Center for High Energy Density Science: Texas Petawatt Laser

University of Texas - Austin

The Texas Petawatt (TPW) Laser facility has been the flagship experimental capability of the Center for High Energy Density Science for nearly a decade for high energy density experiments.  The performance of the machine is now state-of the art enhanced by a year-long $1.5M upgrade that took place in 2015.  The TPW can deliver laser pulses with peak powers exceeding 1 PW to two target areas with distinctly different focusing geometries.  The laser has been in near-continuous operation since 2009 and executed over 45 experimental campaigns.

The TPW system delivers pulses as specified in Table 1 (generally pulses with energy up to 140 J with 140 fs duration). It is internationally unique by delivering pulses with 1-2 orders of magnitude more energy than typical Ti:sapphire based systems, and 3-4 times shorter than those of other Nd:glass based CPA lasers.  This unique high energy, short pulse capability has been accomplished by combining two technologies: (1) high gain amplification on the front end by optical parametric chirped pulse amplification (OPCPA) to retain broad bandwidth up to the 1 J level at center wavelength near 1057 nm and, (2) mixed Nd:glass amplification in which boosting the pulse to ~160 J (before compression) is accomplished using amplifiers with both phosphate and silicate laser glasses whose peak gain wavelengths are shifted from each other allowing a net amplification of more bandwidth and hence shorter compressed pulses.

The TPW laser has been built in a 5000 sq. ft. high bay facility in Robert Lee Moore Hall on the University of Texas campus which houses clean room facilities needed for the TPW laser chain, as well as radiation shielded target areas with controlled access, and a control room for remote operation of the lasers.  The layout of these elements in the underground highbay is illustrated in Fig. 1.

TPW high bay      

Fig. 1:  Current TPW Facility Showing locations of vacuum chambers housing the compressor, TC1 and TC2 beamlines.  Clean room roof and parts of the radiation shield walls are hidden to allow a clear view.

In 2015, the TPW laser was rebuilt to improve temporal pulse contrast, pulse duration, and focusing.  The new TPW design eliminated all large lenses and replaced them with off-axis parabolic mirrors to eliminate discrete prepulses arising from back reflected pencil beams.  The front end was changed to achieve six orders of magnitude gain in a picosecond OPCPA stage prior to full pulse stretching in order to reduce parametric fluorescence and improve the contrast with respect to prepulses 100-ps to 4-ns before the arrival of the main pulse.  Overall the temporal contrast improved by 3 orders of magnitude and it is ~109:1 outside of 100 ps range and1011:1 outside of the ns range. The pulse duration has been reduced by 10%. The laser chain has a 65 mm aperture deformable mirror (DFM) installed at the turn around point of a 64 mm silicate glass rod to correct aberrations imposed by the laser chain.  During the 2015 upgrade a second, 250-mm aperture, DFM was installed at the end of the amp chain in the compressor vacuum chamber.  This mirror has a higher spatial frequency (52 segments) than the DFM that follows the rod amplifier and eliminates the need for strong wavefront correction in the amplifier chain.  The result is an excellent focal spot quality at the 1 PW level.  A Strehl of over 0.75 is achieved on half of the shots.  An f/1.1 focusing parabola yields focused intensity in excess of 2 x 1022 W/cm2.

Two Target Areas

The TPW pulse can shoot into either of two target chambers shown in Fig. 2.  The first target chamber, TC1, normally employs an f/3 dielectric-coated off-axis parabolic mirror that allows the full energy to be focused to >1021 Wcm-2.  Recently, an f/1.1 optic plus diagnostic and focusing system has been added as a focusing option for TC1, reaching a peak intensity of 2x1022 W/cm2.  The second target chamber, TC2, has a unique f/40 focusing geometry.  The focusing mirror has a 10-m focal length and is positioned well outside the target chamber.  This allows a weak focus, extended high intensity interaction lengths, and a large interaction region for experiments such as wakefield electron acceleration, cluster fusion, magnetized shocks, isochoric heating, and high harmonic generation.  Since the focusing optic is outside the target chamber unlike most other systems, there is more flexibility in the choice of the target chamber configuration.

TC1 target chamber  TC2 target chamber

Fig. 2: (left) TC1 target chamber with an f/1.1 off-axis parabolic mirror that can achieve 2x1022 W/cm2 intensities.  (right) TC2 target chamber system with f/40 focusing geometry.

Probe Beams and Other Diagnostics

An independently compressed probe beam line is employed on the TPW, created by extracting 10% of the OPA pulse energy using a beamsplitter.  The beam is downsized and relay-imaged along one of two vacuum telescope lines depending on the final target chamber, toward either TC1 or TC2. The beam is independently compressed with an in-air compressor, allowing for the probe pulse to have a user-defined chirp.

The TPW laser is very well characterized, with over 20 optical diagnostics (nearfield, farfield, spectra, 2nd and 3rd order autocorrelation, wavefront, etc.), multiple energy outputs, current traces of power amplifier PFNs, and pump beam pulse profiles registered automatically on each shot.  An extensive system of data capture is well-established and firing of the laser and all operation of the aligned system can be achieved from the control room.

Texas Petawatt

Parameter Value Unit Additional Information Comments
Center Wavelength 1057 nm    
Pulse duration (I FWHM) 140 fs    
Max energy on target 155 J    
Shot energy stability 8 % rms  
Focal spot at target        
F/number f/3      
intensity FWHM 3.9 μm    
Strehl ratio .7      
F/number f/1.1      
intensity FWHM 2.2 μm    
Strehl ratio .9      
Energy containment 75 % within 2  μm radius  
F/number f/40      
intensity FWHM 60 μm    
Strehl ratio .7      
Pointing Stability 7 μrad rms  
Pre-pulse contrast        
ns scale 10-11   @ 1 ns    
ps scale 10-9   @ 40 ps   
Repetition Rate 1 hr-1