(Left) NIF’s Don Jedlovec captured this spectacular color-enhanced photo of glowing plasma during an Oct. 2 NIF experiment. One megajoule of laser energy irradiated a convergent ablator shock-timing target as it was being measured by eight wedge range filters (WRFs) and a DIM imaging streak camera, or DISC, mounted on diagnostic instrument manipulators (DIMs) inserted into the NIF Target Chamber. Four WRFs are at the pole (top) and four at the equator (lower right), along with the DISC. The WRF units are about 50 centimeters from the imploding target held by the target positioner (TarPos). A shot-triggered, two-megapixel monochrome Basler 641f science camera with a 16mm lens was used to capture the time-integrated image. (Right) CAD image by Paul Bloom shows the configuration of the equipment for the experiment.
(Download Image)MIT’s Mighty-Mite Detectors Prove Their Mettle
One of the key elements in a successful NIF ignition experiment is maintaining the symmetry of the fusion fuel’s areal density as it implodes. Areal density, known as ρR (rho-R), is the combined thickness and density or “stiffness” of the fuel during the implosion. A small but extremely useful diagnostic for measuring ρR symmetry as the fuel is compressed prior to reaching maximum density is the wedge range filter (WRF) developed at the Massachusetts Institute of Technology (MIT).
Used on both NIF and the OMEGA Laser Facility at the University of Rochester, WRFs are precision compact proton spectrometers employed to diagnose ρR asymmetries at “shock flash bang time,” when the fuel has converged (been compressed) to about four times smaller than its initial diameter. The WRF is one of four diagnostic devices created at MIT designed to measure the spectrum and timing of neutrons and protons released during NIF implosions.
Commonly used on all deuterium and deuterium-helium3-fueled NIF implosions, the WRF proton spectrometers, designed by MIT’s Fredrick Seguin, measure the spectrum of proton energy in the range from four million to 20 million electron volts (MeV) using a wedge-shaped aluminum filter placed in front of plastic film called CR39. The WRFs measure the energy downshift of the protons as they leave the implosion to determine the implosion areal density at the shock flash bang time, which happens several hundred picoseconds before the main compression fusion burn.
The energy spectrum of the escaping protons is measured with CR-39 track detectors after they pass through various parts of the wedge. “CR-39 is a special plastic film that is cut up into small ‘coupons’ and used in nuclear diagnostic systems,” said NIF User Facility Manager Doug Larson. “The process was perfected by MIT scientists led by Rich Petrasso (head of the HED physics division at the MIT Plasma Science and Fusion Center).”
The material currently is used in three NIF diagnostics: the WRFs; their even-smaller offspring, the step range filter (SRF) proton spectrometers; and the Magnetic Recoil Spectrometer (MRS), also developed at MIT in collaboration with the Laboratory of Laser Energetics (LLE) at the University of Rochester. All three diagnostics were developed and tested at LLE before fielding on NIF. (The particle-time-of-flight diagnostic, which helps determine the timing of fusion reaction particles using a diamond detector that responds to both protons and neutrons, is the fourth MIT diagnostic used on NIF.)
The nuclear particles leave “tracks” in the CR-39 plastic as they pass through it, and these tracks can later be revealed by etching in a strong base solution. A scanning process counts and characterizes the tracks that were revealed during etching. “The number and diameter of the tracks tell the physics team about the number and energy of the particles,” Larson said. The technique yields valuable data about the dynamics of the final strong shock in a NIF implosion prior to the full compression of the capsule ablator.
The operations and upgrades of the accelerator used to calibrate the WRFs are done by MIT PhD students and postdocs at the Plasma Science and Fusion center under the direction of Maria Gatu Johnson. PhD student Alex Zylstra and former student Mike Rosenberg, who recently received his PhD, are directly involved in working with WRF results. The WRFs are processed in LLNL’s Etch/Scan laboratory in Bldg. 490 by Michelle Valadez under the supervision of Richard Bionta, the physicist for nuclear diagnostics. Minda Cairel of Laser Systems Engineering also has served in the lab.
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