Mau, B

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 159 TOOLS TO CONTROL LARGE SUPERCONDUCTING COLLIDERS Bob Mau Operations Department Head, Fermilab National Accelerator LaboratoryBatavia, Illinois, USA AbstractFermilab’s Tevatron accelerator is a cryogenically cooled four-mile ring ofsuperconducting magnets that controls a 980 GeV beam. Superconductingmagnets are expensive and take time to replace. To protect the magnets,Fermilab developed Quench Protection and Abort systems. To protect thepeople who maintain the magnets, Fermilab implemented ODH policies. Thispaper discusses these problems.1. INTRODUCTIONFermilab is a U.S Department of Energy (DOE) research laboratory, operated under DOE contract byUniversities Research Association (URA). The Fermilab accelerator complex added the Tevatron, acryogenically cooled particle accelerator, into its chain of accelerators in 1983. The Tevatron wasconstructed in Fermilab’s Main Ring tunnel, a ring four miles in circumference. The accelerator is aseparated function synchrotron that raised the operating energy of Fermilab from 400 GeV toultimately 980 GeV.2. IMPACTSOne of the first impacts we saw in operating the new accelerator was that the cycle time increased. TheMain ring’s cycle time ran at 8 seconds while the Tevatron runs between 60 and 200 seconds dependingon the mode of operation. This longer cycle time meant that when tuning you had a much longer wait tofind out whether your change was successful. The experimenters didn’t mind the slower rep ratebecause the duty factor in which they received beam increased dramatically with the slower rate(1 second flat top Vs a 23 second flat top).When tuning the Main Ring it was often quicker to intuitively adjust the tune rather than make acareful calculation of what you wanted changed. But since the Tevatron’s tuning rate was so muchslower it made more sense to actually figure out how much you wanted to move the beam beforemaking an adjustment. In addition, the sensitivity of the superconducting magnets to beam losses meantyou could not use the Main Ring technique of whacking the beam back and forth.3. CRYOGENIC MAGNETSThe second, and by far the biggest, impact to operating a cryogenic accelerator was that these magnetscould easily fail if the conductor went from superconducting to non-superconducting. The Tevatronmagnet coils have a cross section of approximately 12 square mm. We run 4380 amps through thisconductor. If the coil remains superconducting (no electrical resistance) then everything is fine andhappy. But if the coil warms up so that the conductor develops a resistance, that high current will causethe conductor to fail. This is expensive, and worse it takes a week to replace the magnet.1604. PROTECTION & ABORT SYSTEMSTo protect against these kinds of failures, the Tevatron group had to develop a quench protection systemto look for magnets that are to go non-superconducting. They came up with the Quench ProtectionMonitoring system, QPM for short. Some of the requirements for the QPM system were as follows:• To get rid of the beam (initiate a beam abort) in a safe manner• To turn off the power supplies and remove the energy from the magnets before the magnetfailed• To monitor itself and ensure that it was always on line and protecting the magnets• To allow easy troubleshooting• To quickly show what triggered the quench system. (The QPM contains a circular bufferwhere measurements of magnet behavior are recorded. When the QPM system senses aquench, the circular buffer is stopped, which allowed us to back up into time and figure outwhat caused the quench.)Understanding what causes and, more importantly, how to avoid quenches are vital toOperations. Our biggest quench problem is caused by beam scraping. Uncontrolled beam losses, due toscraping, will warm the Tevatron magnet so that it’s no longer superconductive. In this situation, ratherthan waiting for a quench, it is preferable to quickly abort the beam into an external dump. Our Abortsystem has similar requirements to the QPM system:• When triggered by losses or equipment failure, the abort system fires a set of single-turnkickers that remove the beam safely and quickly• To quickly diagnose aborts. (Each Beam Position Monitor and Beam Loss Monitor talks to acircular buffer that store positions and losses through out the beam cycle. When the abortfires, all circular buffers for the BPMs and BLMs stop. This allows us to look back at theTevatron orbit and loss patterns around the entire ring and see what they were doing in smallincrements before the actual abort occurred.)5. BENEFITSOne of the positive side effects of the superconducting magnets was that successful operation preventedus from dumping vast quantities of beam on the magnets. The