A Pulse-Burst Laser System for a High-Repetition-Rate Thomson Scattering Diagnostic D. J. Den Hartog, Department of Physics, University of Wisconsin–Madison A “pulse-burst” laser system is being constructed for addition to the Thomson scattering diagnostic on the Madison Symmetric Torus (MST) reversed-field pinch. This laser is designed to produce a burst of up to 200 approximately 1 J Q-switched pulses at repetition frequencies 5–250 kHz. This laser system will operate at 1064 nm and is a master oscillator, power amplifier (MOPA). The master oscillator is a compact diode-pumped Nd:YVO 4 laser, intermediate amplifier stages are flashlamp-pumped Nd:YAG, and final stages will be flashlamp-pumped Nd:glass (silicate). Variable pulse-width drive (0.3–20 ms) of the flashlamps is accomplished by insulated-gate bipolar transistor (IGBT) switching of large electrolytic capacitor banks. The burst train of laser pulses will enable the study of electron temperature (Te) and electron density (ne) dynamics in a single MST shot, and with ensembling, will enable correlation of Te and ne fluctuations with other fluctuating quantities. Abstract Conducting Shell Surrounding Plasma B T Small & Reversed at Edge B T B P B MST is a medium-size toroidal magnetic confinement device - Ohmically driven Reversed Field Pinch - 1.5 m major radius - 0.52 m minor radius - B < 0.6 T - n e ~ 10 19 m -3 - T e ~ 100 eV to 2.5 keV J. Ambuel, D. Holly, H. Mattison, and P. Robl of the Physical Sciences Laboratory at the University of Wisconsin-Madison have been responsible for the engineering and fabrication of the flashlamp power supplies. D. Dummer, W. Foster, and S. Yaeger of the Physics Department Instrument Shop at the University of Wisconsin-Madison played key roles in fabrication of the amplifier pumping chambers. M. Borchardt, A. Falkowski, R. O’Connell, J. Reusch, H. Stephens and Y. Yang continue to extend the capability of the Thomson scattering system on MST. This work is supported by the U. S. Department of Energy and the National Science Foundation. Acknowledgements Fast Thomson scattering diagnostic with pulse-burst laser will enable new measurement capability • Record fast equilibrium changes, turbulence, electron fluctuations • Goal for system on MST is to measure Te and ne – at a variable rate <50 kHz, for up to 20 ms – at a variable rate between 50 – 250 kHz, for a short burst of 10-30 pulses • Laser is only novel component – Produce a train of ~1 J pulses, low duty cycle (~2 min) for these “bursts” – Use existing Thomson scattering hardware (polychromators, etc) • Adjunct to existing Nd:YAG lasers, use same beamline As currently configured, Thomson scattering diagnostic records a 21-point radial profile four times each shot of MST • Two Nd:YAG lasers each produce two ~2 J pulses (≤ 100 µs separation) during a single flashlamp pulse • Thomson scattered spectrum is recorded with filter polychromators from 21 radial points • Polychromators and data system are able to accept much more data at a higher rate than current four laser pulses Variable pulse width (0.3–20 ms) power supplies will drive flashlamps • Short pulses pump amplifier rods to maximum inversion for production of a burst of 10–30 pulses at pulse repetition rates of 50–250 kHz – rod re-pumps very little between pulses, so must store maximum amount of energy prior to start of burst train • As pulse repetition rates fall below about 50 kHz, Nd:YAG is increasingly able to re-pump – able to produce a continuous train of laser pulses during a 20 ms flashlamp pulse Schematic of variable pulse width flashlamp power supply, designed to deliver a square drive pulse, 0.3–20 ms width, to flashlamp. The insulated-gate bipolar transistor (IGBT) switches the current from the capacitor bank “on” and “off,” delivering up to 20% of the explosion energy of the flashlamp. The flashlamp supplies are controlled and monitored by a system based on single-board computers and expansion cards Single-board computer control system. N. Jiang and W. R. Lempert, Department of Mechanical Engineering, The Ohio State University Pulse-burst laser systems have been developed for high-speed flow diagnosis • Walter Lempert and students have built several pioneering systems at Princeton and The Ohio State University – Current generation laser systems can produce a burst of • 1-99 pulses, • up to several hundred