Transcript Solenoid-free Plasma Startup in NSTX using Transient CHI

Supported by
Office of
Science
Plasma Start-up In NSTX Using
Transient CHI
College W&M
Colorado Sch Mines
Columbia U
Comp-X
General Atomics
INEL
Johns Hopkins U
LANL
LLNL
Lodestar
MIT
Nova Photonics
New York U
Old Dominion U
ORNL
PPPL
PSI
Princeton U
SNL
Think Tank, Inc.
UC Davis
UC Irvine
UCLA
UCSD
U Colorado
U Maryland
U Rochester
U Washington
U Wisconsin
R. Raman, T.R. Jarboe1, D. Mueller2, B.A. Nelson1, M.G. Bell2, M. Ono2,
T. Bigelow3, R. Kaita2, B. Leblanc2, R. Maqueda4,
J. Menard2, S. Paul2, L. Roquemore2
and the NSTX Research Team
1University
of Washington, Seattle, USA
2Princeton Plasma Physics Laboratory, USA
3Oak Ridge National Laboratory, Oak Ridge, TN, USA
4Nova Photonics, USA
12th International ST Workshop
11-13 October 2006
Chengdu, China
*Work supported by US DOE contracts DE-FG03-9ER54519
and DE-AC02-76CH03073.
Culham Sci Ctr
U St. Andrews
York U
Chubu U
Fukui U
Hiroshima U
Hyogo U
Kyoto U
Kyushu U
Kyushu Tokai U
NIFS
Niigata U
U Tokyo
JAERI
Hebrew U
Ioffe Inst
RRC Kurchatov Inst
TRINITI
KBSI
KAIST
ENEA, Frascati
CEA, Cadarache
IPP, Jülich
IPP, Garching
1 Rep
ASCR, Czech
Outline
• Motivation for solenoid-free plasma startup
• Implementation of Coaxial Helicity Injection
(CHI) in NSTX
• Requirements for Transient CHI
• Experimental results from NSTX
– Brief summary of HIT-II results
• Summary and Conclusions
2
Solenoid-free plasma startup is essential for the
viability of the Spherical Tokamak (ST) concept
•
Elimination of the central solenoid simplifies the
engineering design of tokamaks (Re: ARIES AT & RS)
•
CHI is capable of both plasma start-up and edge
current in a pre-established diverted discharge
- Edge current profile for high beta discharges
3
Implementation of CHI in NSTX
Transient CHI: Expect axisymmetric reconnection at the injector to
result in formation of closed flux surfaces
4
Requirements for optimizing Transient CHI
• Bubble burst current*  inj2 / toroidal
• Volt-seconds to replace the toroidal flux
– For
 toroidal
600 mWb,
at ~500V need ~1 ms just for current ramp-up
• Energy for peak toroidal current
1
2
CV 2  12 LI 2
• Energy for ionization of injected gas and
heating to 20eV (~50eV/D)
– For 2 Torr.L injected, need ~2kJ
* T.R. Jarboe,"Formation and steady-state sustainment of a tokamak by coaxial
helicity injection," Fusion Technology 15, 7 (1989).
5
Capacitor bank used in Transient CHI Experiments
•
•
•
50 mF (10 caps), 2 kV
Operated reliably at up to
1.75kV
Produced reliable breakdown
at ~ 1/10th the previous gas
pressure (20 Torr.Liter used
in 2004)
– Constant voltage
application allowed more
precise synchronization
with gas injection
– EC-Pi and gas injection
below divertor used for
Pre-ionization assist
6
Improved pre-ionization to a level that results in
injected gas 10 times less than in 2004
EC-Pi glow
along the
center stack
Divertor
gap
Shot 116565
•
Novel pre-ionization system
– Injects gas and 10-20kW of
18GHz ECH in a cavity below
The small glow shown by
the lower divertor gap
the arrow is in the gap
between the lower
– Successfully tested, achieved
divertor plates and it is
discharge generation at
produced solely by ECinjected gas amount of < 2
Preionization of the gas
Torr.Liter
injected below the lower
divertor plates. No
• Fast Crowbar system
voltage is applied.
