*Work sponsored by DOE/NNSA

M. Myers, J. Giuliani, J. Sethian, M. Wolford, T. Albert 1), M. Friedman1), F. Hegeler1), J. Parish1), P. Burns2), and R.

Jaynes3)

Naval Research LaboratoryPlasma Physics Division

Washington, DC 20375 USA

1) Commonwealth Technology, Inc., Alexandria, VA 22315 USA 2) Research Support Instruments, Lanham, MD 20706 USA 3) Science Applications International Corp., McLean, VA 22102 USA

High Average Power Laser ConferenceWashington, DC

Oct. 30 - 31, 2007

Development of a Large Area, DurableElectron Emitter for High Average

Power KrF Lasers*

High average power KrF lasers require repetitively pulsed, fast rising, uniform electron beams to be generated with little

plasma produced.

In experiments done on the Electra main amplifier, first generation strip-velvet cathodes met the requirements for rise time, gap closure (plasma production), and uniformity but for <103 shots at pulse repetition frequencies (PRF) < 1 Hz.

Efficient electron beam pumping, achieved with the strip cathode geometry, produced 700 J of laser energy in oscillator mode.

At PRF > 1 Hz copious gas/diode plasma is produced and the velvet material is quickly compromised. In addition, the floating edge reducers and the metallic cathode frame see an average electric field > 100 kV/cm and eventually become sources of emission.

Cathode development was pursued along two parallel paths

Velvet cathodes(monolithic and

strip)

Create stripemitters

w/o metallicedge reducers

Reduce averageelectric field on

non-emittingstructures

Build full-sizedcathodes w/ometallic frame

Reduce gasformation w/ocompromising

uniformity

Find more robustemitter w/o

compromisinguniformity

Investigate coat-

ings/surface treat-

ments that prevent

emission.

Temperaturemanagement

capability

High efficiencycathode capable of many

shots at PRF upto 5 Hz

Upper Path: A strip cathode geometry can be created by using a high permeability material like 10-10 steel.

Strategically placed high permeability 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.

Because the beam strips are focused by the local magnetic field, a larger emitting area can be used. This allows a larger AK gap meaning a lower average electric field on the cathode.

The beam edge effect is also mitigated by the focussing gradient. This eliminates the need for the floating, metallic edge reducers used in previous strip cathodes.

MAGIC simulation: steel bars placed -20, -60, and -100 mm positions.

A velvet cathode with 10-10 steel bars was used to test the concept

10-10 steel bars (H ~ 20 at 1 kG) are imbedded in a velvet cathode and tested on Electra.

Time-integrated radiachromic film image of emission from the steel bar cathode shows nicely partitioned strips of beam electrons.

Steel bars spaced4.4 cm apart

Various bar coatings and coverings were tested for emission suppression

The first velvet/iron-bar cathodes were used to consistently produce 700 J of laser energy for single shots.

At PRF > 1 Hz the high electric field and the interaction of the bars with cathode plasma caused the bars to emit which reduced laser pumping efficiency.

The 10-10 steel bars were then tested with different coatings (porcelain, various powder coats, SiOx deposition) or covering strips (glass, alumina, silicon carbide, carbon).

None of the coatings were robust enough to suppress bar emission at 2.5 Hz for more than 1500 shots.

SiC strip coverings were the most desirable in preliminary tests at 1 Hz, but are costly. High density graphite strips performed nearly as well as SiC at 1 Hz and are much cheaper.

Note that the strips are “pre-rotated” 5 - 6 so that they precess in the magnetic field to vertical strips as they cross the AK gap in the diode.

alumina stripsgraphite strips

We will be testing a full sized strip cathode that uses carbon fibers as the emitter.

• Carbon - carbon “Type C” developed with Energy Science Laboratories, Inc., San Diego, CA. • High density graphite strips cover the iron bars.

graphite strips

Lower Path: Experiments* show that inserting a slab of ceramic honeycomb in front of a large area cathode:

Decreases the beam current rise and fall times.

Maintains a more constant diode impedance.

Improves the uniformity of emission.

* M. Friedman, M. Myers, F. Hegeler, S. Swanekamp, J. Sethian, and L. Ludeking, Appl. Phys. Lett., 82, 179 (2003).

----- Patent Pending -----

Five consecutive 10,000 shot runs at 1 Hz

The ceramic honeycomb cathode has several potential advantages

in KrF laser applications The ceramic honeycomb is a secondary emitter of electrons. There is no surface plasma that can create gap closure issues at high PRF. Plasma from the primary emitter is confined by the honeycomb.

Faster rise and fall times mean less pressure foil heating and longer foil lifetime.

Mitigation of emission center screening allows the use of more robust (i.e. non-velvet) primary emitters which typically have a higher turn-on threshold, E0. This results in fewer low energy electrons being emitted and longer foil lifetime.

Uniform secondary emission from the honeycomb results in better temporal and spatial uniformity in the laser beam pulse and less “hot spots” that may decrease foil lifetime.

The -alumina coating on the ceramic supplies a reactive sponge “reservoir” which may increase the cathode lifetime.

The ceramic honeycomb mechanism only requires the primary emitter to supply electrons for a small fraction of the beam pulse thus increasing cathode longevity.

The ceramic dielectric also suppresses the growth of the transit time instability in the electron beam diode. (Friedman, Myers, et al., J. Appl. Phys., 96, 7714 (2004)).

However, many electrons are lost to hibachi ribs when full-sized cathodes are used. In order to meet efficiency requirements, the ceramic honeycomb must be converted to a strip geometry.