mJ/pulse, • <100 ns pulse duration, and • pulse rep rate ≤ 1 MHz. Successfully operated Nd:YAG amplifier chain with compact diode-pumped Nd:YVO 4 oscillator • Substituted vanadate laser into existing Nd:YAG amplifier system at OSU – Pulses from vanadate oscillator are efficiently amplified by YAG – Vanadate oscillator much better that YAG at rep rates >50 kHz – Suppressing amplified spontaneous emission (ASE) will be a major challenge • ASE reduces the amount of energy available to amplify pulses • Techniques to suppress ASE: – large amplifier spacing – spatial filtering – phase-conjugate mirror (PCM) a) Amplification of a burst of 20 pulses at 250 kHz pulse rep rate, average pulse energy ~65 mJ/pulse. b) Individual pulse width ~70 ns. Also amplified a burst of 10 pulses at 100 kHz to ~250 mJ/pulse (not shown). 4 mm Nd:YAG 1/4 inch Nd:YAG 9 mm Nd:YAG 12 mm Nd:YAG 16 mm Nd:glass 16 mm Nd:glass isolator Nd:YVO 4 spatial filter phase-conjugate mirror beam expander 30 cm Construction is proceeding on a pulse-burst laser system for fast Thomson scattering on MST • Laser is a master oscillator, power amplifier architecture – Diode-pumped Nd:YVO 4 oscillator – Four Nd:YAG amplifier stages – Two Nd:glass (silicate) final amplifiers An overview of pulse-burst laser system being constructed for Thomson scattering on MST. The 4 mm and ¼ inch Nd:YAG rods share a pumping chamber with a single flashlamp. The ends of the Nd:glass rods are cut at a 4° angle to prevent self-lasing. The phase-conjugate mirror will be installed only if necessary to reduce ASE. Amplifier 4, the final Nd:YAG stage, contains a 12.2 mm diameter by 3 inch long rod pumped by four 4 mm bore linear flashlamps. Amplifier 5, one of two Nd:glass stages, contains a 15.9 mm diameter by 10.5 inch long rod pumped by six 8 mm bore linear flashlamps. capacitor- charging power supply + trigger transformer electrolytic capacitor bank IGBT Programmable IGBT controller Pulse width adjustable 300 μs – 20 ms flashlamp flashlamp trigger and simmer module • All 19 flashlamps in the full laser system will be individually controllable for pulse width, delay, and energy from a single display screen – Rabbit Semiconductor single-board computer for embedded control – Programmed with Dynamic-C, LabView user interface
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A Pulse-Burst Laser System for a High-Repetition-Rate Thomson Scattering DiagnosticD. J. Den Hartog, Department of Physics, University of Wisconsin–Madison
A “pulse-burst” laser system is being constructed for addition to the Thomson scattering diagnostic on the Madison Symmetric Torus (MST) reversed-field pinch. This laser is designed to produce a burst of up to 200 approximately 1 J Q-switched pulses at repetition frequencies 5–250 kHz. This laser system will operate at 1064 nm and is a master oscillator, power amplifier (MOPA). The master oscillator is a compact diode-pumped Nd:YVO4 laser, intermediate amplifier stages are flashlamp-pumped Nd:YAG, and final stages will be flashlamp-pumped Nd:glass (silicate). Variable pulse-width drive (0.3–20 ms) of the flashlamps is accomplished by insulated-gate bipolar transistor (IGBT) switching of large electrolytic capacitor banks. The burst train of laser pulses will enable the study of electron temperature (Te) and electron density (ne) dynamics in a single MST shot, and with ensembling, will enable correlation of Te and ne fluctuations with other fluctuating quantities.
Abstract
Conducting ShellSurrounding Plasma
BT Small & Reversed at Edge
BTBP
B
MST is a medium-size toroidalmagnetic confinement device
- Ohmically driven Reversed Field Pinch- 1.5 m major radius- 0.52 m minor radius
- B < 0.6 T- ne ~ 1019m-3
- Te ~ 100 eV to 2.5 keV
J. Ambuel, D. Holly, H. Mattison, and P. Robl of the Physical Sciences Laboratory at the University of Wisconsin-Madison have been responsible for the engineering and fabrication of the flashlamp power supplies. D. Dummer, W. Foster, and S. Yaeger of the Physics Department Instrument Shop at the University of Wisconsin-Madison played key roles in fabrication of the amplifier pumping chambers. M. Borchardt, A. Falkowski, R. O’Connell, J. Reusch, H. Stephens and Y. Yang continue to extend the capability of the Thomson scattering system on MST.
This work is supported by the U. S. Department of Energy and the National Science Foundation.
