RA2. Accelerator System Status
ORIC Operations and Development (B. A. Tatum)
ORIC was shut down for maintenance during much of the period in conjunction with the tandem tank opening. The new 13.8kV vacuum breaker for starting the main field motor-generator set (mentioned in the Feb. 2007 newsletter) was installed and has operated reliably. Generator brushes were also replaced, a routine task that is necessary after around 1500 hours of operation. Another old power supply was replaced: the 1975-vintage 850 amp Transrex unit used to energize vertical bending magnet BMV_1_1, which is located at the end of the ORIC extraction system. Improvements were also made to the rf system phase probe.
ORIC provided proton beam to the High Power Target Laboratory (HPTL) in February to support development of Al beams from SiC targets. As we were preparing to resume RIB operations in April, the ORIC PA tube failed with a grid-to-screen internal short circuit. A new spare tube was installed. The failed tube had been in service for over 7 years with around 20,000 hours of operation. Tube lifetime can vary widely, up to 60,000 hours. ORIC and neutron-rich RIB operations resumed in May and continued through the end of June with ORIC running extremely well.
RIB Injector Operations and Development (P.E. Mueller)
During the period from 1 January 2007 to 30 June 2007, the 25-MV Tandem Electrostatic Accelerator delivered radioactive ion beams of
70 kpps [20.9 MV 14+/25+ terminal foil / high energy foil stripped] 465 MeV 95% 132Sn, 40 kpps [21.3 MV 14+/25+ terminal foil / high energy foil stripped] 475 MeV 95% 132Sn, 50 kpps [22.4 MV 14+/25+ terminal foil / high energy foil stripped] 500 MeV 95% 132Sn, 50 kpps [22.5 MV 15+/27+ terminal foil / high energy foil stripped] 540 MeV 95% 132Sn, 23 kpps [23.2 MV 14+/27+ terminal foil / high energy foil stripped] 550 MeV 95% 132Sn, 1.8 kpps [21.4 MV 14+/25+ terminal foil / high energy foil stripped] 480 MeV 22% 134Sn, 1.6 kpps [22.4 MV 14+/25+ terminal foil / high energy foil stripped] 500 MeV 34% 134Sn, and 2 kpps [23.0 MV 14+/26+ terminal foil / high energy foil stripped] 530 MeV 30% 134Sn to the time-of-flight endstation in Beam Line 23, and 72 Mpps [23.3 MV 16+ terminal foil stripped] 396 MeV 80% 132Te to the general purpose endstation in Beam Line 21.
These beams were produced via proton induced fission of 238U by bombarding a pressed powder uranium carbide target coupled to an Electron Beam Plasma (positive) Ion Source (EBPIS) with 15 - 19 uA of 50 MeV 1H. Additionally, the high purity tin beams were produced by passing a positive tin sulfide beam through the recirculating cesium jet charge exchange cell and selecting the negative tin beam resulting from molecular breakup.
Tandem Operations and Development (M. Meigs)
The Tandem Accelerator was operated for almost 1700 hours since the last report. The machine ran at terminal potentials of 4.95 to 23.3 MV and the stable beams 1H, 11B, 32S, 54Fe, 58Ni, 74Ge, 124Sn, 130Te and 197Au, 238U were provided. 238U was provided from the RIB injector uranium carbide source. Radioactive beams of 132,134Sn, and 132Te accounted for more than 630 hours of beam on target. Conditioning was done for about 210 hours, first to return the column to proper form after having the low energy tube at atmosphere for a short time and then to try to combat an unusual deconditioning phenomenon in the top of the machine. This conditioning phenomenon typically occurred any time the voltage was changed by a substantial amount; not just after sparks! It was traced to units 21/22 which will be thoroughly inspected during the next tank opening. Fortunately, the conditioning stops after a short time and allows us to run at normal voltages but the machine is a little more prone to sparks and tics.
At the beginning of the reporting period, the tank was open for scheduled maintenance and it remained open until February 26. Persistent leaks (persistent in not being able to find it and persistent in still showing SF6 after the tank was up to air) in the terminal caused five more tank openings. Two more tank openings were caused by loss of communication to D4. On March 27, a tube truck delivered 20,000 pounds of SF6. This should allow higher voltages after we fix the errant 21/22 pair.