ELECTRA PRE-AMPLIFIER: A REPETITIVELY PULSED, ELECTRON BEAM PUMPED, KrF LASER*

* Work sponsored by DOE/NNSA/DP

•Naval Research Lab•Matt Myers•John Sethian•John Giuliani

•CTI•Frank Hegeler•Tom Albert•Moshe Friedman•James Parrish

•RSI•Warren Webster

•SAIC•Matt Wolford•Areg Mangassarian

•Titan/PSD•Dave Morton•Doug Weidenheimer

The Electra Laser System

A - Oscillator bench (LPX-305i) - 20 ns pulse at 248 nm ( ~ 1 J)

B, C, D - Split beam into 2 pulses and line up end to end (40 ns)

L - Single pass through pre-amplifier ( ~ 30 J)

E, F - Turning mirrors

G, H, I - Multiplex: 6 x 20 ns beams in pulse train

K - double pass through main amplifier ( ~ 700 J)

Main Amp

Pre-amp

Osc.

Electra Pre-Amplifier: Design Constraints

ORESTES KrF physics code estimates Electra Main Amp requires ~30J input to develop ~700 J output energy.

Allowing for losses, pre-amp must supply ~36 J.

Oscillator input to pre-amp will be 0.5 J, 20 ns FWHM pulse from commercial discharge KrF (Lambda Physik LPX305i).

A double-sided, e-beam pumped system is less technologically risky than pure discharge or e-beam assisted discharge systems.

Incorporate advanced pulsed power architecture fast marx/PFL/magnetic switch/TTI marx initially uses conventional gas switches marx retrofitted w/ advanced solid state switch

Demonstrate pulse shaping necessitates single pass system requires low system jitter (< 1 ns 1) requires zero jitter between e-beams

Opt for 10 cm x 10 cm Square Aperture

Why 10 cm x 10 cm square? beam relaying to main amp easier can use off-the-shelf, durable windows flexible multiplexing more compact pulsed power less x-ray shielding smaller magnets good optical size for other applications (i.e. lithography)

Implications: dictates 150-175 kV e-beams for efficient deposition at ~1 Atmosphere low voltage means thin/transparent hibachi foils

reinforced or diamond coated aluminum maximize performance initially use aluminum foil for acceptable performance

estimated hibachi efficiency 63-87%; pump power ~ 1200 kW/cc need ~80 kA/side at ~ 175 kV

design for 20 ns rise/40 ns flat-top/20 ns fall pulse minimize foil heating by slow electrons establish a well-defined pump before amplification flexible timing of pulse-train ASE not thought to be a problem

Orestes: Predicted Laser Yield vs. Pressure and Composition for Electra Pre-Amp

Ar

(To

rr)

100

cm

10 cm

10 c

m

120

cm

Advanced Pulsed Power Architecture

Fast Marx(LGPT switching)

Pulse Forming Line Magnetic Switch

Transit Time Isolator

Electron Beam Diodes

• High efficiency• Low $/E-beam joule cost• Excellent durability

Pulse Sciences Division

Electra Pre-Amp: Conceptual Design

Top View

Side View

Transit Time Isolators: water sections

Magnets

Laser Cell10cm x 10 cm

x 100 cm

Optical path

Cathode: 10 cm x 100 cmWater Joints

60” beamheight

Marx(gas switches) PFL Mag

Switch

Scale (100 cm)

Nominal: 175 kV, 80 kA, Z = 2.2 Low V: 150 kV, 68 kA, Z = 2.2 PRF: 5 Hz20 ns rise, 40 ns flat-top, 20 ns fall

Pulse Sciences Division

Transit Time Isolators: oil sections

Electra Pre-Amp: Performance

DEMONSTRATED Pulsed Power Performance With Resistive LoadOutput: 175 kV, 160 kA, 40 nsec flat pulse (< 20 nsec rise)Rep Rate: Single shot to 5 HzDurability: >100,000 shots before maintenance (Marx switches) 1 Jitter (Single Shot): 600 ps1 Jitter (5 Hz, 10k Shots): 850-1200 ps

Electra Pre-Amp: Performance With Electron Beam Diode

-200

-150

-100

-50

0

50

10 ns/div

Vdi

ode

(kV) Idi

ode

(kA)

Diode performance with 1200 cm2 velvet cathode, 2.1 cm AK gap.

No electron beam rotation or shear with Bext=2.1 kG.

50 shot, 5 Hz jitter ~ 800 ps 1.

Diode voltage and current very reproducible.

voltage

Advanced Components

In addition to demonstrating the advanced pulsed power architecture the Electra pre- amplifier will serve as a test bed for advanced KrF laser components.

