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The first warm "L-Band" accelerator structure for ILC project
:: 14 August, 2007
The Klystron Microwave Department has finished fabricating the first warm "L-Band" accelerator structure to fill a unique niche in the International Linear Collider (ILC) project. L-Band structures use a frequency of 1.3 GHz to accelerate electrons and positrons. For most of the ILC's 31 kilometer length, the structures are cold, functioning at a few degrees above absolute zero. But the ILC needs a warm, or room-temperature, L-Band structure just after the positron source because producing positrons generates huge amounts of heat and "capturing" them requires strong focusing.
The prototype room-temperature structure is currently being installed at the Next Linear Collider Test Accelerator in End Station B, where it will be tested with one millisecond pulses at an accelerating gradient of 15 mega-volts per meter.
"The challenge is trying to run a beam that is optimized for a long-pulse cold machine using warm structures," said Project Manager Chris Adolphsen of the ILC Experiments and Prototypes group.
SLAC has a long history of designing and building warm structures, including the S-Band structures that power the SLAC linear accelerator, and the X-band structures that were proposed before superconducting technology was chosen for the ILC.
The fabrication of this structure comes from the Klystron Microwave Department's unique capability in assembling and processing high-power RF (radio-frequency) equipment, said Department Head Chris Pearson.
Juwen Wang, Deputy Head of the Accelerator Technology Department, originated the basic RF design for the structure and Zenghai Li of the Advanced Computation Department worked out the precise details. The engineering and mechanical design was done by Erik Jongewaard and Gordon Bowden, who faced the daunting challenge of removing 25 kW of power from the structure while limiting its temperature rise to less than one degree Centigrade. Juwen and Gordon also worked as liaisons with the Klystron Department during the structure fabrication, and worked on tuning it with Jim Lewandowski.
News inside News:
About Klystrons-
Scientific accelerators or colliders are powered, almost exclusively, by klystrons. Linear accelerators generally employ pulsed tubes, while storage rings require CW power. At Stanford, the original Mark III electron accelerator (1947) triggered the development of the first megawatt-level S-band klystron. Subsequently (1963) , the development at Stanford of the two-mile 25-GeV accelerator used 250 25-MW klystrons, and the upgrade of that machine to a 100-GeV center-of-mass collider (SLC-1984) necessitated replacing the original klystrons with 65-MW tubes, also at S-band. At the time, they were by far the most powerful pulsed klystrons in the world. Another production tube in the department is the BFK, a 1.25 MW CW klystron for the SLAC B-Factory, which eventually will require as many as 20 of these 15 ft-long tubes, currently the most powerful klystrons in production anywhere. SLAC physicists (together with physicists in several other countries) are now planning a new collider eventually with a center-of-mass energy of at least 1 TeV, for which a number of technical approaches have been advanced. SLAC's Next Linear Collider (NLC) version is a 25-km machine, operating at 11.4 GHz. With the present intermediate energy goal of 500 GeV center-of-mass, the NLC requires approximately 4000 klystrons, each producing 75 MW at 11.4 GHz, with pulses of 1. 6 microsecond duration and 120 Hz repetition frequency.
Two additional imperatives, have rendered the tube development task even more daunting:
The cost of the klystrons must be a fraction of prevailing commercial prices for much lower power tubes, in small quantities.
The klystrons must be very efficient. The electromagnets normally employed to focus and confine klystron electron beams consume too much DC power (80 MW for the NLC) and are not affordable. The NLC tubes employ permanent magnets in a periodic configuration (PPM).
NLC klystron development program at SLAC was initiated in 1989 and has produced good results. Both solenoid-focused and (PPM) focused klystrons have been produced at 50 and 75 MW respectively.75-MW PPM klystron has met full specifications.
An alternative to the pencil-beam 75-MW PPM klystron is an SBK, a 75-MW sheet-beam klystron (which with a double beam could be operated at 150 MW) has been in development for the last several years. Sheet beam klystrons have beams of much lower current density, employ overmoded cavities and are also focused. Furthermore, their fabrication is much simpler than conventional klystrons and requires considerably fewer parts. Since their cathode loading is much lighter, they can be expected to have a much longer life as well. The SLAC Klystron Department will soon be completing an intensive simulation program on the SBK and proceeding to the manufacture of a prototype 75-MW tube. Manufacturing klystrons for the SLC has been the principal mission of the SLAC Klystron Department for the past 15 years. Producing them with good yield and affordable cost, has required the installation of a superior manufacturing facility and the establishment of stringent quality standards. Nevertheless, the development of an NLC 75-MW klystron, operating at a frequency four times higher than S-band represents a formidable technical challenge.
The Microwave Engineering & Maintenance Group supports a wide variety of RF systems for both future and existing accelerator projects:
PEP-II RF System Support and Development
SPEAR3 RF System Support and Development
LCLS LINAC and Injector RF
Accelerator Maintenance R. F.
The Microwave Engineering group has responsibilities for the 2856 MHz, pulsed power klystrons on the main SLC LINAC and the 476 MHz, CW klystrons for the PEP-II and SPEAR3 accelerators.
The group is also involved in modeling the next generation of high pulsed power sheet beam klystrons for the NLC project.
Computational modeling of the existing 1.2 MW, 476 MHz CW klystrons has allowed the group to overcome operational limitations on PEP-II, due to oscillation driven instabilities and peak power saturation effects
In The Images-
1.Some of the people who contributed to the first warm "L-Band" accelerator structure fabricated in the Klystron Microwave Department.
Release link: http://today.slac.stanford.edu/
Tags: Linear Collider , room-temperature , L-Band structure , Prototypes group ilc , superconducting technology , high-power RF , structure fabrication ,
