1. Space Charge Issues in the SNS Linac and Ringby
Sarah Cousineau,
on behalf of the SNS project
Oak Ridge National Laboratory, USA
Coulomb05 Workshop,
Senigallia, Italy, September 12 – 16, 2005

2. The SNS Accelerator Complex
At peak : ~500 People
worked on the construction
of the SNS accelerator
Oak Ridge Accelerator Systems: Integration, Installation, Commissioning, Operation

3. The Spallation Neutron Source
The SNS is a short-pulse neutron source with a single-purpose mission of neutron science, under construction at ORNL
SNS will be the world’s leading facility for neutron scattering research
The peak neutron flux will be ~20–100x ILL
The SNS will begin operation in 2006
SNS is funded through DOE-BES and has a Baseline Cost of 1.4 B$
It will be a short drive from HFIR, a reactor source with a flux comparable to the ILL

4. SNS Design Parameters Average proton power on target: 1.44 MW
Beam energy: 1 GeV
Pulse parameters: 1-ms pulse, 60 Hz repetition rate (6% duty)
Beam current:
26 mA average macropulse current
38 mA peak H- current
1.6 mA average linac beam current
Ring accumulation: 1 ms pulse compressed to 695 ns in 1060 injected turns
Ring intensity: 1.5x1014 protons
945 ns
period
Chopping structure 38 mA 26
1 ms long
Macropulse structure
20 to 50 ms
ramp
time
mini-pulse

5. SNS Accelerator Complex
Front-End: Produce a 1-msec long, chopped, low-energy H- beam
LINAC: Accelerate the beam to 1 GeV
Accumulator Ring: Compress 1 msec long pulse to 700 ns
H- stripped to protons
Deliver beam to Target
2.5 MeV
86.8 MeV
186 MeV
387 MeV
1000 MeV
Ion Source
RFQ
DTL
CCL
SRF, b=0.61
SRF, b=0.81
Chopper system makes gaps
945 ns
Current
1ms
mini-pulse
Current
1 ms macropulse

6. Ion Source Beam Distributions
H- ion source, capable of >38 mA and 60 Hz operation.
Emittance measurements show that the beam leaving the ion source has “wings” or tails; caused by optical nonlinearities in electrostatic lenses.
Measured distributions used to generate input distribution for simulations.
Horizontal
Vertical

7. MEBT Halo Scrapers
MEBT scrapers allow cleaning of beam tail coming from source.
No scraping
With scraping

8. Matching the lattice with space charge
Linac lattice must be matched for a specific current.
Challenge during commissioning runs, when beam current and quad settings were often changing.
MEBT + DTL
SC Matched case: 20mA beam with 20mA lattice.
MEBT + DTL
SC Mismatched case: 38mA beam with 20mA lattice.

9. Linac Beam Profile Measurements
Measured profiles as function of last two MEBT quad setpoints.
Even nominal (design) cases shows some “halo” (could be ion source tails). Here we define halo as any non-Gaussian tails.
See that non-gaussian tails can be “tuned” by matching into DTL1.
DTL3 wirescanner – various quad settings
DTL3 wirescanner – best matched case

10. Linac Beam Profile Measurements
Main commissioning concern is rms beam size, not halo.
Preliminary studies underway with Parmilla to investigate halo for future operation.
Benchmarked results: Qualitatively good (trends agree), quantitative fair.
Example benchmark of DTL3 wirescan data

11. Linac Beam Profile Measurements
Small halo seen for design case, large halo for mismatched case.
Halo seems to be large in DTL, but dies of in CCL.
Simulations show this is because core grows and consumes halo in CCL.
Design quad settings
Mismatched quad settings

12. Mismatched Beam Simulations
For design lattice settings, previous studies show strong dependence on initial distributions.
For mismatched case, final distribution out of warm linac is independent of the initial distribution.
Nominal
Parabolic
MEBT
DTL3
DTL5
CCL4

13. Other Linac High Intensity Beam Challenges
Beam loading a major issue at high intensity (≥ 20 mA)
Adaptive feed forward necessary for good bunching, low losses.
Beam loading prevented normal acceleration of beam with >20mA peak current
Beam loading effect eliminated by means of Adaptive Feed Forward.

14. SCL Commissioning Results
4K commissioning run: Reached 860 MeV, 180us, 20mA on Aug 21
4 low energy SCL cavities out of tuning range at 4K.
Missing cavities lead to high losses in transition region from doublet to FODO structure.
Commissioning the SCL relied on tuning on losses!

15. SCL Commissioning Results
2K commissioning run: Reached 910 MeV on Aug 30th!
Losses way down with missing upstream cavities included.
Beam is Ring Ready!

