of high-energy laser beams in the beamlet demonsfration
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UCRL-JC-119004
Absolute wavefront measurements of high-energy laser beams in the Beamlet Demonsfration Project
‘for the National Ignition Facility .
J. Thaddeus Salmon Erlan S. Bliss
Michael W. Kartz B m o M. Van Wonterghem
Lawrence Evermore National Laboratory Livennore, California
This paper was prepared for submittal to CLEO/QELS ‘95
Baltimore, Maryland May 22-26,1995
DISCLAIMER
This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial. product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute .or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
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I
Absolute wavefront measurements of high-energy
laser beams in the Beamlet Demonstration Project
for. thqNationa1 Ignition Facility
J. Thaddeus Salmon, Erlan S. Bliss,
Michael W. Kartz, and Bruno M. Van Wonterghem
Lawrence Livennore National Laboratory, P. 0. Box 808, Ms L463
Livermore, CA 94550, ph. (510) 422-9529, fax (510) 423-3143
Abstract
We measure the absolute wavefront of Beamlet output pulses and actively
correct for slowly varying aberrations present when the power ampMers are not-
fired. Provision is also made for active premmpensation of pump-induced
ampiifer aberrations.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livennore National Laboratory under contract no. W-7405-Eng-48.
Absolute wavefront measurements of high-energy
laser beams in the Beamlet Demonstration Project
for thqNational Ignition Facility
J. Thaddeus Salmon, Erlan S. Bliss,
Michael W. Kartz, and Bruno M. Van Wonterghem
Lawrence Livermore National Laboratory, P. 0. Box 808, MS L-463
Livermore, CA 94550, ph. (510) 422-9529, fax (510) &3143
Summary
The conceptual design of the National Ignition Facility (NE) laser
passes each of its 192 be&s through power amplifiers four times to maximize
the energy extracted from the glass slabsl, and the aberrations induced in the
slabs by the flashlamps are added to the beam on each of the four passes.
Hence, operating this system with beams near the diffraction limit requires
the use of active optics that are set to the conjugate wavefront of the
aberration expected on the next laser pulse.
As part of the technology development effort for wavefront control on
the NIF laser, we have designed and installed an active optical system in the
Beamlet Demonstration System. The design of Beamlet is described by Van
Wonterghem et al.2 The active optics system, shown schematically in Fig. I,
has a deformable mirror before the main &pass amplifier and two Hartmann
sensors for closed-loop control. The Hartmann sensors sample the beam
immediately after the deformable mirror and at the laser output immediately
before the harmonic generating crystals. The deformable mirror has 39
page 1 of 4
J. T. Salmon et al. "Absolute wavefront measurements ..."
actuators arranged in a triangular pattern as shown in Fig. 2. The influence
function for each actuator is nearly Gaussian with an amplitude at each
adjacent actuator that is 30% of the central peak. A third Hartmann sensor
with a larger number of lenslets (high-density Hartmann sensor) is located
adjacent to the output Hartmann sensor and provides an independent,
higher resolution wavefront diagnostic for the beam entering the crystals.
Figure 3 shows the reconstructed wavefront measured by the high-
density Hartmann sensor on a single full system shot. The wavefront was
calculated from centroid displacements from finite difference formulas -given
in Salmon et al.3 Aberrations other than those caused by amplifier pumping
during the shot were compensated prior to the shot by the active optical
system controlling on 0.2 Hz pulses from the front-end laser upstream of the
deformable inirror. The remaining pump-induced aberration is primarily
fourth-order in one axis and second-order in the other axis and has an
amplitude of 0.9 waves at a wavelength of 1.06 p. The pump light is
entering the amplifier from the front-left side and from the rear-right side as
shown by the arrows. The active optical system can compensate for
aberrations up to fourth-order. Compensating these aberrations depends on
how reproducible the aberrations are. We'are now preparing the closed-loop
control system to precorrect for the pump-induced aberrations.
*$ . .. I - - '
.
We will present these results, along with results of closed-loop tests of
the active optics system on high-energy pulses in the system, and we will
discuss the applicability of active optics to large inertial confinement fusion
laser systems.
page 2 of 4
J. T. Salmon et al. "Absolute wavefront measurements ..."
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References
1. National Ignition Facility Conceptual Design Report, LJNL publkation
UCRL-PROP-117093, August, 1994.
2. B.M. Van Wonterghem, J.R. Murray, D.R Speck, and J.H. Campbell; - - -
'Performance of the NIF Prototype Beamlet," L W Report UCRL-115580,
1994 American Nuclear Society Meeting and 11th Topical Meeting on the
Technology of Fusion Energy, New Orleans, LA, June 19-23,1994.
3. J.T. Salmon, J.W. Bergum, M.W. Kartz, R.W. Presta, and C.D. Swift, SPIE
Proceedings 1920,20 (1993). .
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J. T. Salmon et al. "Absolute wavefront measurements ..."
., * * < + ' Figure Captions
Fig. 1 Schematic of the Beamlet demonstration system for the National. .
Ignition Facility showing the location of the front-end optics, the
deformable mirror, the Hartmann sensors for control (HS), the high-
density Hartmann sensor (HDHS), the optical switch, the output
polarizer, and the frequency converter.
Fig. 2 Configuration of the 39 actuators on the deformable mirror, along
with the mapping of the 77 lenslets of the Hartmann sensors and the
footprint of the laser beam.
Fig. 3 Reconstructed wavefront of a single high-energy laser pulse in
Beamle t.
page 4 of 4
booster amp I ifi er
\ \ \ o&al
spatial 8 laser output
I I,’ I
frequency converter
I IT amplifier
deformable mirror HS
front-end laser system
Fu I I-po we r Actuator \ Half-power
point Hartmann
Absolute Wavefront (waves)
0.5
0.0
-0.5
Pump Light
p-v = 0.94 waves rms = 0.21 waves
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