The OMEGA Magnetic Recoil SpectrometerApril, 2009
The Magnetic Recoil Spectrometer (MRS), a novel neutron spectrometer under development since 2001, was installed on OMEGA in late 2008 to assess implosion performance. In collaboration with the Massachusetts Institute of Technology–Plasma Science and Fusion Center (MIT–PFC), the MRS provides absolute measurements of the neutron spectrum in the range 5 to 30 MeV, from which fuel areal density (ρR), ion temperature (Ti), and neutron yield (Yn) can be determined. Much of the research and development, as well as the instrument optimization of the MRS on OMEGA, is directly applicable to the implementation of the MRS at the National Ignition Facility (NIF). In particular, the same precise magnet, with minor adjustments, can be used on either OMEGA or the NIF. Measurement of fuel areal density (ρR) at the NIF will be essential for assessing the success of implosions relative to simulated results during all stages of development; from warm diagnostic shots to cryogenic ignition-failures to, finally, fully igniting capsules.
The MRS is comprised of three basic components. The first component is a plastic foil (chemically composed of either CH or CD) positioned very close to the implosion (10 cm from the target on OMEGA and 26 cm at the NIF) to produce recoil protons (or deuterons) from incident neutrons. The foil must be positioned as close as possible to the implosion so that:
- the detection efficiency for a defined energy resolution is maximized
- the number of incoming neutrons, at the foil, that have scattered off the target chamber and internal structures is negligible in comparison to the number of down-scattered neutrons coming from the implosion.
The second component of the MRS is a focusing magnet, located outside the target chamber (both at OMEGA and the NIF), used for energy dispersion and for focusing forward-scattered recoil particles onto a detector plane. The focusing of the recoil particles permits a clear mapping between their position in the plane and the energy of the scattered protons or deuterons, and consequently the energy of the neutron that scattered them. The third component is an array of detectors using a unique plastic (CR-39) as a sensitive charged particle recording media. These detectors are positioned at the focal plane of the spectrometer, which record the energy of each recoil particle with a detection efficiency of 100%. Options are available for configuring the MRS even after the component positions are set. The foil composition establishes whether proton or deuteron recoils are used and, as such, allows the foil area and thickness to be adjusted to change the energy resolution and detection efficiency.
In December 2008, the MRS accurately measured the spectrum of scattered neutrons from a high-yield cryogenic deuterium-tritium (DT) implosion on OMEGA. The spectrum of <14-MeV neutrons produced by elastic scattering and (n,2n) reactions of 14-MeV neutrons in DT fuel was used to infer a burn-averaged fuel areal density. This first-ever measurement of the scattered neutrons from a cryogenic-DT implosion (carried out in collaboration with MIT–PSFC) validates the MRS technique, which will form the basis for diagnosing the areal density in the early implosion campaigns on the National Ignition Facility.