CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
LASER TRIGGERED SYNCHRONIZABLE X-RAY SYSTEM
FOR REAL TIME STUDY OF SHOCK WAVES IN CONDENSED
MATERIALS
J. Paul Farrell, K. Batchelor, V. Dudnikov, T. Srinivasan-Rao*, J. Smedley* and
J. McDonald**
Brookhaven Technology Group, Inc. 120 Lake Ave. South, Nesconset, NY 11767
*Brookhaven National Laboratory* Upton, NY 11973
**Pacific Northwest National Laboratory, OPO Box 999 Richland, WA 99352
Abstract A laser excited, sub-nanosecond, pulsed, electron beam system is described. The system
consists of a high voltage pulser and a coaxial laser triggered gas or liquid spark gap. The spark gap
discharges into a pulse forming line designed to produce and maintain a flat voltage pulse for 1 ns or
greater duration on the cathode of a photodiode. A synchronized pulsed laser is used to illuminate the
photo-cathode to produce an electron beam with very high brightness, short duration and current at or
near the space charge limit. The system can be configured to operate at energies from less than 500
keV to 1 MeV and pulse width from less than 10 ps to 1000 ps and higher. This laser controlled
electron beam system can be used to produce synchronizable monochromatic fluorescent or broad
spectrum Bremsstrahlung x-rays for shock wave studies.
INTRODUCTION
Dynamic studies of shock compression utilize
fluorescent x-rays from flash x-ray sources to
characterize the change in lattice constant and other
transient properties (1). These real time x-ray
diffraction
measurements
require
close
synchronization of the x-ray source with the
transient shock wave. The times of interest for these
studies are a few nanoseconds to sub-nanosecond
and lower. The laser excited photo-diode system
described here produces synchronizable short pulse
electron and photon beams in this time interval.
Since the system uses laser excitation to generate an
axial electron beam, the x-ray pulse duration,
photon number and source dimension are
determined by the corresponding characteristics of
the laser. Since the cathode and anode are not
damaged in the pulse discharge, they can be used
for thousands of shots without the need for
replacement.
TECHNICAL DESCRIPTION
The system described here was initially designed
as an electron gun for advanced high- energy
electron accelerators (2). Figure 1 is a block
diagram of the complete system. It is comprised of
a master timer, a laser system that includes a laser
amplifier and optical pulse compressor, a high
voltage pulse power supply, a pulse forming line
and a photo-diode electron gun. In this high voltage
(2 MV) variant, the output of the high voltage
power supply is terminated in a spark gap that
discharges into the pulse forming line. In a low
voltage (300 kV to 500 kV) system, all spark gaps
would be replaced by solid-state switches.
Figure 1. Block diagram of the laser controlled x-ray system,
In operation, the master timer sends trigger
pulses with appropriate delays to the laser and the
high voltage pulser. The laser output is split into
two components; one (100 ps at 1064 nm) travels
up the axis of the high voltage pulser and is timed
to arrive at the triggered spark gap at or near the
peak of the high voltage output waveform. The
second component is amplified, time compressed
and its frequency is adjusted (~ 255 nm) to optimise
photoemission from the cathode of the photodiode.
The pulse forming line (PFL) is terminated with a
low inductance characteristic impedance to prevent
reflection. In the 2 MV high voltage pulser, the PFL
is designed to produce a sustained flat top (< 5%)
output voltage of ~ 1 ns duration. The second laser
component is time delayed to arrive at the photocathode during the 1 ns pulse on time. By triggering
the output spark gap with the same laser that is used
to photo excite the cathode, a very high level of
synchronization ( < ~ 100 ps jitter) in emission of
electron current and arrival of the voltage pulse is
achieved.
Forming System
A general view showing the shape and
dimensions of the 2 MV pulsed power supply is
shown in Figure 2. The pulser is an integral unit
comprising the following components:
•
•
•
•
A metal casing
A pulse generator (100 kV) for exciting
the primary winding of the pulse
transformer
A pulse transformer
A pulse forming line for generating the
short (~1 ns) high-voltage pulse
Loser Trigger input
HV Pulse Transformer
A
Figure 3, Photograph of the pulsed power supply system.
