Accelerator Test Facility

Laser Systems

Marcus Babzien, BNL

Outline

· Nd:YAG system: history, current performance, upgrades

· Future goals

· GW CO₂ performance

· TW CO₂ project: history, status

Nd:YAG Improvements

Past Activity

· New class 10,000 clean room adjacent to gun with dedicated lab space

· Replaced Nd:YAG oscillator with newer Nd:YVO₄ oscillator

· New SHG crystals and Pockels cell for smoother beam profile

· Improved thermal monitoring and regulation on table

· Identified and corrected alignment instability before preamp

· Improved diagnostics for harmonic generation and gun hutch

Current Status

Capabilities

I Photocathode and CO₂ slicing fully available on-demand

I Electron beam-synchronized optical pulses available for users:

5 mJ, 14 ps @ 1064 nm in laser lab (exclusive of slicing)
50 mJ, 10 ps @ 532 nm in laser lab or FEL room (not implemented)
50 mJ, 8 ps @ 266 nm in gun hutch and laser lab

I Delivered light on 112 of 130 days since October 1, 1999 (~86% of available time), for a total of 1300 running hours.

I System typically ready for gun operation within 15 minutes, including data collection.

Demonstrated YAG Laser Performance

Energy (dual pulse mode)
UV on cathode	0-30 mJ
IR at CO2 table	7 mJ
Laser output: total IR	30 mJ
IR into 2w	5 mJ / pulse
Green	1 mJ / pulse
UV	200 mJ
Repetition rate	1.5, 3 Hz
Pulse duration (FWHM):
Oscillator IR	7 ps
Amplified IR	14 ps
Green	10 ps
UV	8 ps
Beam on cathode (FWHM)	0.2 - 3 mm
Profile Uniformity (P-P)	<20%
Shot-to-shot stability (rms):
Timing	<0.2 ps
Energy	2 %
Pointing (fraction of beam )	<1%
Drift (8 hour P-P)
Timing	<2 ps
Energy	<20 %
Pointing (fraction of beam )	<5%

New Nd:YVO₄ Oscillator

Improved stability in both energy, phase, and pointing

High reliability reduces maintenance - realignment should be unnecessary

Old oscillator provides emergency backup in case of failure

	LWE-131	GE-100
Pulse duration (FWHM)	20 ps	7 ps
CW power	~65 mW (currently)	500 mW
Amplitude stability	<0.3% (10kHZ-10 mHz)	<0.2%
Pointing stability	< 50 microradian	<30 microradian
Phase stability	<1 ps p-p (minutes) < 1ps/hour drift	<0.3 ps p-p (minutes) < 0.5ps/hour drift
Estimated laserdiode lifetime	> 30,000 hours*	>10,000

*-has operated 36,000 hours

Diagnostics

Continuously on-line:

I Scanning autocorrelator for oscillator monitoring

I 14 CCD cameras for transverse mode and position measurement with beam profile analyzer*

I 5 pyroelectric energy measuring probes with pickoffs and calibrated readout*

I High speed silicon photodiodes

I Laser-RF Phasemeter*

I 16-channel thermocouple temperature monitoring

Additional:

I 2 ps resolution streak camera (Instrumentation Div.)

I CW laser power meter

I 2 GHz digital sampling oscilloscope

* - output available facility-wide

Performance Tests

Pulse Contrast After Cathode (March 99)

Oscillator Upgrade

GE-100 Phase Stability

· CW phasemeter (DC-10 Hz bandwidth) shown over 4 minutes

Pulse Durations

IR after amplification

Pulse duration 14±2 ps FWHM

Green before quadrupler

Pulse duration 10±2 ps FWHM

Streak Camera Results

UV in laser room

Pulse duration 8±2 ps FWHM

Future Prospects

Short Term

I Complete transition to Nd:YVO₄ oscillator

I Complete projects for transverse beam shaping

I Improve temporal diagnostics for all wavelengths

I Implement pulse shortening

I Improve passive stability or implement active feedback where required

I Improve imaging in gun hutch

Future Prospects

Long Term

Significant changes to laser system must not result in significant facilty-wide shutdown ® develop second drive laser in parallel

Optical synchronization between electrons and laser offers increased advantage as CO₂ pulse duration decreases ® retain "single-laser" philosophy

Reliable oscillators now available in bulk and fiber with phase feedback ® freedom to choose gain medium for amplification

Arbitrary longitudinal shaping of photocathode pulse requires gain medium with larger bandwidth (~100 fs) for "pulse stacking" ® Nd:YAG not optimum

Direct diode-pumping should be used for passive stability, reliability ® Ti:Al₂O₃ not optimum

Saturation mechanisms, active feedback should be planned from start

Beam Quality

Cathode Monument

266 nm

Radial Beam Shaping

Gaussian Reflector

Radial Beam Shaping

Gaussian Reflector

Radial Beam Shaping

Variable Intensity Filter

f=155, q=45

f=145, q=35

f=115, q=115

f=15, q=5

SHG Crystal Distortion

Imaged at Photocathode Conjugate Plane

Temporal Shaping

Saturable Absorption

Saturable Absorber

Testing with ATF laser

Temperature Control

On-table Monitoring

TW CO₂ Status

Progress

I System accepted after on-site work by vendor

I Pressure vessel modified, safety reviewed again to ensure compliance

I Discharge at 8 ATM optimized, gain demonstrated

I Pulse chopping to 30 ps tested, measured with GW seed

I GWÛTW transport line completed

I Safety improvements: exhaust, sheilding, plumbing

I Simulations completed

CO₂ Pulse Shape

From Compton Experiment

TW CO₂ Future Work

Towards Commissioning

I Complete final safety documentation

I Reassembly of amplifier pressure vessel, optics, plumbing

I Re-establish discharge conditions (Optoel)

I Provide seed pulse

I Test output, optimize

I Deliver 30J!!!

CO₂ System Upgrades

I Oscillator replacement: parametric generation from YAG

I Increase gain bandwidth of GW preamplifier

ß ß

isotopes 10 ATM vessel

I Compression to shortest possible pulse duration