1. CBETA Vacuum System
and Beam Stop
Yulin Li, CLASSE

2. Outline Vacuum System Design Scope
Vacuum System Requirement and Considerations
Vacuum System Layout and Sections
Vacuum pumping and simulations
Vacuum system construction, installation and operation

3. Design Scope & Requirement
Provide vacuum beampipes for 4-turn ERL electron beams with sufficient aperture, while not interfere with beam transport magnets.
Through proper design, construction, installation and vacuum pumping, achieve adequate level of vacuum to minimize beam losses due to residual gas scattering.
Provide necessary flexibility to allow accelerator reconfiguration during CBETA commissioning and physics run.
Integrate required beam instrumentation and diagnostics, such as BPMs.

4. Design Considerations
Beampipe materials: Aluminum (6061) preferred for good electric and thermal properties, non-magnetic and no residual radioactivity. Stainless steel may also be used if there is sufficient cost and schedule benefit.
Provide sufficient large beam apertures, while allowing magnets position adjustments.
For high beam current (40x7=280 mA in FFAG sections) operations, design efforts will be made to keep low beam impedance, including smooth beampipe inner profiles, RF shielded bellows and gate valves, gentle transitions between different beampipe cross sections, etc.
Draw experiences from CESR and ERL Photo-Cathode injector designs to minimize needs for vacuum R&D efforts.

5. Layout and Sections
Photo-Injector and dump beamline to be re-used with minor modifications. Most of their components were successfully tested in December 2016 with electron beam up to 5 mA, after they were re-located to the CBETA area.
MLC is a self-contained component, only need vacuum pumping through beamline when warm.

6. Laser Entrance, Buncher, etc.
The Injector
ERL photo-injector (including the photo-cathode electron gun and the super-conducting cavity cryo-module, ICM) performed very well for the Cornell ERL prototype injector project.
The photo-injector has been relocated in the CBETA area (L0E) to serve as the CBETA injector. The relocated injector was operated satisfactorily since.
Only minor modification (mainly the merger) is expected.
ICM
Laser Entrance, Buncher, etc.
Gun

7. Electron Beam Stops 600-kW Aluminum Main Beam Stop
An aluminum 600-kW beam stop was constructed and performed very well for the Cornell ERL prototype injector project.
This main beam stop, together with its protection setup, has been relocated into the L0E area for CBETA project.
Many low power beam stops were also constructed and performed well, and can provide flexibility in staged commissioning of CBETA.
Low Power Beam Stops (Faraday-Cup)
600-kW Aluminum Main Beam Stop

8. Main Beam Stop Beamline
BPMs
Quad Detector
Quad 2
Quad 1
Raster
80 TCs map beam stop body temperature
To dilute power density, a pair quads expand the electron beam, and a raster magnet orbits the expanded beam at 60 Hz. (Detail of the beam stop design is described in: X. Liu, et al, NIM A 709 (2013) 37-43)
A pair of BPMs and a quad-detector monitor beam trajectory and envelop.
Two large ion pumps and three TiSPs handle high gas load from electron-induced desorption.

9. Splitter Section
4 different energies beams will be split into four separated beampipes and then recombined into a single beampipe for optics and timing reasons.
One of important features of this section is to provide beam path length adjustments for each of four energy electron beams.
Various reconfigurations are expected during staged commissioning (from 1-turn to 4-turn). Low cost, non-RF shield components will be used for the initial low beam current stages.
MLC
FFAG

10. FFAG Arcs and Straight
In the FFAG arcs and straight, permanent magnets are arranged in more or less periodic double-magnet cells, with relatively short drifts between magnets reserved as much as possible for vacuum pumping, beam instrumentation.
Single-bore beampipe is through permanent magnets used in the FFAG sections to contain electron beams at all planned energies. So the vacuum system here is to be designed as one-stage installation.
Consideration has been taken in the lattice and magnet design to allow ‘kink-free’ beampipe with uniform apertures, for low fabrication cost and low beam impedance.
It is efficient use of these drifts by constructing beampipe assembly through multiple FFAG magnet cells, reducing number of beampipe flanges.

11. 4-Cell FFAG Beampipes for FA/FB Section
A 4-cell beampipe for the hybrid FFAG magnets was fully designed, as shown.
A revised 4-cell beampipe for the Halbach FFAG magnets is underway, but will have similar functional components, including 4 BPMs, 1 pumping port and two instrumentation ports.

12. 4-Cell FFAG Beampipes Construction
4-Button BPM (for Halbach Cell)
For 4-cell FFAG beampipes in the arcs (FA/FB/TA/TB), a ‘block-and-straight’ approach is planned in construction.
The FFAG 4-button BPMs (108 units + spares) will be ‘mass-produced’. Survey and support features are build into the BPM housing for beampipe assembling, and support on girders.
During beampipe construction, four BPM ‘blocks’ can be precisely located on a jig-plate, according to the FFAG cell geometry.
Straight beampipes (with functional ports and end flanges) will then ‘slide’ onto the BPM blocks, and welded to complete the 4-cell beampipes.
The ‘block-and-straight’ approach avoid beam pipe bending, which is difficult (if not impossible) to achieve required accuracy, especially for the TA/TB sections, where bending curvature varies cell-to-cell.