The 2 MV system shown here is 2.5 meters long
and 1.22 meters high by ~ 1 meter wide. The
welded casing forms the framework onto which all
of the other parts of the pulser are mounted. The
upper section, which is not sealed, houses a solid
dielectric cylinder that forms a support for the pulse
transformer winding. The side and end plates of this
section of the casing are detachable to allow access
to the components of the low voltage 100 kV pulse
generator that drives the high voltage pulse
transformer in the upper section.
Figure 2 General view of the 5 MV Pulsed Power Supply
System
When operated as a synchronizable flash x-ray
source, electrons emitted from the photo cathode
travel 2 to 3 mm to the anode, which is also the x-ray producing target. Since the cathode does not
dump all its charge into the anode, the electron
energy remains nearly constant during the current
pulse. This results in increased fluorescent photon
yield and virtually eliminates destruction of the
anode and cathode surfaces that is observed in
standard flash x-ray devices.
FLUORESCENT X-RAY SOURCE FOR
DIAGNOSTIC OF SHOCK WAVES
The pulse length and spot size of this beam
based x-ray source is determined by the
corresponding pulse length and spot size of the
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laser beam on the photo-cathode. A typical source
spot size would be ~ 1 mm to 2 mm in diameter.
This small spot size provides a very high photon
flux density at the source.
arrival of the shock wave and the diagnostic x-ray
pulse.
CONCLUSION
The same basic design concepts of laser excited
electron emission and synchronized high voltage
pulsed systems that are used in this state-of-art high
voltage electron gun can be used to produce
synchronized x-ray pulses for real time study of
shock waves in condensed materials. A complete xray system operating at 300 kV to 500 kV uses all
solid-state components.
ACKNOWLEDGEMENT
This work is supported, in part, by the U.S.
Department of Energy in the following contracts:
DE-FG02-97ER8233, DE-AC02-98CH10886 and
DE-AC06-76RLO-1830.
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REFERENCES
Figure 3. Dependence of K x-ray yield from thick targets of
Z = 4 to 79 on incident electron energy. (From F. H. Attix
1986.)
1. Y.M. Gupta, K.A. Zimmerman, P.A. Rigg, E.B.
Zaretsky, D.M. Savage and P.M. Bellamy,
Experimental developments to obtain real-time x-ray
diffraction measurements in plate impact
experiments. Rev. Sci. Instr. 70, No. 10, p 4008-4013
(1999).
Using a conventional solid-state laser system, an
electron charge of ~ 50 nC can be drawn from the
cathode without significant voltage droop. The K xray yield from thick targets of Z = 4 to 79 are
shown in Figure 4 (3). From the figure it can be
seen that the yield of 4.5 keV photons peaks at ~ 2
x 10~3 x-rays/sr/electron at an electron energy of ~
150 keV. This corresponds to ~ 0.6 x 10y
photons/sr/pulse for a 50 nC electron bunch.
2. Kenneth Batchelor, J. Paul Farrell, R. Conde, T.
Srinivasan-Rao and J. Smedley, A Laser Triggered
Synchronizable, Sub-Nanosecond Pulsed Electron
Source, Proc. of International Conf. on Future
Accelerators, Stony Brook, NY (June 2001). To be
published.
Experiments are needed to determine if the total
photon yield from this short pulse high brightness
photon source is sufficient signal for real time x-ray
diffraction studies.
3. J.H. Sparow and C.D. Dick, The development and
application of monoenergetic x-ray sources. Report
NBS SP456 (1976) and reproduced in Introduction to
Radiological Physics and Radiation Dosimetry, F.H.
Attix, John Wiley & Sons, New York p. 209 (1986).
An advantage of this laser synchronized x-ray
source is that the system is easily adapted to include
both plate impact and laser induced shocks. The
same laser that is used to induce a shock in the
sample could be used to photo-excite the cathode of
the electron gun To produce laser induced shocks.
This approach has the possibility of achieving a
very high degree of synchronization between the
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