The Texas Petawatt (TPW) Laser facility has been the flagship experimental capability of the Center for High Energy Density Science for over 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, 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 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.
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.
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 is 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.
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.
For additional information, see our webpage: http://texaspetawatt.ph.utexas.edu/
Facility Manager: Dr. Sandra Bruce, firstname.lastname@example.org
Target Area Scientist: Dr. Hernan Quevedo, email@example.com
|Pulse duration (I FWHM)||135||fs|
|Max energy on target||130||J|
|Shot energy stability||5||%||r.m.s.|
|Focal spot at target|
|ns scale||10-11||@ 4 ns|
|ps scale||5x10-8||@ 100 ps|
The Glass Hybrid OPCPA Scale Testbed (GHOST) laser was designed as a small-scale version of the TPW – with the same oscillator characteristics, OPCPA front end, and mixed glass amplification at higher energies. This may be used by experimentalists for whom TPW would be well above their necessary or desired on-target intensity, or who would like to perform a proof-of-concept or smaller scale test experiment in preparation for a later TPW campaign. GHOST boasts a 1/min repetition rate, which can yield statistical data in experiments that TPW is less able to achieve. The specifications for GHOST are shown below. Note: GHOST may be run with or without its final glass amplifier rod, yielding different output pulse characteristics.
In a collaboration with Sandia National Laboratories, CHEDS has available a 200 kA (400 kA planned in a future upgrade) microsecond-scale pulsed power device capable of producing magnetic fields of over 10 T (>20 T after upgrade) for pre-magnetized high energy density plasma experiments.
As part of a major renovation supported by the University of Texas, CHEDS is in the process of developing a support facility, dubbed the Metrology Room, for cleaning, characterization, and staging of optics and laser targets. With a target completion date of mid-September 2021, the facility will boast a cleanroom-like environment with filtered positive pressure air in the entire space, as well as two HEPA blower units over wet and dry table stations within this room. Ultra-pure, type 1 deionized water will be available on demand from an installed filter/deionizer unit. A temperature-controlled ultrasonic bath cleaning unit and various other cleaning/inspection equipment will also be at hand. Two optical tables will provide space for inspection and staging of optics and laser targets.