An H- stripping laser using commercial, diode-pumped Nd:YAG amplifiers Russell Wilcox Laser Stripping Workshop, April 11, 2011

This laser scheme was proposed at the last workshop Assume 1 micron operation (high energy H- beams) –Frequency multiplication could be added A buildup cavity would be used, with ~120x enhancement –Peak power goal is 6MW in buildup cavity SNS reprate and pulse length chosen as an example –10ps micropulse at 400MHz, 1.2ms macropulse at 2Hz This work reports success in testing the 1 micron laser –450W average during 1.2ms pulse (112.5kW peak, ~2x spec) –Saturation characterized, is the limiting factor for increased power –Good beam quality, no transient thermal lens Further testing with a cavity will verify adequacy of the laser modelocked laser chopperamplifier buildup cavity (6MW) 1W0.5W 200W 50W 0.5x100x 4x 120x (250W)(50kW) 24kWaverage in 1ms (peak in 10ps)

Optical schematic Second amp was configured for possible double pass oscillator variable attenuator modulator amp 1 amp 2 isolator half wave plate quarter wave plate 2x telescope diagnostics E camera 1mm beam, 2mm rod 2mm beam, 4mm rod Oscillator85k Amp 125k Amp 250k Other parts40k Total200k

Equipment Power supplies maintain “CW” power level during the pulse, which is much longer than an energy storage lifetime High current from capacitors, which limit the total pulse length power supplies chillers

Electronics and controls The modelocked oscillator is not synchronized, though it has the hooks –Easy to do, commercial box or home made 10ps pulses at 400MHz from laser oscillator, chopped to 1.2ms macropulse –10ps pulse phase within macropulse is random –Laser amplifier time constant is 230us 78MHz oscillator RF modulator pulse generator delay generator RF amplifier A-O modulator laser amplifier power supplies laser amplifier laser oscillator trig 1.2ms 400MHz 2Hz 1.4 or 1.5ms

Energy vs pump current for two stages 451 Watts, 0.54 Joules –11,250W in 10ps Operating point chosen at 250W –80A on both amplifiers Fluence: 17J/cm^2 Intensity: 3.6MW/cm^2 Calculated nonlinear phase shift: ~10^-4 radians stage 1 single pass stage 1 double pass stage 2 single pass 451W 10.4W 79.6W

By adjusting timing, the pulse can be made flat Early, the amp is still “charging” the YAG rod Late, the rising edge is seeing the unsaturated pulsed gain Since these processes are related by the Nd lifetime, they can cancel Small overall droop may be due to capacitors discharging +30us -40us op. point

First stage saturation (80A) From the fit, one finds Go and Isat –OK for G>>1 So far, fits give Isat ~2.5x too large –Same for second stage linear data fit data fit cross sectionlifetime photon energy

Second stage saturation (80A) Linear gain based on Go from fit Simple fit breaks down at low gains –Also, beam shape may be changing, not accounted for linear data fit data fit

Thermal lens appears at higher average power, but no transient lens seen If a lens develops during the pulse, it will focus more power into the diode At 2Hz, no power variation was seen during the pulse 2Hz 3.3Hz 5Hz 10Hz oscilloscope With increasing reprate, the higher average power resulted in focusing At 10Hz, energy per pulse dropped to ~80% of the 2Hz value Some features in profile are interference in attenuating filters amplifier aperture photodiode

Why saturation increases beam size When second amplifier was activated, beam appears to expand Same level of gain and saturation applied to Gaussian “expands” beam This distortion may not make much difference to the cavity

Conclusions, suggestions Laser works better than expected, makes >100kW in 10ps –Try double pass last amp for 1J, 1kW in 1ms, since there’s gain left –At 162.5MHz, 10ps, 450W average is 277kW peak, requiring only 22x buildup cavity for 6MW peak (not including losses) We will build a cavity and control system to demonstrate usable peak power –If this works, it’s a ready solution for 1 micron (PX) Could be a front end for a higher power system (SNS) –Assume 10x loss in conversion to UV –Need 10x 1micron power, or 10J, 10kW –10J diode-pumped slab amplifier exists –10kW diode-pumped YAG laser exists –With 10kW average, 400MHz, 10ps, 10x UV loss, peak power is 250kW –Only 24x cavity enhancement needed for 6MW –Enlarge cavity for 4 pulses, maintain finesse 100, etc.