13. Conceptual 4-Cell FFAG Assembly
Two 4-cell beampipes (on two girders) is connected via sliding joints
One of the vacuum beampipe design challenges is satisfying both the beam aperture and magnet clearance requirements, while hosting a large suite of functional components (BPMs and beam instrument, vacuum pumping and gauging)
The 4-cell beampipe designs for the hybrid magnets (shown here) meet these requirement.
This work will be repeated when the Halbach FFAG cell geometry/lattice becomes available.

14. A Design for FFAG Sections
RF-Shielded Bellows
Bellows with RF shields are needed to provide adequate flexibility of the vacuum system in vacuum component installation and operations.
A – CESR-Style
A Design for FFAG Sections

15. Aluminum Flanges
Each FFAG 4-cell beampipe required at least 6 functional ports.
Experiments on all-aluminum flanges were carried out, with very positive results.
We plan to use these all-aluminum (Alloy 6013-T6) flanges for most of the functional ports on the FFAG 4-cell beampipes.
6013-T6 is readily available (from McMaster, for example) at very low cost.
Machined a pair of 2-3/4” CF flanges, and a CESR oval CF flange for testing.
Performed welding and 120C bakeout cycles.
Performed multiple (10+) sealing tests (with purchased A1100-H14 gaskets), without any leak.

16. Fabrication & Integration
All vacuum beampipes will be fabricated following stringent ultra-high vacuum (UHV) procedure and practice.
All beampipe assemblies will be certified to be leak-free, and will be baked in vacuum up to 150C.
Chemically filtered N2 will be used for venting and purging, during integration to girders at BNL and installation at L0E, so that in situ bakeout can be avoided.
Vacuum processed FFAG beampipes will be back-filled with filtered nitrogen and pinched off. These beampipes will then delivered to BNL to be assembled onto the girder units.
During initial staged low current operations, regular UHV gate valves will be used. Further cost/benefit analyses will determine the necessity of the RF-shielded valves.

17. Pumping and Instrumentation
Compact NEG/SIP pumps will be used (pumps with extensions in the FFAG sections).
Additional ion gauges and RGAs for monitoring and control
Compact pumps fit in FFAG cells and splitter beamlines. No mechanical supports are needed for these pumps

18. CBETA Vacuum – Pumping & Instrumentation
GUN
ICM
MLC
Most existing critical accelerator components are already equipped with RF-GVs. ICM and MLC have no build-in vacuum pumping and instrumentation, so need to be staged into the CBETA vacuum system.
Signals from ion pumps are to be used to monitor (and interlock) vacuum system performances, but a few additional vacuum gauges will be installed as cross checks.
A pair of new RF-GVs is to be installed to isolate FFAG sections from the splitter sections.

19. Gate Valve Interlocking – An Example
GV Enable
Sector 1
Sector 2
“Rough VAC1” “OK”
NexTorr 11 “<10-7 torr”
NexTorr 12 “<10-7 torr”
Sector1 “Ready”
(Pirani Contactor)
“Rough VAC2” “OK”
NexTorr 21 “<10-7 torr”
NexTorr 22 “<10-7 torr”
Sector2 “Ready”
(Pirani Contactor)
GV Ready

20. Designed CBETA Vacuum Level
Assume no in situ bakeout, and NEG pumps near full pumping speed:
Initial vacuum pressure (2~ 3 days of pump-down)  ≤ 10 nTorr  Gas composition: 20% H2O / 30% (CO/CO2-eqv.) / 50% (H2)
‘Stable’ vacuum pressure ( 3 ~ 7 days of pump-down)  ≤ 1 nTorr  Gas composition: 10% H2O / 15% (CO/CO2-eqv.) / 75% (H2)

21. Vacuum Simulations
3D Test-Particle Monte-Carlo program, MolFlow+, will be used to optimize vacuum pumping and to simulate vacuum performances of CBETA vacuum system. For CBETA, only material thermal outgassing is considered.
Simulation showed that one pump per 4 FFAG cells is sufficient
Simulated pressure evolutions in FFAG arcs
Courtesy of Cara Zhao (Cornell Undergrad, Computer Science)

22. Example of Fast Pumpdown
CESR LINAC Gun Cathode Replacement V1 V2 V3 N2
Self-Sealed Quick Connects
NANOCHEM Filter H2O/THC ~ 0.1 ppb!

23. Conclusion
A vacuum design presented to meet the physics requirements of the CBETA project.
Vacuum designs will draw our experiences from the Cornell Prototype Photo-Cathode Injector project, as well from CESR operations.
A staged build up of CBETA vacuum system is expected.
Technical design of vacuum system has started (with functional building blocks, such as the BPMs, pumping port, flanges, etc.), and still need information on the magnet layout to finalize the beampipe design.

24. Thank you!

25. Splitter Chambers
To keep low beam impedance, the beam splitting and combining vacuum chambers may be made of aluminum alloy (6061-T6) with smooth beam path transitions, as used in the ERL Photo-Cathode Injector

26. Beam Path Length Adjustments
Path Length Adjustment with 3 bellow sets.  Non-RF shielded for initial config. w/ large variation  RF-shielded for high beam current ops
Courtesy of David Burke (CLASSE)

27. Ion Clearing Electrode
Ion trapping may not be avoidable without active clearing method, due to the nature of in the final CBETA CW beam operations.
Low impedance clearing electrodes may be deployed at various locations to reduce ill-effect from the ion trapping. Thermal-spray thin electrodes have been successfully implemented in Super KEKB, CesrTA and ERL Injector.