Phoebe Rounds

April 2003

Many of LLE's Summer High School Program participants go on to submit their research findings to various scientific competitions. Of these competitions, the Intel Science Talent Search is generally considered the most prestigious. This year Phoebe Robeson Rounds was selected as a finalist in this competition for research she performed while participating in LLE’s 2002 Summer High School Research Program. As a finalist in the national Intel Science Talent Search, Phoebe was awarded a $5000 scholarship, a high-performance computer, and an all-expenses paid trip to Washington, DC to compete against 39 other finalists for additional scholarships up to $100,000. Irondequoit High School will also receive $1000 to support its math and science programs as a result of Phoebe’s nomination. Phoebe is the second student from the LLE program to advance to the finals in this nationwide competition.

Original doubler-tripler configuration utilized on the 24-beam OMEGA laser system. This technology led to higher energy absorption by the targets.

Last summer, Phoebe developed a systematic method for generating new tripler designs for lasers such as OMEGA. High-powered laser fusion systems utilize crystals known as doublers or triplers to convert laser beams from their initial infrared (IR) state to the higher-frequency ultraviolet (UV) beams necessary for fusion. (The frequency of UV light is three times that of IR light; therefore, the frequency is "tripled.") Phoebe’s new design would use additional tripling crystals in the frequency-conversion portion of OMEGA to increase the bandwidth and thus the smoothness of the laser beams. "Smoother" or more-uniform laser beams maximize the effectiveness of target implosions.

Two frequency-conversion crystals mounted in the OMEGA laser system. The optic sits in a gimbal allowing the tilt to be adjusted so that optimum frequency conversion can be achieved.

OMEGA uses neodymium (Nd)-doped glass to generate its laser pulses. Nd-glass lasers emit beams in the infrared range of the spectrum. These beams are ineffective for inertial confinement fusion (ICF) research because a large fraction of the energy absorbed by the targets is converted to fast electrons which detrimentally preheat the target. Therefore, all Nd-doped glass laser systems utilized for ICF research employ some method of frequency conversion to convert the beams to the more-efficient UV wavelength. OMEGA’s beams are passed through at least two potassium dihydrogen phosphate (KDP) crystals—"doubler" and a "tripler"—to perform the conversion to UV. The original frequency-conversion crystal design for OMEGA involved only one tripler crystal. While this design was efficient, a wider bandwidth of laser light was desired to improve the effectiveness of smoothing by spectral dispersion (SSD), a technique developed by LLE and implemented on OMEGA for the creation of smoothly focused laser beams. A second tripler was added to each beamline of the system in 2000 to increase the bandwidth from 0.3 to 1.0 THz in the UV (4 Å to 12 Å in the IR).

For the current 60-beam OMEGA laser system, the optimum four-tripler design would increase bandwidth by a factor of 2 to 3.

Because even broader bandwidth on OMEGA is still desirable, it was wondered if additional tripling crystals could broaden OMEGA’s bandwidth yet again. Phoebe modified a previous summer high school program participant’s code to accommodate multiple-tripler designs and then developed a systematic method for creating broadband designs for any number of triplers. Her method involves the optimization of tripler thicknesses, tilts, and spacings. Phoebe used this method to generate designs for three, four, and five triplers, designs that can in theory significantly broaden the conversion bandwidth on OMEGA. Her four-tripler designs would convert an IR bandwidth of 30 Å to 2.5 THz in the UV with no loss in conversion efficiency. In other words, Phoebe’s design creates a broader bandwidth without losing beam energy. Her multiple-tripler designs could be implemented on OMEGA or any other high-power laser system to generate smoother laser beams.

Proposed configuration (from left to right). The infrared beam passes through the doubler crystal where 67% of the IR energy is converted to green energy. The triplers that follow combine the green and remaining IR energies to generate ultraviolet energy.