The gas switches used in the Marx will be replaced by solid state LGPTs (laser gated and pumped thyristors.* This retrofit will significantly improve reliability, efficiency, and durability.

Ceramic honeycomb cathodes are used with 1010 steel focusing bars to segment the electron beams so they pass through the foil support structure (hibachi) with minimal attenuation allowing high efficiency deposition in the laser gas.**

A new hibachi that minimizes foil clamping stress and maximizes conduction cooling of the foil is being designed.***

* D. Weidenheimer, et al., “Advanced pulsed power concept and component development for KrF laser IFE”, Conference Record of the 25th International Power Modulator Symposium, 2002, p. 165.

** M. Friedman, et al., Jour. Appl. Phys. 96, 7714, 2004.

*** J. Giuliani, see poster this HAPL Conference.

Advanced Components: LGPT (Laser Gated and Pumped Thyristor)

p

n+

n-

n++

p++

Diode Laser

D Laser

SiliconThyristor

CONCEPT:•All solid state•Diode lasers flood entire thyristor with photons

•Fast switching times (< 100 nsec)•Continuous laser pumping reduces losses

PROGRESS:• > 1,200,000 shots (multiple runs)• Required specs: 16.4 kV, 5 Hz, 1 kA/cm2

• Switch has run @ 50 Hz

shot 220,000vs

shot 1,000,000

Power and Energy

16.4 kV

LGPT Switch

7 cm

1/2 Capacitor

1/2 Capacitor

1/2 Capacitor

1/2 Capacitor

Laser Drive ElectronicsLaser

Application- Ultra Fast Marx (+/- 16.4 kV stage)

Oil Insulation

Configuration withConventional

Switches

Advanced Components: LGPT (Laser Gated and Pumped Thyristor)

Tests of monolithic ceramic honeycomb cathodes show promising durability without compromising rise time, gap closure, and uniformity.

Conversion of the full-sized ceramic honeycomb cathode to a strip geometry may be accomplished using soft iron bars.

Tests using iron bars with velvet emitter have been very successful up to 1 Hz.The next step is to adapt iron bars to ceramic honeycomb geometry and test at 5 Hz.

Strategically placed iron bars interact with the external magnetic guide field to produce local focussing fields along the emitter surface. The overall guide field is not perturbed.

The local concentration of magnetic field focuses the emitted beam electrons into vertical strips that propagate across the AK gap and through the openings in the hibachi.

Advanced Components: Ceramic Honeycomb - Iron Bar Cathode

Advanced Components: Ceramic Honeycomb - Iron Bar Cathode

Use 2.54 cm thick, 325 ppi cordierite ceramic honeycomb with -alumina wash coat.

Use 1010 low-carbon steel bars for high H (10 - 100).

Cathode is 10 cm x 100 cm.

Advanced Components: Conduction Cooled Hibachi

Simply cool foil by conduction to ribs using materials with high thermal conductivity and properly managed fluid flow. (see poster by John Giuliani)

Scalloped foil clamping design reduces tensile stress significantly.

Combination of cooler foil with less clamping stress allows use of thinner foils of lower Z material thus improving efficiency and durability.

H = 0.9-2.3W/cm2

H = 0.9-2.3W/cm2

b =.250" a = .295"

X = 0.841"

P = 30 psi

a f=.320".150"

f = 40º

r =

.459

.187 dia

.194 r

Foil Span: 1.50 cmRibs: .635 cm wide x .762 deepRib Spacing: 2.135 cm

f

.150

.032 wall

.045

.300"

b =.250" a = .295"

X = 0.841"

P = 30 psi

a f=.320".150"

f = 40º

r =

.459

.187 dia

.194 r

Foil Span: 1.50 cmRibs: .635 cm wide x .762 deepRib Spacing: 2.135 cm

f

.150

.032 wall

.045

.300"

Resistive load testing complete

10,000 shot run w/ 1 jitter at 800 – 1200 ps Marx gas switches require maintenance after ~ 50k – 100 k shots Experimental diode performance nicely follows simulations.

E-beam testing into cooled anodes going well

50 shot bursts at 5 Hz with velvet cathodes diode voltages and currents very reproducible and consistent 1 jitter at 800 ps no beam shearing or rotation at Bext = 2.1 kG

longevity testing at 5 Hz will commence with Health Physics approval.

Advanced component development

LGPT has met design goals and pre amp Marx is due for retro-fit circa 2006. This will improve lifetime to 106 – 107 shot range. Longevity testing of velvet-iron bar cathodes in progress. Ceramic honeycomb – iron bar cathode design complete. Mock up being built.

Conduction cooled hibachi design complete. Drawings finalized next week.

Electra Pre-Amp: Status