17th international stellarator / heliotron workshop princeton, usa overdense plasma operation in...
TRANSCRIPT
17th International Stellarator / Heliotron Workshop
Princeton, USA
Overdense Plasma Operation in WEGA Stellarator
Matthias Otte, H.P. Laqua, S. Marsen, Y. Podoba,
T. Stange, D. Zhang, F. Wagner and the WEGA-team
Max-Planck-Institut für Plasmaphysik, Greifswald branch, EURATOM Ass.
Wendelsteinstr. 1
D-17491 Greifswald, Germany
Matthias Otte Princeton, 13 October 2009 2
Outline
• Motivation on over-dense plasma generation
• Overview WEGA
• Setup of 28GHz ECRH system
• Results from OXB mode heating
• Summary and outlook
Overview of actual WEGA results will be
presented on poster P-D02
during workshop’s poster session from Monday
till Wednesday.
Motivation
Why OXB Mode Heating ?
• Electromagnetic waves used for resonant electron or ion cyclotron heating can not penetrate the plasma above associated cut-off density ➽ problem for high-density operation
• Electrostatic Bernstein waves (EBW)
- no density limit but need a medium for propagating
- excitation via mode conversion process (X-B or O-X-B)
- damped on electrons at fundamental or harmonic Doppler-shifted
electron cyclotron resonance
• Plasma is optically thick for EBWs even at low temperature (<10 eV)
• Interesting physics:
- Wave field physics
- Generation and confinement of fast electrons
- Current drive in overdense plasmas
Matthias Otte Princeton, 13 October 2009 3
The OXB-mode Conversion Process
Matthias Otte Princeton, 13 October 2009 4
Requirements
• O-wave launched with θopt in respect to the
magnetic field vector
- correct polarisation
- angular window width ~ 1/k0Ln (with L = n / grad
n)
• Density ne above cut-off density ne,cut
• Existence of UHR (ω > ωc)
Example:
Ray tracing
calculation of slow
X-mode conversion
process
at W7-AS
θo
pt
0B
en
y
Matthias Otte Princeton, 13 October 2009 5
WEGA = Wendelstein Experiment in Greifswald zur Ausbildung (for education)
• Hybrid tokamak/stellarator experiment in Grenoble/France in the 1970s and ´80s
for development of LH heating
• Installation at IPP Greifswald in 2000/2001
as classical stellarator
• October 2009: about 33000 pulses
with typical duration of 10 - 60 s
• 0.5 T operation for 30 s possible
• Up to ~100 pulses / day
Overview WEGA
Matthias Otte Princeton, 13 October 2009 6
Btoroidal
Bpoloidal
Bvertical
Setup and Magnetic Properties
Vessel• Two half-tori with R = 0.72 m, r = 0.19
m • 100 ports ( ≤ 92 mm)
Magnetic field coils
• 40 toroidal field coils: Bmax (cw) = 0.34
T
and 0.9T for pulsed operation• Helical field coils: l = 2, m = 5• Rotational transform = 0.1 - 1
amax = 11 cm, Vmax = 0.15 m3 (limiter
configuration)
a <= 5 cm for high iota (separatrix
configuration)• Vertical field and error field
compensation coils
Plasma heating• 20 + 6 kW magnetrons @ 2.45 GHz
(cw) • 10 kW gyrotron @ 28 GHz (cw)
• 5-arm transformer with 0.44 Vs
Working gases• Helium, Argon, Hydrogen
Relevant 28GHz Diagnostics
• Interferometer (80 GHz, single channel)
• Sniffer probe (28 GHz)• 12-channel bolometer array• Radiometer with 12 channels (23 – 40
GHz) for ECE, EBE and Reflectometry
• Spectrum analyzer up to 40 GHz• Soft X-ray (PHA)• Langmuir probes
Matthias Otte Princeton, 13 October 2009 7
Bolometer array
Sniffer probe
ECE antenna system
Low field side
High field side
Microwave Diagnostic
Matthias Otte Princeton, 13 October 2009 8
Temperature measurements
• ECE: problem ➽ over-dense and optically thin
• Electron Bernstein emission EBE: accessible via
BXO conversion, optically thick even at low
temperature, no density limit
➽ oblique alignment of antenna necessary
Oblique alignmentof EBE antenna (55°)
EBE horn
HFS-ECE horn
Setup of 28GHz ECRH system
Matthias Otte Princeton, 13 October 2009 10
ECRH Components
Installation of a 28GHz ECRH for B0=0.5T operation in cooperation with CIEMAT/Spain and IPF Stuttgart
4.3 m
Gyrotron 10kW cw(20 kHz modulation) Transmission line
Mirror system inside vessel for X2 and OXB mode
28 GHz Gyrotron (CPI VGA-8028)0.110 kW CW
15 kW 10s20 kW 1s
80 dBDirectional
Coupler
TE02-TE01
ConverterDC Break & Mode
FilterTE01-TE11
Converter
HE11
WaveguideHE11
UptaperTE11-HE11
Converter
VacuumWindow
WEGAVacuumVesselArc
DetectorArc
Detector
AntennaForwardPower
Detector
ReflectedPower
Detector
SmoothWaveguide
TE02 TE01
TE11
TE11
Uptaper
TE11 HE11
HE11 HE11
Ø41 mm
Ø63.5 mmØ44.45 mm
Ø63.5 mm
Ø32.6 mmØ32.6 mm Ø32.6 mm
Matthias Otte Princeton, 13 October 2009 11
ECRH Components: In-Vessel Mirrors• Switching between X2 and OXB mode heating possible by exchanging A-port
mirror
• Quasi-optical Gaussian beam in equatorial plane at vertical elongated symmetry plane
(steepest gradient expected) under angle of 55° to magnetic field line
• Steerable OXB-mirror for finding optimum OXB conversion (up to now venting of vessel necessary)
flux surfaces
Gaussian beam
port
toroidal coil
elliptical focus
OXB-Mirror
Discharges at B0=0.5 T up to 30s at <= 0.4
OXBA-port X2
Matthias Otte Princeton, 13 October 2009 12
OXB – Ray Propagation
Ray-tracing calculations by J.
