December 21, 2015
Our upgrade project is called the 12 GeV CEBAF Upgrade Project.
At the time CD-4A was achieved, we demonstrated 2.2 GeV per pass. This was 12 GeV! Well, not quite. In fact with more than one pass, we limited ourselves to a little more than 6 GeV with three passes, and to 10.5 GeV with 5.5 passes. It was not felt to be prudent to demand 12 GeV out of the machine immediately after turn on.
Operations in the spring of 2015 at high energy, ~10.5 GeV, came to a premature end when a power loss event led to damage of one of the Central Helium Liquefier cold compressors. A replacement was obtained on loan from Oak Ridge National Lab, which has a cryo-system similar to ours. During the summer, that compressor was installed and this fall the full CHL suite returned to operation.
Also during the summer of 2015, a number of other projects were completed, including the final steps of the civil construction part of the upgrade, which involved installation of the tunnel air conditioning. While several aspects of the Utilities Infrastructure Modernization project, cooling towers and communications were on the task list, there was other work more directly related to the operation of the accelerator.
One of the causes of trips in superconducting cavities is irregularities on the internal surfaces of the cavities. These can develop into centers of field emission, which can lead to a breakdown, a spark. The process is analogous to the way that boiling in a pan starts on small specks on the bottom of the pan. Some of these field emitters are created by specks of dust that sneak into the cavities over time. It has been found that by introducing helium into the cavities, some of the field emitters can be “burned off”. The treated cavity can then sustain a higher gradient without breakdowns and trips. At Jefferson Lab, with its long operating experience, protocols have been developed to deal with the slow degradation of performance and to judge when it is beneficial to treat the cavities. This applies to the oldest C20 modules, and also to the C50 cavities, but the much younger C100’s are still under study.
Nevertheless, one of the main goals of the summer shutdown was a substantial program of helium processing, which was completed.
So, the pieces were in place for the fall run, with a goal of establishing an operational CEBAF with energy at, or close to, the design energy of 12 GeV.
First moves involved bringing up the ensembles of cryomodules in the two separate linacs, to try to establish an acceptable operating gradient, ~1090 MeV in each linac, initially without beam. This took a couple of weeks. Then, beam was introduced to the accelerator. In measured steps, beam was established, calibrated and optimized, half a turn at a time through 5 passes. December 11, beam was extracted at 11 GeV to Hall A. Continuous Wave beam, which the operators refer to as “CW beam” then followed.
Attention turned towards Hall D, and the new ARC 10 magnets joined the game. Once more down the north linac, and 12 GeV beam was extracted from the machine to the Hall D tagger dump. As a final touch, the 12 GeV electron beam was used to generate a photon beam that was transported to the GlueX experiment, which then commenced a series of studies.
The ability to extract beam at full energy to all the four halls depends on a new 750 MHz separator configuration. This development was derived from a project funded under the American Recovery and Reinvestment Act and was demonstrated to work. At this stage:
- 12 GeV CW beam has been delivered to Hall D
- 70 microamps of 11 GeV CW beam (770 kW) has been delivered to Hall A
- The RF trip rate of ~5 per hour with “spring 2016” running conditions has been established
- Beam emittance within the “initial years” specification has been achieved
- 11 GeV beam separation has been demonstrated.
All this is an amazing outcome, which was not by any means a given at the start of the run. Many issues were resolved on the way, and this also allowed the exploration of the operating parameters. The machine aficionados will tell you that it is not perfect, but we expect that our users will be happy that we now have a machine that can deliver on the energy they desire. We now look forward to the spring run!
CEBAF. 12 GeV!