A Facility for Accelerator Physics and Test Beam Experiments Experimental Program Advisory Committee Roger Erickson for the SABER Design Team December 4, 2006
The Problem: FFTB is gone! The Final Focus Test Beam (FFTB) has been dismantled to make room for the LCLS. What can be done to replace its functions? SABER: South Arc Beam Experiment Region A proposed facility for experiments requiring compressed, focused beams of electrons or positrons. To be built in the Instrument Section in the SLC South Arc tunnel. Description: SABER White Paper (December 2005, revised August 2006) http://www.slac.stanford.edu/grp/rd/epac/loi/saber.pdf SABER Workshop (March 2006) presentations: http://www-conf.slac.stanford.edu/saber/present/default.htm SABER Optical Design, R. Erickson, K. Bane, P. Emma, and Y. Nosochkov, Proceedings of the EPAC06 (June 2006) https://www-internal.slac.stanford.edu/ad/adsaber/saber_files/wepch061.pdf
SLAC Overview with SABER SABER consists of three main components: 1. Experimental area with final focus and beam dump in SLC South Arc tunnel. 2. Linac Pulse Compressor upgrade to compress positron bunches. 3. Bypass Line to deliver e - or e + beams to SABER, bypassing the LCLS. LCLS injector is being installed at Sector 21, downstream of existing positron source.
SABER in SLC South Arc Tunnel
SW Adit Entrance Ramp View from the parking lot below MCC A trailer or small counting-house building could be installed adjacent to the ramp.
SW Adit Entrance to SABER South Arc tunnel at the bottom of the ramp. A concrete maze has been designed for this area.
Architectural Drawing of South Arc Area SABER MCC Linac SW Adit MCC 40 ft 3.73 Tunnel pitch angle
South Arc Tunnel People can work safely in the South Arc tunnel, independently of PEP-II or LCLS operations. Easy access through upgraded SW Adit entrance. Convenient space for user building in existing paved area beside SW Adit ramp. Radiation safety issues are easier than in FFTB (SABER is deep underground).
South Arc Tunnel Instrument Section, looking upstream.
South Arc Tunnel Looking downstream, approaching dump area.
Current Status of South Arc Magnets appear to be in good shape. Arc achromats 1 and 2 (BSY to SABER) have been electrically isolated from the rest of the South and North Arcs, and pass high-pot and resistance checks. A smaller power supply (30 V, 3000 A) has been identified for powering Achromats 1 and 2. Installation is expected to begin shortly. New dc cables have been installed for the arc circuit in the BSY area, replacing some that appeared to be radiation-damaged. Control system is functioning; arc magnet movers respond to control system commands. Some magnet power supplies in the BSY zone and non-interlocked corrector magnets in the S ARC zone have been powered up.
Current Status of South Arc - continued LCW flow has been maintained, except for a few devices in the BSY. To reduce power consumption for LCW pumps, flow restrictors have been fabricated to reduce pressure to the SLC arcs, but maintain full pressure for SABER systems. Vacuum system integrity is good in the arc achromats and SABER area. A section of the S. Arc beamline in the BSY had leaks that have been repaired; this section is being re-integrated with the BSY system this week.
Personnel Protection System Phase 1 Goal is to restore South Arc PPS to operating condition. Combine S Arc and SFF zones into one SABER zone. Status: Design complete. Maintenance in progress. Phase 2 Goal is to add a new controlled-access entrance portal at the SW Adit, and move the portal at the SE Adit to a new location in the tunnel closer to SABER. Status: PPS design work in progress. Rad Physics Dept studying entrance maze design.
PPS Phase 1 Status: Ready for certification January 2007.
PPS Phase 2 SABER Focal Point Emergency Off Buttons Utility Shaft
Plans for the Coming Year Phase 1 Restore South Arc through Instrument Section as it was in 1998. Upgrade to meet new safety standards. in progress. Install smaller power supply for arc magnets. design complete. Install temporary beam dump. done. Deliver first beam to temporary dump in early 2007. Phase 2 Reconfigure Instrument Section for SABER final focus. conceptual design complete. Install PPS entrance in SW Adit. detailed design in progress. Install shielded high-power beam dump. A suitable dump from a dismantled beam line has been located. Demonstrate focused and compressed electron beam.
Configuration for Initial Beam Tests Incoming Beam Temporary Beam Dump
Schematic Map Incoming Beam No changes to here Focal Point 12/4/06 Achro 3 EPAC Review
SABER Final Focus Magnets SABER quads (8) from SLC Final Focus Final focusing quad from FFTB Dump line quads from FFTB Dump line dipoles from FFTB
Experimental Area Current working design: Nominal beam position: 42 inches above the tunnel floor. 39 inches from south tunnel wall. Downward pitch 3.730 deg. Focal point is 2m downstream of face of last quadrupole. The focal point is approximately 100 feet from the next arc AG magnet (where the tunnel starts to curve to the south). This allows room for an imaging spectrometer system.
Ease of Access SW Adit entrance: Ramp is approximately 10 ft wide. Room enough for a 39 inch door (same as FFTB), followed by 4 ft wide PPS maze for personnel and equipment access. Two vertical cable penetrations carry cables to MCC for power supplies and instrumentation. One is near the upstream end of the IS and reaches the surface outside the west door of MCC. The other is near the proposed IP and reaches the surface outside the east door of MCC. A cable tray could be installed through the SW Adit (through the maze or above the shielding blocks) to a trailer or other building on the paved area near the SW Adit ramp.
Positron Compressor Chicane The existing Sector 10 compressor chicane has been used successfully for several years, but cannot be used with positrons (because electrons are required to make the positrons, and only one charge can pass through the present chicane). Chicane will be modified to be symmetric for electrons and positrons. Two new dipoles with wider poles are needed for the first and fourth positions.
Compressor Chicane Components Incoming beams e - e + New positron chicane Existing components with control system nomenclature
Proposed Symmetric Chicane e + e -
First Dipole of Compressor Chicane Chicane path Undeflected linac trajectory
Bypass Line A Bypass Line from Sector 20 will allow delivery of 30 GeV electrons to BSY without passing through last third of linac. SABER will then be independent of LCLS operations. Optics design is compatible with connection to the South Arc. Trajectory past Sector 30 and NIT/SIT breakout point passes through a congested area, but can be built with careful design. Cost could be reduced by using one of the PEP-II injection transport lines between Sectors 21-29 for this purpose.
Electron Transport Line for PEP-II e - Beam to PEP-II SABER Bypass Line geometry and components will look similar to PEP-II e- inject. system. e - Beam to PEP-II Linac Protection Collimator Bend Magnets Linac e + Return Line (only upstream of Sector 19) First Dipole of Compressor Chicane
PEP-II Transport Lines in Linac Tunnel e- e+
PEP-II Transport Lines e + to PEP-II e - to PEP-II Steering correctors BPM Quadrupole
Linac Tunnel with Transport Lines PEP-II e - PEP-II e + Linac Bypass Line Bypass quadrupoles are offset in z from PEP-II e+ system.
SABER Bypass Line Geometry
Bypass Trajectory Approaching South Arc Match at existing 51B2 dipole To LCLS To SABER Dipoles: red Quads: blue Sextupoles: green To PEP-II LER
Linac Tunnel Approaching BSY Bypass Line LINAC Bypass Line trajectory will pass over SIT Line through congested area. SIT Line to PEP-II
Bypass Trajectory in BSY near 51AGF Plan view of SLC arcs diverging downstream of 50B1. SLC South Arc Trajectory Linac Beam Bypass Line
Bypass Trajectory into 51B2 North Arc South Arc LCLS Beam Bypass Line New Dipole (52B2 from N. Arc)
B2 Dipoles in 51- and 52-lines South Arc Linac (LCLS) Trajectory 51B2 in S. Arc 52B2 in N. Arc To SABER
Optical Functions: Sector 20 to Focal Point 1 st Dogleg: Linac to Bypass 2 nd Dogleg: Match to Achro 1 Sector 20-5 Bypass Line Arc Achromats 1 & 2 Final Focus D (m) Prepared by Yuri Nosochkov.
