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

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

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

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

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

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

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

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.

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

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

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

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

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.

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!

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

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.

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

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

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

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)

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.

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

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.

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.

SNS Power Upgrade

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.

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

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

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.