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.

DISCLAIMER

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produced from the best available original document.

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