– Rapidly reduces the injector
current after the CHI
discharge has elongated into
Shot 116570
the vessel.
ECH: T. Bigelow (ORNL)
7
Closed flux current
generation by Transient CHI
6 ms
8 ms
10 ms
12 ms
15 ms
17 ms
• Plasma current amplified
many times over the
injected current.
• The sequence of camera
images shows a fish eye
image of the interior of the
NSTX vacuum vessel.
The central column is the
center stack, which
contains the conventional
induction solenoid. The
lower bright region seen
at 6ms is the injector
region.
Hiroshima University (N. Nishino) Camera
8
Images: R. Kaita (PPPL)
Discharges without an absorber arc show high
current multiplication ratios (Ip / Iinj) of 60
9
Dramatic improvement in closed flux current
generation from 2005
2006 discharges operated at higher capacitor bank voltage and
higher toroidal field
LRDFIT (J. Menard)
10
Electron temperature and density profiles
become less hollow with time
Profile becomes less hollow with time
Plasma and Injector current
120814: Black: 8ms, Red: 12ms
120842: Black: 8ms, Red: 10ms
Thomson (B. LeBlanc)
11
Data indicates that ~200kW of ECH would increase Te to ~100eV
• Thomson scattering data indicates Te drops to 50% in 35ms  TauE ~ 4ms
• Zero-D estimates indicate 200kW ECH would increase
Te ~ 60eV in 8ms and 100eV in 20ms, assuming TauE
does not increase.
• Consistent with Radiated power levels of <100kW
• Consistent with low electron densities of ~2x1018m-3, for
impurity burn through. Li a possibility for controlling
Oxygen.
12
Some discharges persist for as long as the
equilibrium coil currents are maintained
Fast camera: R. Maqueda
13
Movie of a high current discharge
Fast Camera:
R. Maqueda &
L. Roquemore
14
15
Favorable scaling with machine size
Attainable current multiplication is given as ,
For similar values of BT,
I P  Iinj ( T / inj )
 T NSTX ~ 10  T HIT II
So current multiplication in NSTX should be 10x HIT-II, which is observed
Next step STs would have about 10x the toroidal flux in NSTX,
Which means current multiplication ratios in excess of 100 is not unrealistic in larger STs
Potential for high current multiplication in larger STs
16
Allowable injector currents determined by maximum voltage

Assuming constant ,
 inj
I inj  Vinj  (
For similar values of
I inj
T )
 inj, at the same voltage,
in HIT-II is about 10 times higher than in NSTX
Consistent with
I inj
~15-20kA on HIT-II vs ~2kA in NSTX
Also consistent with the bubble burst relation,
2
I inj  2 inj
/(o2 d 2 I TF )
Which requires 10x more current in HIT-II than in NSTX
10x more injector flux of that in present NSTX 60kA experiments with 10x more
injector flux leads to >2MA startup currents with 20kA injector current in future
larger machines.
17
Full 2kV capability in NSTX would increase Ip ~ 300kA
Best results from NSTX 2005 and 2006
HIT-II data: R. Raman, T.R. Jarboe et al.,
Nuclear Fusion, 45, L15-L19 (2005)
Voltage, flux optimization allowed HIT-II to
increase closed flux current as capacitor
charging voltage was increased
18
Record non-inductive plasma startup currents in a tokamak
(160kA in NSTX) verifies high current feasibility of CHI for
plasma startup applications
The significance of these results are:
1)
4)
demonstration of the process in a vessel volume thirty times
larger than HIT-II on a size scale more comparable to a reactor,
a remarkable multiplication factor of 60 between the injected
current and the achieved toroidal current, compared to six in
previous experiments,
results were obtained on a machine designed with mainly
conventional components and systems,
indicate favorable scaling with machine size.
•
NSTX high current discharges not yet optimized
2)
3)
–
•
Extension to ~300kA should be possible at 2kV
Future experiments to explore coupling to OH
–
–
200kW ECH to heat the CHI plasma
Coupling to RF and NBI
19