pulse shaping & energy capabilities of angularly-multiplexed krf lasers 17 th hapl meeting naval...

Pulse Shaping & Energy Capabilities of Angularly-Multiplexed KrF Lasers

17th HAPL MeetingNaval Research Laboratory

October 30-31, 2007

R. H. LehmbergRSI/NRL Washington, DC

Work supported by USDOE/NNSA/DP

One of the primary requirements for laser-driven inertial fusion energy (IFE) is the ability to produce high energy pulses with the temporal shapes needed to control hydrodynamic instabilities and target preheat. Although this capability has been well established on glass lasers such as the National Ignition Facility (NIF), it is less certain on Krypton Fluoride (KrF) lasers, where the large multi-beam angularly multiplexed amplifiers tend to have heavy saturation, high gains, & complicated gas kinetics. Saturation can produce severe pulse distortion, and high gains may allow excessive target preheat due to near-axial amplified spontaneous emission (ASE). This poster reviews the steps required to minimize pulse distortion & ASE, then describes and simulates a robust technique to produce the desired pulseshape by pre-compensating the distortion.

Although the pulse shaping and ASE considerations are applicable to any large KrF laser, the simulations presented here apply to the ~30 kJ system designed for the recently-proposed NRL Fusion Test Facility (FTF).

Introduction

Using the Orestes code, we have modeled the energy and pulse shapingcapabilities of the KrF laser in our proposed Fusion Test Facility (FTF)

We developed a simple & stable iteration technique for calculating the pre-distorted input pulseshape required to achieve the desired output pulseshape.

The simulations show that KrF amplifiers can behave as quasi-storage lasers for 1 ns pulsewidths & pulse spacings

Our FTF design allows energies up to 30 kJ from each of our 20 amplifier systems without excessive ASE prepulse on target

It may be possible to generate the complicated pre-distorted input pulses by using an optical Kerr gate

Main Results

28 kJ KrF laser Amp

(one of 20)

Containment

Vessel

Laser Beam Ducts

Reaction

Chamber

28 kJ KrF Laser Amp(one of 20)

ContainmentVessel

Laser Beam Ducts

ReactionChamber

Conceptual Design of the FTF(M. W. McGeoch, Plex Corporation)

Laser energy on target: 500 kJFusion Power: 30 - 150 MWRep Rate: 5 HzChamber radius: 5.5 m

FTF Optical Block Diagram (one of 20)T. Lehecka, Penn State Electro-Optics Center

Front End

Amp 1 Amp

MirrorFixed

AmpMirrorFixed

Amp 2

225 ns, ~1 kJ

225 ns, ~30 kJ

Final Amplifier(100 x 100 cm2)

Driver Amplifier (30 x 30 cm2)

~ 2030 J input in 90 sequential pulses, one/beam

~ 30 kJ out

~ 1.2 kJ

~ 28 kJ in 90 synchronized pulses,one/beam Target Chamber

X6 15Lenses

6X15Convex Mirrors

6X15 Flat Mirrors

6X15

Convex

Mirrors

Oscillator5 Hz Rep

Rate

Beam Smoothing

Pulse Shaping

2.5 ns plus foot pulse

Object

15 beam splitter

.5 2Amp 4

ns ~2 J

15 beam Multiplexer – 38 ns

90 beam splitter

Amp 3 38 ns

20 - 30 J 90 beam Multiplexer

225 ns

90 beam De-multiplexer

225 ns 2.5 nsOutput optics

The simulations deal primarily with the driver & final amplifier results

One of 90 Target BeamsASE Rays (cw)Mirror Pumped KrF

(not drawn to scale)

Input Array (6 X 15)

Output Array (6 X 15)

Because of its large angular divergence & cw time dependence, ASE can preheat the target via beam channels intended only for earlier pulses

M. Karasik, et al, J. Appl. Phys. 98, 053101 (2005)

On-target ASE intensity ramps up as: IASE(t) GDRVR(t)GFINL(t)NB(t)

GDRVR(t)GFINL(t) are the gains & NB(t) is the increasing number of beams whose demultiplexed paths allow light to reach the target at time t

280 cm

Performance: flat-top laser energy: 30.1 kJ flat-top intrinsic efficiency: 12.7%

mirror

Pulsed powerfeed (1 of 16)

Laserlight

e-beam

Cathode and feed detail

e-beam(s)

cathode pair

Power feed(insulator)Laser cell

window(100 x

100 cm2)

