A Time-Resolved High-Resolution X-ray Spectrometer for NIF

An unusual collaboration among U.S. Department of Energy (DOE) laboratories pursuing different approaches to nuclear fusion has produced the first time-resolved high-resolution x-ray spectrometer for use in NIF inertial confinement fusion and high energy density experiments.

The instrument, the Diagnostic Instrument Manipulator-based High Resolution Spectrometer, or DHIRES, was designed by LLNL’s Andrew MacPhee based on his work at the UK’s Rutherford Appleton Laboratory. The design was verified and implemented by physicists Kenneth Hill, Manfred Bitter, and Lan Gao of the Princeton Plasma Physics Laboratory (PPPL) in collaboration with Joe Kilkenny of General Atomics, who contributed to the dual-channel aspect of the design.

The systems engineering and design team included LLNL’s Jay Ayers and Shannon Ayers along with David Nelson from the University of Rochester’s Laboratory for Laser Energetics (LLE). The spectrometer was assembled and a careful, absolute calibration performed by PPPL backed by image-plate calibration at LLNL.

DHIRES, which is one of the "transformational" diagnostics, is designed to record high-resolution spectral data from krypton doped into the fuel of imploding capsules used in ignition and HED experiments. The spectrometer uses two conically curved crystals to focus the krypton spectral lines onto the slit of a streak camera with a spectral resolution of about 10 electron volts. The streak camera, known as DISC, was recently redesigned to improve its spatial and temporal resolution.

A third DHIRES channel records spectra from a cylindrically curved crystal onto image plate and is used for an in-situ calibration of the streaked data. The spectra can be analyzed to diagnose the electron temperature and density of the stagnating plasma in a NIF implosion.

DHIRES was first tested on a NIF polar direct drive exploding pusher target on Sept. 28. The data successfully demonstrated that DHIRES, along with the improved DISC, could meet the diagnostic requirements.

Experiments to meet the physics requirements for DHIRES began on Nov. 2. In this campaign, DHIRES will be used to accurately measure electron temperature and density during NIF implosions.

"Measuring these conditions is key to achieving ignition of a self-sustaining fusion process on NIF," said PPPL physicist Lan Gao. "The fusion yield is very sensitive to temperature," added Marilyn Schneider, leader of NIF’s Radiation Physics and Spectroscopic Diagnostics Group.  "The spectrometer will provide 

the most sensitive temperature measurements to date. The device’s ability to plot temperature against time will also be very helpful."

PPPL, which studies magnetic confinement fusion, uses spectrometers to analyze the electromagnetic spectrum of plasma, the hot fourth state of matter in which electrons have separated from atomic nuclei, inside doughnut-shaped fusion devices known as tokamaks. These devices heat the particles and confine them in magnetic fields, causing the nuclei to fuse and produce fusion energy. In contrast, NIF’s high-energy lasers cause fusion by compressing and heating tiny pellets filled with isotopes of hydrogen.

NIF experiments are relevant to projects that include the U.S. Stockpile Stewardship Program, which maintains the nation’s nuclear deterrent without full-scale testing; other national security-related studies; and fundamental Discovery Science explorations of astrophysics, planetary physics, material science, and many other fields.

LLNL researchers Schneider, Shirley Person, Dave Bradley, and Perry Bell also contributed to the project, along with PPPL physicist Phil Efthimion, head of PPPL’s Plasma Science & Technology Department, and PPPL graduate student Brian Kraus.