16. SNS Ring: Loss-loss design philosophy
The ring was designed with a low loss philosophy.
Designed centered around mitigating losses from:
Injection.
Extraction.
Space charge.
Other collective effects: impedances, e-p, etc.
Ring will require uncontrolled losses ≤ 0.01% of the total beam intensity.

17. Phase-Space Painting with Space-Charge Horizontal Phase-Space: Px vs. X
Injection painting scheme optimized to minimize space charge.
Paint with hole in the center to help create uniform density.
Also try to keep circulating beam foil intercepts to a minimum (~6 foil hits per proton).
200 Turns
600 Turns
No Space Charge – 1060 Turns
1060 Turns

18. Phase-Space Painting with Space-Charge Real Space: Y vs. X
Correlated painting scheme chosen over anti-correlated because:
Smaller number of foil hits.
Less space charge halo observed in simulation.
Footprint suites target requirements.
No Space Charge – 1060 Turns
600 Turns
1060 Turns
200 Turns

19. Lattice Tune Chosen to Avoid Resonances
Design lattice tune for 1.4 MW operation: Qx=6.23, Qy=6.20.
Intensity limitation for this tune is half-integer coherent resonance.
Chromaticity adds another Q = 0.07 in spread.
Sextupoles in ring for correcting chromaticity.
SC Tune Footprint
N=0.5*1014 – 263 turns
N=1.0*1014 – 526 turns
N=2.0*1014 – 1052 turns

20. ORBIT Simulation of Baseline Accumulation Scenario
Beam broadening from space charge observed:
Paint to  = 165, space charge broadens to 175
Emittance Distributions
Fraction larger than emittance
No Space Charge
With Space Charge
No Space Charge
With Space Charge
Emittance ( mm mrad)

21. Alternative Lattice Tune
Alternative lattice tune studied: Qx=6.4, Qy=6.30.
SC Tune Footprint
Crosses resonances:
3Qx = 19; normal sextupole
2Qx + Qy = 19; skew sextupole
Sextupole correctors available in the ring for correcting sextupole errors.

22. Anticipated Loss Distribution in the SNS Ring
Space charge induced beam halo will be intercepted by collimation system.
Final loss distribution determined by collimation system (except for injection, extraction losses).
Losses > 1 W/m in collimation straight.
Simulated Loss Pattern in Ring

23. The Remaining Commissioning Schedule
CD-4 deadline
30/Jun/06
Jan/05
Jan/06
Jan/07
Linac, to linac dump
25/Jul – Sept/05
HEBT/Ring/RTBT, to extr. dump
2/Jan – 19/Feb/06 (47 days)
RTBT, to target
1/Apr – 28/Apr/06
Compared to original plan, ring commissioning will be performed within a much smaller time frame and with a very reduced suite of diagnostics.

24. Post – CD4 Intensity Ramp-Up
We will commission the beam with low intensity, ~2×1013 ppp (10mA, 1 Hz).
We will ramp up beam power gradually.
Should reach 1.4 MW by 2010.
Plans for second target station in ~2010.

25. SNS Power Upgrade

26. SNS Power Upgrade Technical Issues
Accelerator Issues
Cryomodule Acquisition Approach
Nine (9) HB Cryomodules
SRF Facility to Support Acquisition Approach
Full Production vs Maintenance/Testing
Front End/Ion Source R&D
Reliability/Higher Current – 75 mA
Ring Injection Issues Dump Upgrade
Foil issues: Need new material, multiple foils, or laser stripping.
New Magnets: Scaled for 1.3 GeV
Injection Dump: Capacity of 150kW may need upgrade to 300kW.
Target Issues
Target module designed for 1 MW
R&D (Bubble Injection) to extend to > 2 MW
Accelerator Physics R&D projects
Laser stripping proof of principle experiments.
Active feedback system experiments at PSR.

27. Active Feedback System for Upgrade
Active feedback system planned for intensity upgrade.
Can stabilize e-P and other instabilities resulting from collective effects.
Proof-of-principle experiments done at PSR in spring, 2005 (SNS, Argonne, LANL, Indiana University collaboration).
Active Feedback System
Kicker
Pick-up
Circulating beam
Courtesy C. Deibele

28. First results of e-P feedback experiments
Results summary:
Instability suppression observed.
During normal operation, high RF voltage used to suppress instability.
With damper on, RF voltage could be reduced by 13% to 18%.
System still needs optimization.
Courtesy S. Henderson

29. Summary SNS is on track for completion in 2006.
So far, SNS warm and superconducting linac has been commissioned. All major beam commissioning milestones have been met.
Have observed some space charge and high intensity affects in during linac commissioning.
Space charge effects will become more apparent during ramp up to high intensity.
SNS has been approved for a beam power upgrade to 3 MW beginning in ~2010.