TeV magnets demanded better tuning,and the QPM and Abort systems dumped the beam at spots we specified. As a result, the residual lossesin the Tevatron were far lower than during the days when the Main Ring was the final stage ofacceleration.6. ODH PROBLEMSThere are other problems related to superconducting magnets. They require large volumes of internalmagnet cryogens. If a cryogenic relief valve opens on one or more of the magnets there is a possibilitythat the cryogens will displace oxygen in the tunnel, possibly creating an oxygen deficiency hazard(ODH). The whole cryogenic system is considered to be a pressure vessel. (During the Tevatron’sdesign stage a considerable amount of attention had to be paid to this by various safety committees.)This in turn makes accesses into these beam enclosures complicated. Fixed oxygen alarms wererequired throughout the Tevatron beam enclosures.Anyone accessing the Tevatron tunnel is required to take a personal oxygen monitor that willalarm whenever the oxygen levels get below 19.5%. They also must carry a small tank that will givethem 5 minutes of air for escape.A two-man rule became mandatory for all accesses.161The cryogens also required the laboratory’s medical department to certify that all workers whoenter ODH enclosures are medically qualified to do so. To this end, a database had to be set up and theOperations Department required use it to check training and medical approval records for every workerbefore allowing tunnel access.In addition to all this, moving heavy objects above or around cryogenic components in the tunnelrequired additional procedures to prevent accidental rupturing of the insulating vacuum or puncturingcryogenic supply lines.7. OTHER PROBLEMS1. Four miles takes too long to walk and takes too much time transporting equipment around tocomplete work, or to interlock, especially when everyone is waiting for beam. So operatorsand technicians drive the ring in electric golf carts. After a safety analysis was performed andfixed bumpers were added around vulnerable areas to prevent colliding golf carts fromrupturing cryo and vacuum systems.2. Ground faults are much harder to locate unless a cryogenic magnet ruptures its insulatingvacuum line and spews its insulation around to be noticed.3. Vacuum is much harder to leak check in cryogenic magnet systems. The Vacuum techniciansneed to rely more on time of flight methods for locating vacuum trouble spots. The number ofvacuum connections went up a factor of five from the conventional system to the cryogenicvacuum system.4. If cryogenic magnets warm, cryo-pumping within the vacuum chamber can releasecontaminates into the vacuum system. These contaminates can then block the smallerapertures in the cryo system causing flow problems. The fix requires a warm up and adecontamination cycle for that part of the ring affected, which typically causes a week downtime.5. We have more rotating equipment, in the form of compressors and expansion engines thatrequire constant attention and higher maintenance.6. We are more susceptible to power outages. If our cryo equipment is not started within 30minutes of a site wide power outage or glitch, we are down for a week while we purify and re-cool the ring. Thunderstorms have a much bigger impact on cryogenic accelerators thanconventional accelerators.7. Control software is more complex and sophisticated due to the complexity of cryo systems,power supply systems, and beam tuning controls.8. Our cryogenic machines use helium for cooling purposes; it is a non-renewable resource. Wehave developed methods of capturing gas accidentally lost due to quenches and such, but wemust be regularly resupplied.8. RESPONSIBILITESOperators must be trained to work with the fire department for response to any ODH emergency. Thisrequires being medically approved to use the Self- Contained Breathing Apparatus (SCBA) provided bythe Fire Department. In addition to all this, the use of cryogenics requires more training for OperationsDepartment personnel due to its controls system.9. CONCLUSIONEncourage the development of air temperature superconductors.162Cryogenic machines are much more temperamental than conventional machines. Our systemuses helium, which is a non-renewable resource. Cryogenic machines require refrigerators, acryogenics group for maintenance, sophisticated quench protection systems, more time to replace failedmagnets, has a bigger impact on operations due to weather conditions, and a much more complex andexpensive set of instrumentation is needed for efficient beam diagnoses. And finally, it takes more timeto train operators to deal with cryogenic systems and procedures.

Tools to Control Large Superconducting Colliders

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