1010 50 50 10 1050 5040

Intensity(MW/cm2)

ore-Beam

Deposition(MW/cc)

time (ns)

t1 = 50 t2 = 27515

10

5

0

IOUT

10 x IIN

10 x PEB

10 x IASE

0 100 200 300 400

Distance from rear mirror (cm)

I(→)

ISAT

t = 150 ns

10 x PEB

I(←)IASE

0 100 200 300

15

10

5

0

280 cm280 cm280 cm

Performance: flat-top laser energy: 30.1 kJ flat-top intrinsic efficiency: 12.7%

mirror

Pulsed powerfeed (1 of 16)

Laserlight

e-beam

Cathode and feed detail

e-beam(s)

cathode pair

Power feed(insulator)Laser cell

window(100 x

100 cm2)

1010 50 50 10 1050 5040

Intensity(MW/cm2)

ore-Beam

Deposition(MW/cc)

time (ns)

t1 = 50 t2 = 27515

10

5

0

IOUT

10 x IIN

10 x PEB

10 x IASE

0 100 200 300 400

Distance from rear mirror (cm)

I(→)

ISAT

t = 150 ns

10 x PEB

I(←)IASE

0 100 200 300

15

10

5

0

The simulations are based on our 30 kJ final amp design

Baseline Case: Desired pulseshapes (2.5 ns main pulse + 6 ns foot pulse) into the driver amplifier

Time (ns)

I IN (

MW

/cm

2)

Control Beam

Buffer Beam (I0)ITOT

Beams I1,I2,..,I90

Time (ns)

I IN (

MW

/cm

2)

PEB PE

B (

MW

/cm

3)

Control beam loads down the amplifiers during e-beam ramp-up, thereby preventing high gains & large target ASE at early times

Buffer beams (I0 & I90) minimize distortion of earliest & latest beams

Total input energy of target beams (I1,..,I90) is 90 x 0.25 = 22.5 J

Time (ns) Time (ns) Time (ns)

I OU

T (

MW

/cm

2)

PEB (MW/cm3)

IASE (MW/cm2) @ Final Amp

ITOT

I1 I3 I88 I90

In spite of their contiguous placement, the pulses out of the final amp are still distorted by saturation

Total output energy of the 90 target beams is 30.4 kJ

Time (ns) Time (ns) Time (ns) Time (ns)

I OU

T (

MW

/cm

2)

I OU

T (

MW

/cm

2)

Pulse to pulse variations remain small, in spite of changes in the gas kinetics (e.g., Fluorine burnup)

This suggests possible pre-compensation with a single input pulseshape

Iteration TechniqueFirst, estimate highest energy available from the final amp within constraints on driver input(Saturation small changes in output energy require much larger % changes in input)

EIN > 40 J would require an additional front end amplifierEIN < 20 J would lower efficiency and give higher driver gain & ASE

(Estimates require only a single beam with a rectangular 225 ns pulse.) Then scale the intensity IS(t) of the desired pulseshape to give the estimated output energy in 90 beams.

0th iteration: Initially, the driver input pulses are the same shape as the desired output (baseline case) but scaled down to the 20-30 J input energy: IIN

(0)(b,t-tb) = η IS(t) (η ~ 10-3) for each beam b = 0,1,..,90,91. The final amp output pulses IOUT

(0)(b,t-tb) will be distorted.

Subsequent iterations: Choose one of the 92 beams bREF to represent all others (e.g., bREF = 34) Replace each driver input pulse by its partially compensated version; e.g. the 1st iteration uses

IIN(1)(b,t-tb)=IIN

(0)(b,t-tb)[IS(t)/IOUT(0)(bREF,t-tREF)]

then recalculate to update the output pulses IOUT|(1)(b,t-tb). Continue this procedure, replacing

IIN(N)(b,t-tB)=IIN

(N-1)(b,t-tb)[IS(t)/IOUT|(N-1)(bREF,t-tREF)]

in each driver beam until the output IOUT(N)(bREF,t-tREF) of the reference beam is close enough to

the ideal pulseshape IS(t). The pre-distorted input pulse IIN(N)(bREF,t-tREF) is then stored in a data

file and subsequently used to generate the full 92 beam simulations shown here.

This procedure could be modified to pre-correct each beam independently, if necessary.

