHCD Experiment on TST-2
200 MHz transmittersCombline antenna
(11 elements)
Initially, the combline antenna used on JFT-2M, adapted for use on TST-2, will be used to excite a unidirectional fast wave with n = 12 (corresponding to n|| = 5).
Direct excitation of the LH wave is planned in the future. The fast wave can mode convert to the LH wave and drive current.
(200 kW x 4, from JFT-2M)
TORICLH/TST2/101/One0 = 1 x 1017 m-3
Te0 = 1 keVIp = 10 kAn||0 = 7ant = 0
Collaboration with J. Wright, P. Bonoli (MIT)