Initial Operation of a Pulse-Burst Laser System for High

Initial Operation of a Pulse-Burst Laser System for High-Repetition-Rate Thomson Scattering
W.S. Harris, D.J. Den Hartog, and N.C. Hurst, Department of Physics, University of Wisconsin-Madison
Nd:YVO4 Master Oscillator Characterization
4
6.4
9
12
16
16
1
1a
2
4
6
6
4 × 67
4 × 67
4 × 67
4 × 67
8 × 250
8 × 250
Set-up
Double-pass
Double-pass
Double-pass
Single-pass
Single-pass
Single-pass
aThese stages share a single flashlamp within a single pumping chamber.
bThis stage has been installed, but not yet operational.
cThis stage has not been installed.
80
100
0.8
2.5
2.0
1.5
• 7 ns pulse width
7 ns pulse width
1.0
• Pulse energy is 1.5 J after subtracting
ASE contribution to the energy meter
0.5
0.0
20
40
60
time(ns)
80
b)
0.5
c)
0.5
Sync Pulse
0.
1.0
d)
0.5
0.
0
50
100
150
Time (µs)
200
250
Requirements for Master Oscillator Energy Stability
• Laser rod must be at a constant temperature for energy stability to be achieved
– Does not occur for an isolated burst of pulses
• The (unamplified) master oscillator is run continuously prior to a burst, and turned
off briefly (100-200 µs) while the flashlamps are pumped
• The pumpdiode is turned off while the Q-switch is off to prevent heating of the
oscillator crystal
• 18 amplified Q-switch pulses
0.4
• Ignoring the first and last pulse
– Average pulse energy is 0.53 J
0.2
– Relative pulse deviation ∆E/E is 4.6%
0.0
0.0
b)
45 ns pulse width
• Pulse width at 250 kHz is 45 ns
(FWHM)
0.2
– Pulse width decreases with Qswitch frequency
0.0
200
400
600
Time (ns)
800
1000
• Burn paper patterns after the fourth
Nd:YAG amplifier stage
2.0
2.5
• Flashlamps operating at 15% explosion
energy
• 250 µs flashlamp pump pulse
• Uniform beam profile
2.0
• Beam diameter is 12 mm
12mm
• Photodiode output through a flashlamp pulse
• Amplified laser pulses observed at
10 kHz (100 µs spacing)
• Baseline signal due to ASE increases throughout flashlamp pumping time (500 µs) and decreases after
lamps turn off
20
15
10
5
0
-5
0
1.0
1.5
time (ms)
Pulse Energies at 100 kHz Operation
0.6
0.4
0.5
• Pulse to pulse energy stability ∆E/E
is 4.6%
Flashlamp Current (A)
Triggering Sequence:
1. Nd:YVO4 Q-switch (orange) continuously pulses prior master trigger, with pump diode (green) energized
2. Master trigger (blue) sent to CompactRIO to begin amplified pulsing
sequence
3. Q-switch and pump diode turn off
briefly to allow flashlamps to pump
amplifier stages
4. Flashlamps (red) in amplifier stages
are pulsed
5. Master oscillator pump diode turns
on
6. Q-switch turns on to produce amplified laser pulses
• 2 ms flashlamp pump pulse
100
200
400
600
Time (µs)
800
400
200
0
t0−ε
• 150 µs wide flashlamp pump pulse
600
500
400
300
200
100
0
-100
0
• tL is a time window chosen to be much longer than the flashlamp pulse
• t0 is the time of the Q-switched laser pulse
• 2 × ε is the width of the laser pulse
The integral over the interval [t0 −ε, t0 +ǫ] gives the contribution of the laser pulse, I pulse,
to the total energy measured by the energy meter, Etotal, while the rest of the integral,
IAS E , is the contribution due to ASE.
The energy of the amplified pulse is then taken to be
I pulse
Etotal.
(2)
E pulse =
I pulse + IAS E
1.0
– Average pulse energy is 0.32 J
0.5
– Relative pulse deviation ∆E/E is 6.6%
0.0
0.00
0.05
0.10
time (ms)
0.15
0.20
• Flashlamps operating at 30% explosion
energy
∗ Optimization of flashlamp voltages and trigger timing has not been performed
• Operation at 21% of the flashlamp explosion energy
Future Work
100
200
Time (µs)
300
400
100
Acknowledgements
10-2
10-4
10
-6
10-8
Oscillator
Laser pulse energy after spatial filtering.
• Further testing of the fifth amplification stage (Nd:glass).
• Optimization of the flashlamp charging voltages and Q-switch timing for stable, repetitive pulsing.
• Characterization of the laser pulse energy with five amplifier stages.
• Installation of the sixth amplification stage (if deemed necessary).
• Extend the laser beam path to the MST vessel.
