1. HRIBF Update and Near-Term
(J. R. Beene, M. J. Meigs, R. C. Juras)
The performance of HRIBF during fiscal 2004 (ending October 1) was truly outstanding. We were able to provide a record number of 1670 hours of RIB to experiments, an increase of 35% over FY2003 (which was itself a record year with an increase of 20% over 2002). This result is particularly remarkable given the fact that we are in the midst of the construction of the new High Power Target Laboratory (HPTL). It will not be possible to continue this rapid increase in RIB hours delivered for another year. We have been aware for some time that greater interference between HRIBF operations and the HPTL project was inevitable in fiscal 2005. We were also aware that the replacement of the tandem terminal magnet power supply carried out at the end of FY04 was bound to impact operating hours during the first quarter of FY2005. We did not anticipate a third issue that will impact our beam schedule. This was the failure of several coils in the downstream element of the second stage mass analyzer (Isobar Separator). This is among the newest magnets in the facility and it must operate for us to inject beams from the RIB injector to the tandem. New coils have been ordered (a full set, since there is apparently a design flaw) and will be delivered in late April, 2005. We expect the Isobar Separator to be fully restored to operation by the end of May. (See the following subsetions 1.1 for detailed status of the RIB isobar separator magnet system.) For the time being, we are limited to injecting beams of A less than or equal to 40 from the RIB injector.
As a result of these developments, the neutron-rich campaign planned for early spring has been delayed until summer. We expect the time between now and May to be occupied by a continuation of the present 7Be campaign, followed by a nine-week 17,18F campaign, along with short stable-beam runs and a brief tank opening for tandem maintenance. The remainder of the year will be occupied by a ~4-month neutron-rich RIB campaign.
This has been - and will continue to be - a particularly trying time for HRIBF operations and research staff. We have many irons in the fire and are stretched extremely thin.
We had two major DOE reviews of HRIBF related activities in 2004. An annual review of the HPTL project, including a preliminary review of the new IRIS2 project, was carried out in September, and a Scientific and Technical Review of HRIBF operations and low-energy nuclear physics research was held in November. More detailed discussions on the status of HPTL and IRIS2 can be found in a subsequent article. We do not yet have a written report on the S&T review, but the oral closeout was extremely favorable. Our staff and users are to be commended for the excellent work they have done and continue to do.
1.1 RIB isobar separator magnet system update
The RIB isobar separator magnet system on beam line 12 experienced a partial coil failure in August 2004 while providing 134Sn for RIB-121. Beam instabilities were a direct result of a water leak on the upper coil of the second separator magnet. The run was stopped to try to address the magnet problem. There was an attempt to remove water from the coil and to dry it by passing air through the middle pancake that was determined to be leaking. This pancake was then shorted and 318 amps were applied to the magnet in an attempt to further dry it. At this time, the top pancake in the bottom coil failed and subsequently yet another pancake in the bottom coil failed. The magnet had been specified to run at 360 amps, but evidently there was a flaw in the design or manufacture which caused it to fail at even lower currents. Prior to this point, the magnet had not been run at more than half the capacity for any length of time. Therefore, at this time, three of the six pancakes in the second magnet are shorted which limits transport through the system to masses less than 40. The system is usable for the Be and F campaigns, but not for neutron-rich beams.
Replacement coils are under fabrication and should be shipped at the end of March. Since they will be transported by sea, it will probably be the end of April before they arrive at ORNL. Installation of the new coils will be challenging and will probably require about a month from disassembly to subsequent operation for the experimental program. Specifications and testing for these coils is more stringent than for the original to try to prevent the same type of failure.
1.2 Tandem accelerator terminal magnet update
A new power supply was purchased and installed to replace the original power supply for the tandem accelerator terminal magnet (BM T-1). The original power supply was manufactured in 1976 by Alpha Scientific Inc. and was showing signs of its age. Power supply failures resulted in several tank openings in the previous two years, and replacement parts for the power supply were becoming difficult to obtain and very expensive. For several months before it was removed, the power supply was operating without a full complement of transistors in the 57-transistor regulator bank because we had not been able to obtain suitable replacements. In addition to the problem of obtaining replacement transistors, water passages in heat sinks for the transistors banks, in transformers and chokes, and in the water-cooled shunt were becoming corroded or plugged.
New power supply installation was challenging because the only access to the terminal is by means of the central service platform (CSP) and then access to the second level of the terminal, where the power supply is located, is by means of a ladder through a narrow opening. The original power supply was installed before the terminal was completely assembled. The old power supply was cut into pieces for removal and the new power supply, purchased from Alpha Scientific, was custom designed to be separable into pieces suitable for transport in the CSP and up the ladder.
When the new power supply went into operation, it was initially susceptible to accelerator high voltage transients. Working with the manufacturer, the susceptibilities were identified and corrected. Each repair and modification resulted in significant lost time because the power supply is located in the terminal of the accelerator and requires a tank opening to access. Of nine unscheduled tank openings during the first quarter of FY05, seven were caused by the power supply. The first six failures were on the power-supply regulator amplifier circuit board. These were solved by circuit modifications that would better protect components on the circuit board, and modifications of the power supply enclosure and cabling aimed at reducing any transient energy leading to the circuit board. The seventh failure was caused by failure of an input fuse. All of the input fuses will be replaced by a slower-acting fuse type. Until December 2, every tandem accelerator spark over 20 MV resulted in damage to the regulator circuit board. Since December 2, there have been more than ten sparks over 20 MV with no regulator circuit board damage. The manufacturer will provide a regulator amplifier circuit board that incorporates all of our modifications and will also provide slower-acting fuses and an appropriate fuse block. The circuit board will be packaged in a separate, shielded enclosure. The new parts will be installed during the next scheduled maintenance period.