Implementation of a New Neutron Temporal DiagnosticSeptember, 2013
The neutron temporal diagnostic (NTD) measures the fusion-reaction-rate history of neutrons emitted from DT- and D2-filled targets. The neutron burn width is an important measure of confinement and is used to infer pressures exceeding 30 Gbar in cryogenic DT implosions. During June, the OMEGA NTD was replaced with a new ROSS streak-camera–based system. The impulse response of the ROSS-based system will be better characterized, reducing the uncertainty in the inferred cryogenic implosion pressures. The instrument is based on the fast rise time of a commercially available plastic scintillator. Neutron collisions with the scintillator convert neutron kinetic energy to 350- to 450-nm-wavelength light. The scintillator is shielded against hard x rays using a high-Z Hevimet nose cone (see Fig. 1). The light is collected and transported by an optical system to the photocathode of a fast streak camera, whose output image is recorded by a charge-coupled–device camera. Simultaneously recorded laser fiducial pulses provide absolute timing for the NTD neutron signal. To determine the absolute time calibration, the Hevimet nose cone is removed and replaced with an aluminum nose cone to measure hard x rays from a calibration target.
The original NTD, which was previously deployed on the Nova laser at Lawrence Livermore National Laboratory, could no longer be maintained. It was replaced with more-modern components, including a new ROSS streak camera, a new optical relay system, and a new motioncontrol system for the scintillator nose cone. The installation of the new nose-cone assembly into the target chamber is shown (left). This new assembly makes it possible to change out nose cones without a target chamber entry, which will facilitate more frequent NTD calibration. The complete NTD installation with the nose-cone assembly mounted to the target chamber, the black optical relay box, and the high-density, polyethylene-shielded housing for the ROSS streak camera is shown above.