102
Pulse Energy (J) Gain
a
−9
Oscillator 35×10
1
108×10−6 3000
2
0.12
1000
3
0.69
6
4
1.48
2
a
• Ignoring the first two pulses
b)
Stage
t−ε
1.5
0
Gain Characterization of Nd:YAG Amplifier Stages
Numerical integration of the energy monitor’s analog output, f (t), is broken up into
three intervals in order to determine the pulse energy:
Z tL
Z t−ε
Z t0+ε
Z tL
f (t)dt =
f (t)dt +
f (t)dt +
f (t)dt
(1)
0
• Applied flashlamp voltage (blue) and
current (red)
a)
1000
Amplified Laser Pulse Energy is Determined by Integrating
Energy Meter Output
• 10 amplified Q-switch pulses
600
Pulse Energy (J)
Q-Switch
Pump Diode
Nd:YAG
Nd:YAG
Nd:YAG
Nd:YAG
Nd:glass
Nd:glass
Flashlamp arc
Diameter
Nlamps diameter × length
(mm)
(mm × mm)
a
40
60
Time (µs)
Amplified Spontaneous Emission (ASE) Levels are
Acceptable
Photodiode Level (A.U.)
Trigger
a)
0.5
0.
1.0
A new pulse-burst laser being built is described as follows:
• Nd:YVO4 master oscillator, 5-250 kHz
• Continuous mode
– 20 ms long train of pulses at ≤50 kHz
• Burst mode
– Bursts of 10-30 pulses at frequencies up to 250 kHz
• Output pulse energy 1-2 J
• Master-oscillator power-amplifier whose amplification stages have
the following properties:
20
• Pulse to pulse deviation of energy,
∆E, is calculated from the variance
between pulses, σ
Flashlamp Voltage (V)
Laser Triggering Control Technique for Optimum
Energy Stability
0.
1.0
1
2
3
4
5b
6c
0.2
• Single pulse amplified through all four
Nd:YAG amplifier stages
800
1.0
Pulse-Burst Laser System
• 250 kHz operation of master oscillator
0.4
-0.2
0
Thomson scattering on MST has been performed using two commercial (Nd:YAG) Spectron lasers. Recent upgrades permit operation of
the diagnostic with the following characteristics:
• 2 J pulse energies at 1064 nm
• 1-25 kHz operation
• Multi-point collection system allowing 21 simultaneous radial
measurements
Material
0.6
1.0
Spectron Laser System
Stage
0.8
3.0
Pulse Energy (J)
Thomson Scattering on MST
• Commercial laser delivers 2-8 µJ
pulses at 1064 nm
1.0
0
Stable Pulse Energy at 10 kHz Operation
0.6
a)
0.0
Photodiode Signal (V)
• National Instruments cRIO-9073
• 40 MHz clock for fast timing control
• Eight control modules:
– 2× digital I/O modules - triggering of
master oscillator, flashlamps, digitizers, and beam alignment cameras
– Analog output module - control of
pump diode level of master oscillator
– 5× analog input modules - Flashlamp
voltage and current monitors, 12-bit
and 62.5 kHz per channel
• Synchronization between flashlamps
and Q-switch is made possible by using the cRIO system
Photodiode Signal (V)
1.2
Single Laser Pulses of 1.5 J with Uniform Burn
Patterns are Achieved
Pulse Energy (J)
Laser Control Accomplished Using CompactRIO
Integrated System
A pulse-burst laser has been installed for Thomson scattering measurements on the Madison Symmetric Torus (MST) reversed-field
pinch. The laser design is a master-oscillator power-amplifier. The
master oscillator is a commercial Nd:YVO4 laser (1064 nm) which
is capable of Q-switching at frequencies between 5-250 kHz. Four
Nd:YAG amplifier stages are in place to amplify the Nd:YVO4 emission. Single pulses through the Nd:YAG amplifier stages gives energies up to 1.5 J and the gain for each stage has been measured.
Repetitive pulsing at 10 kHz has also been performed for 2 ms bursts
giving average pulse energies of 0.53 J with ∆E/E of 4.6%, where
∆E is the standard deviation between pulses. The next step will be
to add one of two Nd:glass (silicate) amplifier stages to produce final
pulse energies of 1-2 J for bursts up to 250 kHz.
Energy Meter Output (A.U.)
ABSTRACT
1
2
3
Amplifier Stage
4
The authors would like to thank Mike Borchardt, Adam Falkowski, Josh Reusch and
Ming Yang from the Thomson Scattering group at MST for their help. Mikhail Reyfman
and David Deicher from the electronics shop have also provided valuable assistance. In
addition, the efforts of Jack Ambuel and Phil Robl of the Physical Sciences Laboratory
at the University of Wisconsin-Madison are also recognized.
This work was supported by the U.S. Department of Energy and the National Science
Foundation.