Pre-distortion of the driver input pulses produces the desired output shape in the reference beam

I IN (

MW

/cm

2)

Time (ns)

I OU

T (

MW

/cm

2)

Time (ns)

Ref. BeamIdeal Pulse

Time (ns)

I IN (

MW

/cm

2)

Control Beam

Buffer Beam (I0)

ITOT

Target Beams (I1,I2,…,I90)


The idea works well for all 90 beams


I OU

T (

MW

/cm

2)

PEB (MW/cm3)

IASE (MW/cm2)

ITOT

IREF I45I1 I3 I88 I90



I OU

T (

MW

/cm

2)

I OU

T (

MW

/cm

2)

Magnified views of selected beams showlittle distortion, even in the earliest pulses

On-target ASE is very low because the contiguous pulses continually load the amps and limit the gain

Time (ns)

Inte

nsity

on

Tar

get

Flu

ence

on

Ta

rget

IPULSE[1013 W/cm2]

IASE[105 W/cm2]

FPULSE [105 J/cm2]

FASE [101 J/cm2]

There is enough margin here to allow lower input energies (e.g. ~ 10 J)

I IN (

MW

/cm

2)

Time (ns) Time (ns)

Now apply the idea to something more challenging:an RX pulse with a 2:1 shock ignition spike at the end

I IN (

MW

/cm

2)

I OU

T (

MW

/cm

2)

Ref. Beam Ideal Pulse

Time (ns)

Control Beam

Buffer Beam (I0)

ITOT




I OU

T (

MW

/cm

2)

PEB (MW/cm3)

IASE (MW/cm2)

ITOT

IREF IMID

No unpleasant surprises here



I OU

T (

MW

/cm

2)

I OU

T (

MW

/cm

2)

Significant distortion occurs only in the 1st pulse,which can be fixed by modifying the control beam

Time (ns)

For something even more challenging, the pulses are now only 1ns FWHM and non-contiguous

I IN (

MW

/cm

2)

Time (ns) Time (ns)

I OU

T (

MW

/cm

2) Ref. Beam

Ideal Pulse

I IN (

MW

/cm

2)

Control Beam

Buffer Beam (I0)

ITOT



Time (ns) Time (ns) Time (ns)Time (ns) Time (ns) Time (ns)

I OU

T (

MW

/cm

2)

PEB (MW/cm3)IASE (MW/cm2)

ITOT

IREF IMID

This regime demonstrates KrF storage laser capabilityfor pulses ~1 ns, but enhanced ASE may be an issue



I OU

T (

MW

/cm

2)

I OU

T (

MW

/cm

2)

Shape fidelities are not as good as those of the longerpulses, but the pulse energies are still ~90% as large.

Inte

nsity

on

Tar

get

Flu

ence

on

Ta

rget

FPULSE [105 J/cm2]

FASE [101 J/cm2]

Time (ns)

IPULSE [1013 W/cm2]

IASE [106 W/cm2]

On-target ASE is enhanced because inter-pulse spacing allows high transient gains, but it is still not a problem

z

EI (ISI, t-dep. pol.)

EC (coherent pulse, 1054 nm, 45o pol.)

(3) Medium

EI (ISI cw, 248 nm, x-pol.)

x

EIy (ISI pulse, y-pol.)

x

x

y-Polarizer @ 248 nm

x

So how can we generate those uglypre-distorted input pulseshapes?

Take advantage of NIF fiber optic technology @ 1054 nm, then transfer pulseshape to 248 nm ISI light via an optical Kerr gate

The proposed Kerr gate would allow a coherent pulsed 1 m control beam of high intensity IC(t) to impose the desired pulseshape on the envelope of a cw 248 nm ISI beam via polarization rotation. The time-dependent ISI intensity transmission at the exit of the y-polarizer is:

T(t) = sin2[(t)/2] where (t) L248(xxyy + xyyx) IC(t)

Summary & Conclusions

Using the Orestes code, we have modeled the energy and pulse shapingcapabilities of the KrF laser in our proposed Fusion Test Facility (FTF).

We developed a simple & stable iteration technique for calculating the pre-distorted input pulseshape required to achieve the desired output pulseshape. It may be feasible to carry out this technique experimentally on the rep-rated laser.

The simulations show that KrF amplifiers can behave as quasi-storage lasers for 1 ns pulsewidths.

Our FTF design allows energies up to 30 kJ from each of our 20 amplifier systems without excessive ASE prepulse on target. The ASE is so small that it might be feasible to use lower energy input pulses (at a minor loss in efficiency).

It may be possible to generate the complicated pre-distorted input pulses by using an optical Kerr gate.

pulse shaping & energy capabilities of angularly-multiplexed krf lasers 17 th hapl meeting naval...

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