A full-sized cathode without a support frame was built and tested removing a source of unwanted emission

The electric field on the cathode corners and edges of the full-sized cathode is reduced by con-touring the machinable ceramic.

The new cathode is composed of individually mounted tiles (no longer a monolithic slab) greatly reducing the amount of adhesive (and it’s associated out-gassing and emission).

Using the full-sized ceramic honeycomb cathode and a velvet primary emitter: 7,800 continuous shots were taken at 5 Hz with no damage to the pressure foil.

Number of shots is still limited by out-gassing of the velvet primary emitter which reaches temperatures that approach 300 C.

Increased longevity was achieved by using a “carbon-carbon” primary emitter in place of the velvet and by cooling

the cathode mount

Carbon-carbon type “B”, developed at Energy Science Laboratories, Inc., San Diego, CA., consists of 1.5 mm long, 6 m dia. fibers, “bonded” at ~ 2% packing fraction to 3 mm thick graphite.

Cooled mounting plate maintains primary emitter temperature at < 50 C at 2.5 Hz.

The amount of evolved gas and the cathode conditioning time was drastically reduced.

Using the CC-B emitter with the cordierite ceramic honeycomb tiles we achieved:

25,000 continuous shots at 2.5 Hz

High voltage bushing

for water cooling

Cooled mounting

plate

CC-B emitter Cordiertie tile

Raw, visible light images of emitter surface for velvet and ceramic honeycomb cathodes at various times during diode

voltage pulse.

1 cm/div

-55 15 ns

cera

mic

hon

eycom

b

-40 30 ns -25 45 ns 80 150 ns

80 150 ns20 90 ns-10 60 ns-25 45 ns

velv

et

Xybion Gated Camera Data: Gate: 70 ns Gain: maximum

Although the secondary emission from the ceramic honeycomb is quite uniform, run duration is limited by debris

punctures in the foil• As is, the cordierite ceramic honeycomb tiles are not expected to reach a

106 shot lifetime.• More than 150,000 total shots have been achieved on sets of cordierite

ceramic tiles but the

material becomes brittle and is compromised with increasing number of shots.• Punctures start with a “pinhole” that may grow to 50 m - 100 m before the foil

fails completely.

typical “splat”damage pattern

pinhole damagefrom debris

typical damage pattern seen on foil

and ribs

The source of the punctures is almost certainly the cathode but the mechanism remains elusive

• Emission from foil when voltage reverses: “cathode spot” initiation site is debris on

foil.• Debris particle hot enough to melt foil: “hot rock”.• Particle stuck on foil absorbs energy and heats up with each subsequent pulse.• Late time “hot spots” induced by mis-fires of the pulsed power system.• Local non-uniform emission due to ceramic seams and/or mounting adhesives.• Marco-particle generated by cathode explosive emission ballistically driven

through foil.• Exfoliation of ceramic due to rapid heating of trapped water or gas.

discovery ofa “hot rock”

possible exfoliation

Coating the honeycomb tiles with SiO2 makes them more robust, reduces evolved gases, and improves foil lifetime.

• Achieved longest 2-sided laser run: 16,600 shots at 2.5 Hz.

• Significantly less debris and cathode spots produced.

• No “conditioning” runs required. More consistent results.

• Risetime and emission uniformity unaffected by coatings.

• SiC may help with surface electric fields.

un

-coate

dS

iO2 c

oate

dS

iO2 +

SiC

Visible light, 50 ns gate

Using honeycomb tiles made of zirconia increases durability while maintaining current risetime and emission uniformity.

• Mounting the tiles from the front with ceramic screws eliminates the use of adhesives.

• Zirconia material is very robust and can be coated with SiO2.

• Zirconia material is available in larger tile sizes which will greatly reduce the number of tiles necessary and thus reduce the number of seams.

A process was developed with ESLI that enables carbon fiber material to be applied to contoured carbon surfaces.

• Carbon-carbon type “C”, developed at Energy Science Laboratories, Inc., San Diego, CA., consists of 1.5 mm long, 6 m dia. fibers, “bonded” at ~ 2% packing fraction to contoured graphite.

• Cathodes are operated without ceramic honeycomb tiles.

• No debris or foil damage detected in 4,000 shots at 2.5 Hz.

• No outgassing - operates at 50 x lower vacuum - pressure maintained at 4.3 x 10-6 Torr.

• Unaffected by temperatureShows promise for

0

100

200

300

400

500

600

700

0 500 1000 1500 2000 2500 3000 3500 4000

Shot #In

teg

rate

d S

wit

ch

Cu

rre

nt

(arb

.)

Although the average electric field on the metal cathode shroud is ~ 50 kV/cm there is evidence of “hot spots” and

emission sites at PRF > 1 Hz• The metal shroud could be a source of debris that limits the lifetime of the pressure foil.

Electro-polished stainless steel shroud after ~ 5,000

shots at 2.5 Hz.

Time-integrated image (side view) of visible light produced in diode

on single shot

Design by Ray Allen, NRL Code 6770 using “AMAZE”

• Use a “Chang” profile

for

both cathode and

shroud.

• Maintain electric fields

below 100 kV/cm.

• Implement on next

generation machine.

Re-design the shroud so that its “face” is the emitter surface.

An efficient, durable emitter for high average power KrF lasers - combine advances made along the 2 parallel,

developmental paths.

Honeycomb strip emitter

10-10 steel barswith SiC coating

Individual strip emitter

ceramic screwmounts

contoured CC- type Cprimary emitter

cooled mount

advanced tilecoatings