Tandem Operations

The EN tandem Van de Graaff accelerator at Kansas State University has operated extremely well this reporting period. The usual and unexpected problems have occurred with the tandem which have necessitated tandem openings, but none were of the serious type.

Two major modifications were done to the tandem and the third will be completed following this reporting period. In December of l994 we installed a new National Electrostatics Corporation (model FS6) foil assembly with 254 foil targets in the terminal of the tandem, along with a new foil control system. This gives us a total stripping foil capacity of 310 foils as we left the original 60 position foil stripper in place. The foil control system allows us to switch between our original stripper and the N.E.C. stripper. In May of l995 both the low energy and high energy columns of the tandem were refitted with new column resistor assemblies. The resistor assemblies that were removed were the old resistor assemblies provided by High Voltage Engineering Corporation (currently Vivirad-High Voltage Corporation) when the tandem was installed in the late 1960's (400 megaohms per plane). The new assemblies are of the same design as the ones currently used at the EN at Oak Ridge National Laboratory and at Sandia National Laboratory, however the value of the resistors are different being 800 megaohms per plane. The new resistors were purchased from E.B.G., Inc. We have observed that the new column resistor chain has improved overall beam energy stability and quality.

In October plans are to install a new Balzers model TMU 260 turbo pump in the terminal of the tandem to recirculate gas stripping. We plan to modify the existing H.V.E.C. gas stripper canal by threading its I.D. and, in addition, placing threaded collimators for the entrance and exit apertures of the stripping box, we will create a "differentially" pumped stripping region for achieving higher target density at lower stripping gas flow.

Through this reporting period the tandem has logged 2627 hours of actual beam time operation. Down time during this period totaled 804 hours. Of the 804 hours, 696 hours were used for the two major modifications and the remaining 108 hours were used for routine and unexpected tandem opening.

Ion Source Operation

The EN Van de Graaff accelerator at Kansas State University is injected with two different ion sources. The first is a Middelton high intensity sputter source which has operated again this year at least 90% of the time, producing beams of carbon, oxygen, fluorine, silicon, copper, boron, iodine, chlorine, gold and silver. Our second source, the diode ion source, has produced the remaining 10%, with beams of hydrogen, carbon and fluorine. Both sources have operated extremely well over this past reporting period. We installed a low ripple 60 KV HV power supply on the diode source replacing the original HV power supply whose ripple was too large for producing the pulsed beam required for LINAC operations.

LINAC Operations

After our normal winter shutdown, the LINAC was returned to service in May. During the shutdown we replaced rf power coupling and superconducting solenoid current leads in the cryostats. Cold leak repairs were made in the high-beta cryostat. The cryostat turbomolecular vacuum pumps were replaced with Balzers TPV 330 pumps for reliability and ease of bearing maintenance. We experienced problems with the LHe refrigerator system as its output dropped dramatically in June. After sorting through various scenarios, we found that two separate systems had gone out of calibration thus affecting the mass flow throughput and the setting of the first JT valve. Upon recalibration, the refrigerator returned to its original performance specifications.

We initiated several upgrade projects this last year. The refrigerator cold engines were upgraded by replacing the cylinder heads with the new design which incorporates Kevlar seats for the intake and exhaust valves. A new building is in the final stages of construction. We will move the refrigeration compressor off of its current location on the laboratory roof to the new building commencing October 9. We have added a number of sensors to the refrigerator to facilitate better monitoring and control and we have prepared for remote control of the compressor from the cold box location. Piping runs are under construction and we hope to have the compressor move completed by December of this year.

Acknowledgment

This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy.