Space Charge Issues in the SNS Linac and

Space Charge Issues in the SNS Linac and Ring by Sarah Cousineau, on behalf of the SNS project Oak Ridge National Laboratory, USA Coulomb 05 Workshop, Senigallia, Italy, September 12 – 16, 2005

The SNS Accelerator Complex ~20 M$ ~113 M$ ~177 M$ ~60 M$ ~106 M$ ~63 M$ Oak Ridge Accelerator Systems: Integration, Installation, Commissioning, Operation At peak : ~500 People worked on the construction of the SNS accelerator

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– 100 x 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 Ge. V • Pulse parameters: 1 -ms pulse, 60 Hz repetition rate (6% duty) Chopping structure mini-pulse 38 m. A • Beam current: 945 ns period – 26 m. A average macropulse current – 38 m. A peak H- current – 1. 6 m. A average linac beam current • Ring accumulation: 1 ms pulse compressed to 695 ns in 1060 injected turns • Ring intensity: 1. 5 x 1014 protons time Macropulse structure 26 m. A 20 to 50 ms ramp 1 ms long time

SNS Accelerator Complex Front-End: 2. 5 Me. V Ion Source RFQ Accelerate the beam to 1 Ge. V Compress 1 msec long pulse to 700 ns H- stripped to protons 86. 8 Me. V DTL 945 ns Current Accumulator Ring: 186 Me. V CCL 387 Me. V SRF, b=0. 61 Chopper system makes gaps Deliver beam to Target 1000 Me. V SRF, b=0. 81 mini-pulse 1 ms macropulse Current Produce a 1 msec long, chopped, low -energy Hbeam LINAC: 1 ms

Ion Source Beam Distributions • H- ion source, capable of >38 m. A 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. SC Matched case: 20 m. A beam with 20 m. A lattice. SC Mismatched case: 38 m. A beam with 20 m. A lattice. MEBT + DTL

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 DTL 1. DTL 3 wirescanner – various quad settings DTL 3 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 DTL 3 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 DTL 3 DTL 5 CCL 4

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

SCL Commissioning Results • 4 K commissioning run: Reached 860 Me. V, 180 us, 20 m. A on Aug 21 • 4 low energy SCL cavities out of tuning range at 4 K. • 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 • 2 K commissioning run: Reached 910 Me. V on Aug 30 th! • 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. 200 Turns • 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). 600 Turns No Space Charge – 1060 Turns

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

Alternative Lattice Tune • Alternative lattice tune studied: Qx=6. 4, Qy=6. 30. SC Tune Footprint Crosses resonances: 3 Qx = 19; normal sextupole 2 Qx + Qy = 19; skew sextupole Sextupole correctors available in the ring for correcting sextupole errors.

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

The Remaining Commissioning Schedule Jan/05 Linac, to linac dump 25/Jul – Sept/05 Jan/06 CD-4 deadline 30/Jun/06 HEBT/Ring/RTBT, to extr. dump 2/Jan – 19/Feb/06 (47 days) Jan/07 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 – CD 4 Intensity Ramp-Up • We will commission the beam with low intensity, ~2× 1013 ppp (10 m. A, 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 Ø Cryomodule Acquisition Approach § Nine (9) HB Cryomodules Ø SRF Facility to Support Acquisition Approach § Full Production vs Maintenance/Testing Ø Front End/Ion Source R&D § Reliability/Higher Current – 75 m. A Ø Ring Injection Issues Dump Upgrade § Foil issues: Need new material, multiple foils, or laser stripping. § New Magnets: Scaled for 1. 3 Ge. V § Injection Dump: Capacity of 150 k. W may need upgrade to 300 k. W. • Target Issues Ø 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. Active Feedback System • 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). 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.