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From the Acting Director of the Accelerator Directorate:
X-band Accelerator Technology—Where It Could Be Going

(Photo - Bob Hettel)
(Photo by Brad Plummer.)

In my new role as acting head of the Accelerator Directorate, I have had the opportunity to emerge from the storage ring light source world to learn much more about the extensive R&D program for high-gradient acceleration here at SLAC. For those who do not know much about this program, I recommend visiting the website for the Accelerator Research Division where one can learn about the innovative programs to develop beam-driven plasma wakefield acceleration with the FACET program, direct laser acceleration and high-gradient, normal conducting radio-frequency acceleration. While direct laser and plasma wakefield acceleration could provide extremely high gradients that could revolutionize the world of compact accelerators (1 GeV/m and 10 GeV/m, respectively), the timeline for their practical application is fairly far in the future. Normal conducting high gradient RF technology, on the other hand, is already highly developed and its practical applications are imminent.

By way of background history (or at least my perceived version of it), the high gradient RF accelerator R&D program was spawned by the success of the SLAC Linear Collider, operating at 92 GeV center-of-mass energy. The SLC led to programs to worldwide to devise a linear collider that could reach TeV-scale center-of-mass energies with leptons, as opposed to hadrons as in the Large Hadron Collider. In order to reduce the size and cost of this "Next Linear Collider," SLAC and others embarked on the development of RF accelerator structure and power source development that would have higher gradient than the typical 20 MV/m achieved with SLAC's S-band technology. The NLC Test Accelerator at SLAC was built to develop X-band technology operating at 11.4 GHz, four times the frequency and having roughly four times the gradient (~100 MV/m) than the S-band technology presently used at SLAC. Other teams offered lower gradient (31 MV/m) pulsed superconducting technology was offered as an alternative to normal conducting X-band. Without going into extensive and convoluted explanations as to why (ask your local ARD staff member), the superconducting technology was ultimately chosen for the ILC and the Global Design Effort was launched. This decision left SLAC holding a highly skilled core competency and an investment of >$100M in normal conducting X-band technology development with no obvious place to go with it, at least at first.

Today SLAC is faced with a challenge: what to do with our X-band program, especially given the eventual decline of the ILC program. Fortunately, an answer that could be good news for this program may be emerging. While the competition between normal conducting and superconducting accelerators continues, not only for high-energy physics applications, but also for light source applications where high repetition rate linear accelerators, on the scale of megaHertz and higher, are desired (e.g. high rep-rate FELs and energy recovery linac-based light sources), there is at least one area where the normal conducting technology wins: applications that really require the high gradient that cannot be provided by superconducting accelerators. Interest in compact linear accelerators using X-band technology is rapidly increasing, partly due to the overwhelming success of the Linac Coherent Light Source. Whereas a typical light source facility might have just enough real estate to accommodate a 1.5 to 2-GeV superconducting accelerator that could drive soft X-ray FELs, the same length X-band linac could provide 6 to 8-GeV electrons to drive hard X-ray FELs. The burgeoning science enabled by the LCLS has sparked world-wide interest in such hard X-ray FELs, and now "everybody" wants one, many having only limited space. Light source facilities that have expressed interest in X-band accelerators include the Paul Scherrer Institute in Switzerland, the FERMI facility in Trieste, the Diamond facility in Great Britain, the Shanghai Synchrotron Radiation Facility, the Pohang Accelerator Laboratory in Korea, KVI in Groningen, and even Los Alamos National Laboratory, which is considering building a 50-keV FEL, five times higher energy than the LCLS.

Other applications for compact linacs include inverse Compton backscattering photon sources (e.g. the 250-MeV MEGaray for Lawrence Livermore National laboratory, which fits on the back of a truck), compact injectors for storage rings (e.g. for the NSLS-II), and possibly for a future version of FACET that would fit within the first third of the SLAC linac tunnel. Supporting these compact sources, SLAC is also developing a high-brightness X-band RF gun that would enable these linacs to be driven with a single frequency and would provide electron bunches having lower longitudinal emittance than an S-band gun, reducing the degree of bunch compression in transport line chicanes. SLAC is engaged in collaborations with many of these X-band initiatives, including a collaboration with CERN and KEK on the development of X-band accelerator structures and a collaboration with LLNL on the development of the 250 MeV linac for the MEGa-ray source.

A critical step in realizing these new areas of application will be to find industrial partners that can provide the technology, especially the RF power sources, to diverse customers. At the moment there are no commercial vendors for X-band klystrons, and facilities are reluctant to proceed with X-band implementations until this is resolved. In the meantime, C-band technology, operating at one half the frequency and gradient of SLAC's X-band and for which commercial klystrons are available, is being chosen for high-gradient accelerator projects having short-term schedules. SLAC's Accelerator Research Division is actively engaged with other labs worldwide to promote and develop X-band technology and identify future customers in order to attract industrial partners. To this end, workshops on X-band technology are being held, including one last March here at SLAC, and one is planned for December at the Cockroft Institute in Great Britain.

As a relative newcomer to the world of X-band technology, I do not profess to know all the issues that could ultimately thwart its realization as a viable and ubiquitous high-gradient accelerator technology. On the other hand, there are a lot of people here at SLAC who know this business very well, from research and development points of view, and have had their hands in the hardware for a long time. To me, it seems clear that this effort should continue and funding agencies should be convinced that the prospects of future applications for X-band technology warrant their R&D support.

óBob Hettel
SLAC Today, September 24, 2010