The Radiation Physics Department staff at Stanford Linear Accelerator Center (SLAC) are involved in the design of several large facilities. In this issue, we report on the status of two of these projects, the Stanford Positron Electron Asymmetric Ring 3 (SPEAR3) and the Linac Coherent Light Source (LCLS).
Operation of the SPEAR2 ring in the Stanford Synchrotron Radiation Laboratory (SSRL) will stop on March 31 in preparation for a seven-month-long down-time to upgrade this facility to a third-generation storage ring, SPEAR3. In the upgrade, the ring tunnel floor as well as the entire ring component of the 234-m-circumference SPEAR2 (including vacuum chamber, magnets, power supplies, and radiofrequency [RF] system) will be taken out. The 18-cell SPEAR2 focus drift defocus drift (FODO) lattice will be replaced with a completely new double-bend achromat 18-cell lattice, increasing the circulating electron current from 100 mA to 500 mA. With the new optics, the horizontal beam emittance will be reduced from 160 nm-rad to 18 nm-rad. The betatron function will also be reduced throughout the storage ring, resulting in a beam size that is approximately four times smaller than that in SPEAR2. The dipole critical energy will increase from 4.8 keV to 7.6 keV. The stored beam lifetime at maximum current will exceed 15 hours. The existing SSRL injector system of a 150-MeV linac and the 3-GeV booster ring will remain unchanged. For SPEAR2, the injector is operated at 2.3 GeV. The stored beam energy is then ramped up to 3 GeV. For SPEAR3, the injection will occur at an energy of 3 GeV. The beam will be injected from the booster into the ring at 4 W compared to 1 W of average beam power for SPEAR2.
In SPEAR3, C-shaped dipole magnets with the opening toward the outside of the storage ring remove the self-shielding (~5 cm of iron) provided for the user side of the SSRL that has been present in the H-shaped dipole magnets for the SPEAR2. Another significant restriction in design of the SPEAR3 shielding is that the bulk shielding structure (lateral walls, ratchet walls, and roof) may not be changed. The existing shielding for the SPEAR ring comprises 61-cm-thick concrete lateral walls, a 30-cm-thick concrete roof, and 61- or 91-cm-thick concrete ratchet walls.
The shielding requirements for SPEAR3 are based on the detailed review of beam-loss patterns and study of how local shielding can be used to augment the existing bulk shielding of the SPEAR ring. The next task for Radiation Physics Department staff is the study of the shielding requirements for the photon beamlines and hutches. For more information on the SPEAR3 project, see the project website.
The LCLS project at SLAC recently passed the Department of Energy Critical Decision 1 (CD-1) process, which is the decision to approve preliminary baseline range. With CD-1 approval, LCLS is now authorized to start project engineering design activities, for which $6M has been allocated in the President's budget for fiscal year 2003.
The LCLS project is a multi-institutional proposal for a single-pass, x-ray free-electron laser (X-FEL) using electron beams from the linac and operating in the wavelength region of 1.5 to 15 angstroms. The institutions with major LCLS responsibilities include SLAC, Argonne National Laboratory, Brookhaven National Laboratory, Lawrence Livermore National Laboratory, Los Alamos National Laboratory, and University of California at Los Angeles (UCLA).
The main components of the project are a photocathode RF-gun, the last 1 km of the SLAC linac, two bunch compressors, a 121-m-long undulator, x-ray optics, and experimental stations. The X-FEL will receive a beam of electrons accelerated up to 15 GeV through the last kilometer of SLAC's 3-km linear accelerator. The electron beam will make a single pass through the undulator. The undulator will force the beam of electrons to move from side to side, causing them to emit x-rays that have much higher energies and much shorter wavelengths than the photons that we perceive as visible light.
LCLS will have properties vastly exceeding those of current x-ray sources in peak brightness, coherence, and ultra-short pulses. The peak brightness of the LCLS will be 10 orders of magnitude greater than current synchrotrons; the light will be coherent, enabling many new types of experiments; and, at 230 fs, the pulses will be ultra-short, enabling studies of fast chemical and physical processes. For more information on the LCLS project, see the project website.
After 38 years of dedicated service, W. Ralph Nelson decided to retire from the Radiation Physics Department at SLAC. Ralph is an internationally recognized radiation physicist and a fellow of the Health Physics Society who has played a major role for several decades in the shielding of many accelerator facilities throughout the world. He is a coauthor of the EGS4 Monte Carlo electromagnetic radiation generation and transport code that was developed originally for shielding and is now widely used in medical physics. He is also one of the pioneers of the application of Monte Carlo calculation methods to the shielding of accelerators. He is also a coauthor of the analytical/semi-empirical code, SHIELD11, which is used widely for shielding high-energy electron accelerators and many of the existing and planned synchrotron radiation laboratories. Ralph has also published many important papers regarding the calculation and measurement of photoproduction of muons (which was the subject of his Ph.D. dissertation at Stanford University).
Ralph is an active bluegrass musician, playing bass in the band Wild Oats `n Honey and serving on the board of the Redwood Bluegrass Association. He is currently enjoying a lengthy camping trip in the Arizona desert with his wife Kay and their two cats. He hopes to return to SLAC and continue to make contributions to the field of radiation physics on a volunteer basis.
I am delighted to report that as of June 15, 2003, Alberto Fasso will rejoin the staff of the Radiation Physics Department at SLAC. With the return of Alberto, who is a co-author of the FLUKA code, the Radiation Physics Department will continue its strong commitment to utilizing state-of-the-art Monte Carlo codes for radiation protection in accelerators.