*Work sponsored by DOE/NNSA M. Myers, J. Giuliani, J. Sethian, M. Wolford, T. Albert 1), M. Friedman 1), F. Hegeler 1), J. Parish 1), P. Burns 2), and R. Jaynes 3) Naval Research Laboratory Plasma Physics Division Washington, DC USA 1) Commonwealth Technology, Inc., Alexandria, VA USA 2) Research Support Instruments, Lanham, MD USA 3) Science Applications International Corp., McLean, VA USA High Average Power Laser Conference Washington, DC Oct , 2007 Development of a Large Area, Durable Electron 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 <10 3 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 strip emitters w/o metallic edge reducers Reduce average electric field on non-emitting structures Build full-sized cathodes w/o metallic frame Reduce gas formation w/o compromising uniformity Find more robust emitter w/o compromising uniformity Investigate coat- ings/surface treat- ments that prevent emission. Temperature management capability High efficiency cathode capable of many shots at PRF up to 5 Hz

Upper Path: A strip cathode geometry can be created by using a high permeability material like 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 steel bars was used to test the concept 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 spaced 4.4 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 steel bars were then tested with different coatings (porcelain, various powder coats, SiO x 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 strips graphite 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, E 0. 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 emitterCordiertie 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 ceramic honeycomb -40  30 ns-25  45 ns80  150 ns 20  90 ns -10  60 ns -25  45 ns velvet 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 10 6 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  m before the foil fails completely. typical “splat” damage pattern pinhole damage from 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 of a “hot rock” possible exfoliation

Coating the honeycomb tiles with SiO 2 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-coated SiO 2 coated SiO 2 + 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 SiO 2. 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 Torr. Unaffected by temperatureShows promise for

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 steel bars with SiC coating Individual strip emitter ceramic screw mounts contoured CC- type C primary emitter cooled mount advanced tile coatings