Planar Cryo Target Positioner

November 2002

A new target positioner (planar cryo) has been deployed on OMEGA to measure the properties of condensed gasses at cryogenic temperatures. It uses a closed-loop cooling system that is fully compatible with the infrastructure developed for spherical cryogenic targets. A single planar cryo system can field one target every two hours.

The target positioner is shown installed in a Moving Cryostat Transfer Cart (MCTC). (The MCTC has been partially disassembled for clarity.) For more information about the MCTC, see June 2002, Moving the MCTC.

A representative target cell structure used for equation-of-state experiments. The fill gas condenses/freezes in the cell. Windows and internal components can be changed to suit the experimental requirements. Beryllium windows can be provided for transverse
x-ray imaging.

Each target cell is composed of numerous small components that must be meticulously cleaned, assembled and thoroughly tested.

A close-up view shows target assembly. The target cell is clamped between a pair of retaining rings that prevent hardware from being ejected into the OMEGA chamber. Slots machined at 1 and 7 o’clock are for the fill and vent tubes. Slots machined at 4 and 10 o’clock are provided for transverse
x-ray imaging.

An assembled target (attached to an assembly fixture) is loaded into a room-temperature filling station where vacuum testing, purging and filling are completed. Final fill pressure of the reservoir is approximately one atmosphere.

A planar cryogenic target assembly is ready for filling. The coiled copper reservoir (1) is filled with room-temperature gas (e.g., D2) before attaching the target assembly to the cold head. The target holder (3) is secured to the cold head with the split nut (2), providing conductive thermal coupling.

A filled, room-temperature target is manually installed onto the target positioner. Thermal isolation between the reservoir and target cell permit the cell to be cooled below 10°K while the reservoir can exceed 250°K. The temperature differential and resulting pressure gradient ensure that the cell can be filled with the gas. Given the cell temperature, the density of the fill at shot time can be determined.

Assembly fixtures are used to protect targets during handling operations.

This photograph was taken at the instant a shot occurred. The MC is inserted into the OMEGA chamber (6 o’clock). Approximately 1.5 seconds prior to a shot, the shroud is rapidly removed from the MC. The Active Shock Breakout (ASBO) diagnostic (10 o’clock) is used to measure the velocity of laser induced shock waves that propagate through the target cell.

After securing the target, a shroud is installed to minimize the radiative heat load incident on the target and to reduce the risk of developing condensation on the target cell. The MCTC is then sealed, evacuated, and cooled. Windows permit observation along multiple axes for positioning the target and aligning diagnostics.

A cross-sectional view shows the simplicity of the target/shroud assembly. Heat is conducted through the copper standoff to the cold head.

The target cell is destroyed during the shot. The target holder and reservoir can be recovered and reused.