Around the Lab
The Multi-Terawatt Laser at LLESeptember, 2011
The Multi-Terawatt laser (MTW) is a significant research tool supporting the University of Rochester’s commitment to advancing research in the physics of x-ray and high-energy, directed-particle beams and ultrafast laser science at the Laboratory for Laser Energetics.
Originally built to demonstrate key laser and diagnostic technologies required for the OMEGA Extended Performance (EP) Laser Facility, the MTW provides an experimental facility to perform large-area damage testing and intermediate-scale target experiments with picosecond laser pulses. University of Rochester graduate students and staff within LLE as well as faculty, staff, scientists and students from external institutions extensively use this university-scale facility. Participants in LLE’s Summer High School Research Program also benefit from access to the MTW with the goal of engaging these students in a state-of-the-art environment exposing them to research and learning about careers in science and technology.
The MTW, shown schematically in Fig. 1, is a single-beam, hybrid chirped-pulse–amplification laser system. It combines optical-parametric amplification (OPA) with neodymium (Nd)-doped laser-glass amplification to produce compressed output-pulse energies limited only by the damage thresholds of the gold gratings in the vacuum compressor. The 10-J maximum output energy in a <1-ps-length transform-limited pulse yields peak powers in excess of 10 TW. An ~f/3 off-axis parabolic mirror provides nearly diffraction-limited focusing to ~4-µm (FWHM) spot sizes that yield intensities on target up to 4 × 1019 W/cm2.
A veritable workhorse, the MTW can access experimental regimes beyond those typically reached by conventional tabletop laser systems without requiring the cost and schedule overhead associated with large-scale facilities. The high shot rate and availability of this flexible, experimental platform facilitates parametric studies with the good statistics required to study important physics issues. It also allows novel target experiments to be tested before they are transferred to large-scale lasers like OMEGA EP. Examples of important experimental features conducted by the MTW include:
- high-temporal-contrast laser enhancements and diagnostic development, enabling solid-target experiments
- high pointing stability makes it possible to use small, low-mass targets (Vtarget ≈ 10–6 mm3)
Target-diagnostic development on MTW has included time-integrated Kα photon spectroscopy, high-resolution x-ray crystal imaging, time-resolved x-ray spectroscopy, high-energy ion-emission analysis, and optical-transition-radiation diagnosis. Each diagnostic has been a successful platform for diagnostic development in MTW experiments and for future deployment on OMEGA EP.
One example of MTW target experiments is time-resolved Kα spectroscopic measurements, which have been used to infer the hot-electron equilibration dynamics in high-intensity laser interactions with thin-foil solid targets.1 Figure 2 shows a schematic of the experimental setup. The MTW delivered 1- to 10-J, 0.5- to 1-ps pulses at a wavelength of 1.053 µm on-target intensities in the range 1018 to 1019W/cm2. Kα radiation emitted from the target was measured with an x-ray streak camera with 2-ps time resolution coupled to an HAPG (highly annealed pyrolytic graphite) crystal spectrometer. The toroidally curved HAPG crystal collected x-radiation in the energy band of 7.8 to 8.5 keV. This spectral range covers the 2p → 1s transition in Cu, allowing time-resolved Cu Kα measurements at 8.05 keV.
Figure 3 shows typical time-resolved data obtained in the experiment. The data show Kα signals from 500 × 500 × 20-µm3 Cu foils irradiated at focused intensities of 1.5 × 1018 W/cm2 (dashed line) and 1.1 × 1019 W/cm2 (solid line). These are the first experiments at relativistic laser intensities to show few-picosecond hot-electron relaxation times with Kα-emission periods up to a factor 4× shorter than in previously reported experiments.2
An energy upgrade to the MTW is underway that will push it to the 100-J/100-TW level and increase visibility of the system as a facility for basic high-energy-density (HED) science and foster new discoveries in intense laser–plasma interactions. The upgrade involves adding an additional Nd-doped glass laser-rod amplifier between the OPCPA system and the Nd:glass disk amplifier, and installing multilayer-dielectric (MLD) diffraction gratings with higher damage thresholds in the vacuum compressor to increase the system-compressed output energy. The MTW energy upgrade will also provide the high-energy pump (~100 J at 1053 nm) for an OPA line (OPAL) that will deliver multi-Joule (>5 J), 15-fs compressed pulses. This novel ultrafast optical source will enable advances in high-fluence radiation sources, high-energy particle acceleration, and proton radiography. Ultimately, this work will pave the way for an all-OPA system pumped by the OMEGA EP laser that will be capable of exploring laser-induced nuclear physics and nonlinear quantum electrodynamics experiments, such as pair-plasma production. These areas offer exciting opportunities to leverage existing university-scale facilities and to attract the next generation of laser scientists and HED plasma physicists.
1P. M. Nilson et al., in preparation.
2H. Chen et al., Phys. Rev. E 76, 056402 (2007).