Acknowledgements
Fast Thomson scattering diagnostic with pulse-burst laser will enable new measurement capability
• Record fast equilibrium changes, turbulence, electron fluctuations • Goal for system on MST is to measure Te and ne
– at a variable rate <50 kHz, for up to 20 ms– at a variable rate between 50 – 250 kHz, for a short burst of 10-30 pulses
• Laser is only novel component– Produce a train of ~1 J pulses, low duty cycle (~2 min) for these “bursts”– Use existing Thomson scattering hardware (polychromators, etc)
• Adjunct to existing Nd:YAG lasers, use same beamline
As currently configured, Thomson scattering diagnostic records a 21-point radial profile four times each shot of MST
• Two Nd:YAG lasers each produce two ~2 J pulses (≤ 100 µs separation) during a single flashlamp pulse
• Thomson scattered spectrum is recorded with filter polychromators from 21 radial points
• Polychromators and data system are able to accept much more data at a higher rate than current four laser pulses
Variable pulse width (0.3–20 ms) power supplies will drive flashlamps
• Short pulses pump amplifier rods to maximum inversion for production of a burst of 10–30 pulses at pulse repetition rates of 50–250 kHz
– rod re-pumps very little between pulses, so must store maximum amount of energy prior to start of burst train
• As pulse repetition rates fall below about 50 kHz, Nd:YAG is increasingly able to re-pump
– able to produce a continuous train of laser pulses during a 20 ms flashlamp pulse
Schematic of variable pulse width flashlamp power supply, designed to deliver a square drive pulse, 0.3–20 ms width, to flashlamp. The insulated-gate bipolar transistor (IGBT) switches the current from the capacitor bank “on” and “off,” delivering up to 20% of the explosion energy of the flashlamp.
The flashlamp supplies are controlled and monitored by a system based on single-board computers and expansion cards
Single-board computer control system.
N. Jiang and W. R. Lempert, Department of Mechanical Engineering, The Ohio State University
Pulse-burst laser systems have been developed for high-speed flow diagnosis
• Walter Lempert and students have built several pioneering systems at Princeton and The Ohio State University
– Current generation laser systems can produce a burst of• 1-99 pulses,• up to several hundred mJ/pulse,• <100 ns pulse duration, and• pulse rep rate ≤ 1 MHz.
Successfully operated Nd:YAG amplifier chain with compact diode-pumped Nd:YVO4 oscillator
• Substituted vanadate laser into existing Nd:YAG amplifier system at OSU– Pulses from vanadate oscillator are efficiently amplified by YAG– Vanadate oscillator much better that YAG at rep rates >50 kHz– Suppressing amplified spontaneous emission (ASE) will be a major
challenge• ASE reduces the amount of energy available to amplify pulses• Techniques to suppress ASE:
– large amplifier spacing– spatial filtering– phase-conjugate mirror (PCM)
a) Amplification of a burst of 20 pulses at 250 kHz pulse rep rate, average pulse energy ~65 mJ/pulse. b) Individual pulse width ~70 ns. Also amplified a burst of 10 pulses at 100 kHz to ~250 mJ/pulse (not shown).
4 mm Nd:YAG
1/4 inch Nd:YAG
9 mm Nd:YAG
12 mm Nd:YAG
16 mm Nd:glass
16 mm Nd:glass
isolator
Nd:YVO4
spatial filter
phase-conjugate mirror
beam expander
30 cm
Construction is proceeding on a pulse-burst laser system for fast Thomson scattering on MST
• Laser is a master oscillator, power amplifier architecture– Diode-pumped Nd:YVO4 oscillator– Four Nd:YAG amplifier stages– Two Nd:glass (silicate) final amplifiers
An overview of pulse-burst laser system being constructed for Thomson scattering on MST. The 4 mm and ¼ inch Nd:YAG rods share a pumping chamber with a single flashlamp. The ends of the Nd:glass rods are cut at a 4° angle to prevent self-lasing. The phase-conjugate mirror will be installed only if necessary to reduce ASE.
Amplifier 4, the final Nd:YAG stage, contains a 12.2 mm diameter by 3 inch long rod pumped by four 4 mm bore linear flashlamps.
Amplifier 5, one of two Nd:glass stages, contains a 15.9 mm diameter by 10.5 inch long rod pumped by six 8 mm bore linear flashlamps.
capacitor-chargingpowersupply
+
triggertransformer
electrolyticcapacitor
bank
IGBT
Programmable IGBTcontroller
Pulse width adjustable300 µs – 20 ms
flashlamp
flashlamptrigger
andsimmermodule
• All 19 flashlamps in the full laser system will be individually controllable for pulse width, delay, and energy from a single display screen
– Rabbit Semiconductor single-board computer for embedded control– Programmed with Dynamic-C, LabView user interface
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