MIT Plasma Science Lab Develops NIF Diagnostics
A typical NIF experiment is over in a few billionths of a second. Obtaining meaningful information about what occurs during this extremely brief time period, in and around a tiny target, has required the design and development of a new breed of detectors, cameras, and other diagnostic instruments, many of which have been created through partnerships with universities and national laboratories.
One of these institutions, the Massachusetts Institute of Technology (MIT) Plasma Science and Fusion Center, has played a key role in developing, testing, and calibrating diagnostic equipment for NIF. Among the diagnostics tested and calibrated by the center's Ph.D. students and staff members are three devices designed to measure the spectrum and timing of neutrons and protons.
LLNL Senior Research Scientist Richard Petrasso's group at the MIT center has been creating detection techniques for inertial confinement fusion (ICF) experiments for more than a decade. One important diagnostic for National Ignition Campaign experiments, the magnetic recoil spectrometer (MRS), came about through a successful collaboration between MIT, which designed the device; the University of Rochester's Laboratory for Laser Energetics (LLE), which built it; and LLNL, which provided the setting for fielding the device.
The MRS is used to study the neutron spectrum from an implosion by measuring proton (or deuteron) energy produced in a collision with the neutrons. According to Rich Zacharias, lead engineer for NIF's nuclear diagnostics, "MRS is a critical diagnostic for measuring the energetics or yield and the density of the imploded targets. These are key components of the figure of merit to quantify how well the shot is approaching ignition conditions."
During fusion experiments, the imploding target releases neutrons. For the MRS technique, neutrons collide with protons in a plastic (CH) or deuterated plastic (CD) foil near the target chamber wall, producing protons of equivalent energy traveling in the same direction. These charged particles are then bent by a magnetic field after they travel through an aperture in the Target Chamber wall. The angle at which they bend depends on the energy of the protons, which is a measure of the density of the imploded target.
The stream of protons strikes an array of plastic coupons (detectors) and causes tracking. To study the resulting proton tracks, this array of plastic coupons must be developed using an acid etching technique and scanned with a scanning microscope. Scientists can determine the energy distributions of the neutrons emitted from the target by the density of the tracks distributed across the coupons. The neutron spectrum provides important information about plasma parameters such as ion temperature, density, and velocity. MRS's accuracy also makes it useful for calibrating other neutron diagnostics.
Other effective ways of measuring what happens in and around the target capsules developed by the MIT team include the wedged range filter (WRF), which measures the spectrum of proton energy using a wedge of filter material placed in front of plastic film that is processed using a similar method to the MRS; and the particle-time-of-flight (PToF) diagnostic, which helps determine the timing of charged particles using a diamond detector that responds to neutrons and protons.
MIT scientists have helped NIF build a processing facility for developing the MRS film, and they are actively involved in performing shot data analysis, reporting results, and troubleshooting, as well as enhancing and supporting the equipment they designed. About half a dozen MIT physicists and doctoral and post-doctoral students have an ongoing role in NIF diagnostics and are regularly on site performing research and analysis