Transcript Pulse-burst laser system development for fast Thomson

Advances in time-resolved
measurement of |B| and Te
in low-magnetic-field plasmas
D. J. Den Hartog
University of Wisconsin – Madison
Open Magnetic Systems for Plasma Confinement
Novosibirsk, Russia
8 July 2010
We are developing techniques for internal time-resolved
measurement of |B| and Te in a low-field plasma.
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Brief introduction to the MST Reversed-Field Pinch
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Spectral motional Stark effect (MSE) for magnetic field
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High-repetition-rate Thomson scattering for electron temperature
Co-authors:
J. R. Ambuel, M. T. Borchardt, K. Caspary, E. A. Den Hartog, A. F. Falkowski, W. S. Harris,
J. Ko, N. A. Pablant, J. A. Reusch, P. E. Robl, H. D. Stephens, H. P. Summers, Y. M. Yang
University of Wisconsin–Madison, University of California–San Diego, University of
Strathclyde
This work is supported by the U. S. Department of Energy and the National Science
Foundation.
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The Reversed-Field Pinch: toroidal confinement at low |B|.
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Toroidal field BT is ~10X smaller in RFP than same-current tokamak
Equilibrium substantially determined by self-generated plasma currents
– Internal |B| and Te measurements critical to equilibrium reconstruction
MST is a large RFP operated at moderate current.
R = 1.5 m, a = 0.5 m, I ≤ 0.6 MA
ne ~ 1019 m-3, Te, Ti ≤ 2 keV
Spectral motional Stark effect
Two compact and reliable diagnostic neutral beams are
installed on MST.
•
Two beams from the Budker
Institute
– 50 keV H beam for MSE
(and CHERS)
• 20 ms duration
– 20 keV He beam for
Rutherford scattering
• 3 ms duration
•
Beams have relatively low
divergence and small energy
spread
Spectral MSE diagnostic measures magnetic field using the
hydrogen neutral beam.
• Beam of H atoms is excited and radiates as it traverses plasma
H 0  e -  H 0  H 0  h
plasma

h
n=3
• H0 electron energy levels are split
by motional E = vbeam × B
H
• Measure emitted radiation pattern,
know vbeam, calculate B
n=2
p
s
p-
The MSE diagnostic has an on-axis view and a new midradius view.
Spectral MSE technique measures B ≥ 0.2 T.
•
For low |B|, vbeam  B is small (~1 MV/m)
– Stark components overlap
– Data is fit with sophisticated model of Stark spectrum
New mid-radius view gives both |B| and pitch angle γ
Spectra from mid-radius views with orthogonal polarization
amplitude (counts)
•
400
fit
sigma
300
pi
200
100
0
wavelength
wavelength
|B| = 0.42 ± 0.07 T
g = 40.3° ±
9.4°
The spectral MSE diagnostic enables precise time-resolved
measurement of internal magnetic field.
MSE on-axis |B|, event ensemble
0.42
|B| (T)
0.40
0.38
0.36
100 µsec shuttering
0.34
-2.0
-1.0
0
1.0
2.0
time relative to reconnection event (ms)
High-repetition-rate Thomson scattering
Standard commercial flashlamp-pumped Nd:YAG lasers
have been upgraded to “pulse-burst” capability.
•
A single laser produces a burst of up to fifteen 2 J Q-switched pulses
– repetition rates variable 1–12.5 kHz
Burst of 15 pulses at 1 kHz repetition rate
The Thomson scattering diagnostic on the MST RFP uses
two upgraded lasers.
•
Collect thirty Te profiles during a single MST discharge
– at rates from 1–25 kHz (with two interleaved lasers)
– each Te profile consists of twenty spatial points
– detailed measurements in an overdense plasma
The laser head was a standard commercial flashlamppumped Nd:YAG.
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“As manufactured” operation:
– single flashlamp pump pulse
– single Q-switch
• reliably produced one 2 J, 9 ns laser pulse
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First upgrade was to take direct control of Pockels cell Q-switching
– optimize optical energy extraction from laser rod
Major upgrade was to take control of flashlamp pulsewidth
and repetition rate.
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Original flashlamp drive was a simple inductor/capacitor pulse-forming network
– produced a single 100 µs pump pulse
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New system based on solid-state switching of large electrolytic capacitor banks
– produces multiple pulses of adjustable width at variable repetition rates
Most typical mode of operation is to drive flashlamps with a
burst of fifteen 150 µs pulses at a repetition rate of 1 kHz.
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applied voltage in black
flashlamp current in red
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The Pockels cell is switched near the end of each flashlamp pulse
– produces a train of fifteen ~2 J pulses from the laser
Flexible control of flashlamp drive and Q-switch enables
new Thomson scattering capabilities.
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For example, to measure fast Te dynamics:
– drive the flashlamps with a burst of five wider pulses at 1 kHz rep rate
– switch the Q-switch three times during each flashlamp pulse
– produce three laser pulses (80 µs separation) from each flashlamp pulse
Two lasers interleaved to produce
six pulses at 25 kHz every ms
In MST, structure in the Te profile is strongly correlated with
rapidly rotating magnetic island structure.
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m = 1 helical island chains rotate toroidally at ~10 kHz
Correlate many Te profile measurements with phase of island
Able to reconstruct modulation of Te profile correlated with island position
Flattening
Helical Structure
Next step pulse-burst laser system for fast Thomson
scattering is being commissioned.
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Laser will produce a burst of ~1 J pulses at rep rates up to 250 kHz
– low duty cycle (~2 min) for these “bursts”
Nd:YAG stages are complete, Nd:glass amplifier is being
commissioned.
Summary
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The spectral MSE diagnostic records the entire hydrogen Stark spectrum
– precise time-resolved measurement of internal magnetic field
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Two Nd:YAG lasers have been upgraded to “pulse-burst” capability
– These lasers are used in the Thomson scattering system to
• record the dynamic evolution of the Te profile
• measure Te fluctuations associated with large-scale tearing modes