|Pulse duration (I FWHM)||140||fs|
|Max energy on target||120||J||Energy currently limited by laser damage considerations|
|Shot energy stability||5||%||rms|
|Focal spot at target|
|ns scale||10-11||@ 4 ns|
|ps scale||5x10-8||@ 100 ps|
Center for High Energy Density Science: Texas Petawatt Laser
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 yearlong $1.5M upgrade that took place in 2015. The TPW can deliver laser pulses with peak powers of 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 can deliver pulses as specified in Table 1. During the April 15 to June 27, 2019 period the laser delivered mean laser pulse performance parameters of Energy = 121 ±4 J, Duration = 137 ±11 fs, and Strehl Ratio = 0.66 ±11, which is typical performance. 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 ~140 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 resides in a 5000 sq. ft. high bay facility in Robert Lee Moore Hall on the University of Texas campus. The facility has 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 shown in Fig. 1.
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.
The TPW laser was rebuilt in 2015 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 pre-pulses arising from back-reflected pencil beams. The front end was redesigned 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 pre-pulse contrast in the regime 100-ps to 4-ns before the arrival of the main pulse. The temporal contrast improved by three orders of magnitude and it is ~5 x 10-8 beyond 100 ps range and <10-11 beyond 4 ns. The laser chain has a 65 mm aperture deformable mirror (DFM) installed at the turn around point of a 64-mm silicate glass rod amplifier 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 amplifier 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 0.7 is usually achieved on half of the shots. An f/1.1 focusing parabola yielded a focused intensity in excess of 2 x 1022W/cm2, but this mirror is not generally available to LaserNetUS experiments.
Two Target Areas
The TPW pulse can fire 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 >1021Wcm-2. The second target chamber, TC2, has a unique f/40 focusing geometry. The focusing mirror has a 10-m focal length and positioned well outside the target chamber. This creates a weak focus with long high intensity interaction lengths and a large interaction region which is ideal for many experiments such as wakefield electron acceleration, cluster fusion, magnetized shocks, isochoric heating, and high harmonic generation. Since the TC2 focusing optic is outside the target chamber, unlike most other systems, there is more flexibility in the choice of the target chamber configuration.
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, towards either TC1 or TC2. The beam is independently compressed with an in-air compressor, allowing the probe pulse to have a user-defined chirp.
The TPW laser is 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. Near field and focal images, autocorrelation images, and shot energy are registered automatically on each shot and immediately available to the user. An extensive system of data capture and storage is well-established and firing of the laser and all operation of the aligned system can be achieved from the control room.