Final Focus Optics and Dump Line β (m) D(m) Focal Point
Focal Point (No Bypass Line) β x = 1.5 cm, β y = 15 cm Red is Gaussian fit to tracking data. σ x = 6.9 μm σ y = 6.7 μm σ z = 18.8 μm
Focal Point with Bypass Line σ x = 5.5 μm σ y = 6.1 μm Final parameters are equivalent, whether the beam comes through the last third of the linac or is transported through the Bypass Line. σ z = 19.3 μm
Use of PEP-II Injection Transport Line The Bypass Line design runs parallel and close to the existing PEP-II transport lines, and could easily be modified to match a long section of the electron transport (NIT) line. Nearly 1 km of vacuum chamber, pumps, BPMs, steering correctors, cables, small power supplies, control system hardware, and mechanical supports can be used directly. The PEP-II electron injection transport line was designed for 12 GeV; needs 2.5 times stronger focusing for SABER. This could be achieved by adding quadrupoles with cross-sections identical to existing quads.
SABER Beam Parameters Energy: 28.5 GeV with PEP-II or LCLS with bypass line. Up to 30 GeV with full linac (but not with bypass line). Charge per pulse: 2 x 10 10 e - or e + /pulse with full compression. 3.5 x 10 10 e - or e + /pulse without compression. Pulse length: σ z < 30 μm with 4% momentum spread. σ z < 45 μm with 1.5% momentum spread. Spot size at IP: 10 μm nominal; σ x, y < 7 μm achieved in computer simulations. Momentum spread: 4% full width with full compression. < 0.5% full width without compression. Momentum dispersion at IP: η = 0. η = 0.
Future Opportunities Test Beam Single e+ or e- per pulse A beam of this kind could be produced by passing the primary beam through a foil in the BSY. Dipole B2 and magnets downstream would be set for a lower energy, dumping the primary beam on a collimator in the BSY. Reduce bunch length Studies suggest σ z could be further reduced by about 30% by lowering the damping ring energy from 1.19 GeV to 0.9 GeV. Other Ideas include moving the compressors further upstream and adding an RF photoinjector gun similar to the LCLS gun. These ideas merit further study.
SABER Safety Issues Radiation Beam energy and power are substantially less than SLC, which ran successfully from 1986 to 1998. The Sector 10 positron compressor chicane will be a mirror image of the electron compressor, and will incorporate the same safety features. Electrical Conventional electrical equipment is being inspected and upgraded for code compliance. Arc-flash hazard analysis has been done and new labels applied. Distribution panels and cable tray grounding have been upgraded. Some cables will be replaced. Non-ionizing Radiation No new RF sources involved with SABER. Some future experiments may involve lasers, but these will be reviewed when they are proposed. Seismic Hazard Existing South Arc tunnel will be used. No new civil construction, except for SW Adit PPS maze and additional cable penetrations. The walls of the SW Adit ramp were reinforced earlier this year.
Summary SLC experience proved that low-emittance beams of electrons or positrons could be transported through the arc systems and focused to small spots. FFTB experience proved that beam bunches could be compressed longitudinally to < 100 fsec. Computer simulations show that a final focus system can be built in the South Arc tunnel, and that low emittance, compressed bunches of electrons or positrons can be delivered to users. The electron bunch compressor chicane in the linac will be modified to compress positron bunches, opening up new areas of physics. A Bypass Line from Sector 20 to the BSY can be built to deliver beams to SABER, independently of LCLS. SABER will be constructed using equipment saved from the FFTB and SLC Final Focus, along with new hardware using well established designs.
Acknowledgements Special thanks to: Yuri Nosochkov, who has developed optical solutions for SABER. Karl Bane, who has been investigating wake field effects and demonstrated bunch compression in the South Arc. Lynn Bentson, who has done the preliminary engineering studies and cost estimates. Paul Emma, who did initial optics and tracking simulations for the FFTB and has helped in exploring other options. Patrick Krejcik, who revived our interest in the South Arc option and demonstrated its feasibility. Al Baker, Johannes Bauer, Martin Berndt, Patrick Bong, Dan Blankenship, Alex Chao, Scott DeBarger, Ted Fieguth, Carsten Hast, Rick Iverson, Nanyang Li, Paul Miller, Alyssa Prinz, Mike Saleski, H. Shin, Kristina Turner, Dieter Walz, Mark Woodley, and the CEP, Controls, PED, MFD, and Accelerator Operations staff members who have been helping revive the old hardware and offering help and ideas for SABER.