Preinhaelter and J. Urban (IPP-Prague)
predict high efficiency central heating
at densities above 1 × 1019 m-3 and no
Doppler-shift
Challenge
How to reach 1×1019 m-3 (28 GHz O-
cutoff) ?
• 0.5×1019 m-3 is already reached with
X2-mode
(X-cutoff)
Additional energy sources available
• 20 kW 2.45 GHz resistive R-wave
heating at 0.5 T
• Ohmic heating with transformer
mirror
Results from OXB mode heating
Matthias Otte Princeton, 13 October 2009 14
Reaching OXB mode3.
3. Start OXB conversion above 1 × 1019
m-3 (O-mode cut-off): • central deposition due to
high single pass absorption• EBE signal in keV – range
(radiation but not electron temperature) ➽ supra-thermal electron component
• density peaking up to 1.4 ×
1019 m-3 (33 GHz EBE signal)
4.
4. Switch off additional heating:Plasma sustained by OXB-heating only, strongly peaked radiation profiles
2.
2. Overcome power gap above 0.5 × 1018
m-3 (X2-mode cut-off) by additional resistive 2.45 GHz heating (20 kW)
1.1.Plasma start-up: Plasma generation by 10% X2-mode at resonant field, multi-pass heating with broad deposition
He, #30340
Further Results during OXB Phase
Matthias Otte Princeton, 13 October 2009 15
Langmuir probe
• Peaked profiles with ne,0 = 1.3 ×
1019 m-3
well above cut-off density
• Bulk parameters:
ne = 1 × 1019 m-3 at R = 75 cm
Te = 5 -15 eV
0 2 4 6 8 10 12 14 1605
1015202530354045505560
Pra
d (
mW
/cm
3)
reff
(cm)
#30344/ He /iota 0.3/ECRH 7 kW
28 GHz OXB-heating
28 GHz O2/X2 multipass heating
12-channel Bolometer camera
Reconstructed radiation power
signal:
• Peaked profile during OXB-
phase due to central power
deposition
• Broad, flat profile during O2-
mode heating due to multi-pass
absorption
Matthias Otte Princeton, 13 October 2009 16
Soft X-ray • Pulse height analyzer with range of 0.5keV – 15keV
• Signals above 1keV detected exclusively during OXB phase
• Only Bremsstrahlung, but no characteristic radiation detected (e.g. wall material) except from argon from previous experiments➽ supra-thermal electron component arises in center
as expected by resonant absorption mechanism
0 2 4 6 8 10 12 14
1
10
100
1000
Co
un
ts
Energy [ keV ]
Helium#30340-#30357
smoothed curve
K(Ar) from background gas
Optimization
Gas
• Analogue results in argon even without additional heating but no conversion reached in hydrogen (factor of 3 missing in density)
Rotational transform• iota (iota↑, confinement↑, but plasma size↓) • best iota = 0.3 (still limiter configuration, higher iota not
accessible)
OXB mirror position• October 2009: remotely steering of OXB-mirror possible,
detailed study of emission pattern performed
Matthias Otte Princeton, 13 October 2009 17
Thermal emission pattern for varying toroidal position on target installed at vessel high field side
Summary and Outlook
WEGA has reached a new unexplored physics regime!
• Quasi-stationary overdense plasma heated by 28 GHz OXB mode
conversion exclusively
• Supra-thermal electron component detected in EBE radiation,
confirmed by X-rays detector, however quantitative fraction to be
determined
• Acceleration mechanism of the supra-thermal electron component
➽ maybe auto-resonance cyclotron acceleration of electrons which is not
limited
Supra-thermal electrons
• Fast particle confinement
• Generation of current drive
• Ray tracing with non-Maxwellian
➽ do they exist and how could they be measured?
• Source for highly charged ions
Matthias Otte Princeton, 13 October 2009 18
Acknowledgement
The authors gratefully acknowledge the work of
D. Aßmus, R. Gerhardt, the colleagues of the W7-
diagnostic group and the technical staff of the IPP.
Thank you for your attention!
Matthias Otte Princeton, 13 October 2009 19
EBE Signal
Trad before OXB-heating
Chan 527.3 GHz
Chan 527.3 GHz
Chan 628.5 GHzChan 6
28.5 GHz
Trad during OXB-heating
notchfilter