The Spallation Neutron Source Linac Performance and Operational

The Spallation Neutron Source Linac: Performance and Operational Experience Stuart Henderson Oak Ridge National Laboratory On Behalf of the SNS Team Special thanks to John Mammosser, Sang-ho Kim, Ricky Campisi, Marc Crofford, Yoon Kang 1 2009 Linear Collider Workshop of the Americas Managed by UT-Battelle September 29, 2009 for the U. S. Department of Energy LCWA 2009, September 29,

The Spallation Neutron Source • The SNS at Oak Ridge National Laboratory is the world’s most powerful spallation neutron source, driven by the world’s most powerful proton linac • The SNS construction project, a collaboration of six US DOE labs, began in 1999 and was • We have spent completed on-time and within budget in 2006 at a cost of 1. 4 B$ three busy years in a “ramp -up” phase, increasing beam power from ~5 k. W to 1 MW, availability from ~60% to ~85%, beam energy from 840 Me. V to 930 Me. V • SNS now operates ~5000 hrs/year, supporting a 132 Managed by UT-Battelle for the U. S. Department Energy instrument ofuser LCWA 2009, September 29, 2009

The Beam Power Frontier for Protons Courtesy J. Wei · Central challenge at the beam power frontier is controlling beam loss to minimize residual activation · 1 n. A protons at 1 Ge. V, a 1 Watt beam, activates stainless steel to 80 mrem/hr at 1 ft after 4 hrs 3 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

SNS Accelerator Complex Accumulator Ring Collimators Front-End: Produce a 1 -msec long, chopped, Hbeam 1 Ge. V LINAC Accumulator Ring: Compress 1 msec long pulse to 700 nsec Injection Extraction RF 1000 Me. V 2. 5 Me. V RTBT HEBT LINAC Chopper system makes gaps mini-pulse Current 945 ns Liquid Hg Target 1 ms macropulse 4 Managed by UT-Battelle for the U. S. Department of Energy Current Front-End 1 ms LCWA 2009, September 29, 2009

SNS Linear Accelerator 2. 5 Me. V 87 Me. V H- RFQ DTL 186 Me. V CCL 386 Me. V SRF, b=0. 61 1000 Me. V SRF, b=0. 81 · Reserve Front-end system – H- volume production source – 4 -vane 402. 5 MHz RFQ – two-stage chopper system · Front-end design parameters: – 38 m. A peak current – 68% beam-on chopping – 1. 0 msec, 60 Hz, 6% duty 5 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

SNS Linear Accelerator 2. 5 Me. V 87 Me. V H- RFQ DTL 186 Me. V CCL 386 Me. V SRF, b=0. 61 1000 Me. V SRF, b=0. 81 · Reserve SNS linac architecture consists of – Conventional normal conducting structures to 186 Me. V – Superconducting structures to 1 Ge. V · · 6 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009 402. 5 MHz Drift Tube Linac to 87 Me. V 805 MHz Coupled Cavity Linac to 186 Me. V

SNS Linear Accelerator 2. 5 Me. V 87 Me. V H- RFQ DTL 186 Me. V CCL · 81 independently-powered 805 MHz SC cavities, in 23 cryomodules SRF, b=0. 61 1000 Me. V SRF, b=0. 81 Reserve World’s first high-energy superconducting linac for protons · 386 Me. V · Medium beta cavity Space is reserved for additional cryomodules to give 1. 3 Ge. V High beta cavity 7 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

Linac RF Systems · All systems 8% duty factor: 1. 3 ms, 60 Hz · 7 DTL Klystrons: 2. 5 MW 402. 5 MHz · 4 CCL Klystrons: 5 MW 805 MHz · 81 SCL Klystrons: 550 k. W, 805 MHz · 15 solid-state modulators each providing 1 MW average power · Digital RF controls with feedback and adaptive feedforward 8 Managed by UT-Battelle for the U. S. Department of Energy 81 SCL Klystrons High Voltage Converter Modulators DTL Klystrons LCWA 2009, September 29, 2009 CCL Klystrons

Accumulator Ring and Transport Lines · Accumulates 1 -msec long H- beam pulse by multi-turn charge exchange injection via a stripper foil Circum Energy frev Q x, Q y Accum turns Final Intensity Current 9 Managed by UT-Battelle for the U. S. Department of Energy 248 m 1 Ge. V 1 MHz 6. 23, 6. 20 1060 1. 5 x 1014 26 A Collimation Extraction Injection RF RTBT HEBT Target LCWA 2009, September 29, 2009

SNS Beam Power Performance History Power on Target [k. W] 1 MW beam power on target achieved in routine operation 10 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

Major Parameters Achieved vs. Designed Design Individually achieved Highest production beam Beam Energy (Ge. V) 1. 01 0. 93 Peak Beam current (m. A) 38 40 36 Average Beam Current (m. A) 26 26 24 1000 825 60 60 60 Beam Power on Target (k. W) 1440 1030 Linac Beam Duty Factor (%) 6. 0 5. 0 1. 5 x 1014 1. 6 x 1014 1. 1 x 1014 81 80 80 Parameters Beam Pulse Length ( s) Repetition Rate (Hz) Beam intensity on Target (protons per pulse) SCL Cavities in Service 11 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

Smooth Running… 12 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

Downtime by System SNS Availability FY 07: 66% FY 08: 72% FY 09: 80% 13 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

SNS SCL Operations and Performance • The first high-energy SC linac for protons, and the first pulsed operational machine at a relatively high duty • We have learned a lot in the last 5 years about operation of pulsed SC linacs: – Operating temperature, Heating by electron loadings(cavity, FPC, beam pipes), Multipacting & Turn-on difficulties, HOM coupler issues, RF Control, Tuner issues, Beam loss, interlocks, alarms, monitoring, … • Current operating parameters are providing very stable and reliable SCL operation – Less than one trip of the SCL per day mainly by errant beam or control noise • Beam energy (930 Me. V) is lower than design (1000 Me. V) due to high-beta linac gradient limitations • No cavity performance degradation has occurred to date – Field emission very stable • Several cryomodules were successfully repaired without disassembly 14 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

How is the SNS Linac Relevant to the ILC? • While the SNS cavity and CMs were Jlab-designed, many features from Tesla technology were adopted, providing the platform upon which the SCL was built – – HOM coupler scaled from TTF Tuner assembly Fast piezo-electric control Some aspects of cavity processing incorporated into procedures • In many ways the SNS SCL is an ILC linac “in-miniature”: – pulsed operation: 1 msec, 60 Hz, 6% duty – utilizes modern digital LLRF control, – experiences Lorentz-force detuning and incorporates active compensation – we routinely confront gradient limitations and their ramifications • Largest pulsed SC linac in operation • SNS provides a real-world example of operational limitations that the community is wise to learn about 15 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

SNS Cavities and Cryomodules b=0. 81 Specifications: Ea=15. 8 MV/m, Qo> 5 E 9 at 2. 1 K b=0. 61 Specifications: Ea=10. 1 MV/m, Qo> 5 E 9 at 2. 1 K Medium beta (b=0. 61) cavity High beta (b=0. 81) cavity Helium Vessel Field Probe Fast Tuner HOM Coupler Slow Tuner Fundamental Power Coupler 16 Managed by UT-Battelle for the U. S. Department of Energy 11 CMs LCWA 2009, September 29, 2009 12 CMs

Cavity Gradient Performance History: August 2006: 7 cavities off-line; 850 Me. V; 5 Hz Large fundamental frequency coupling through HOM coupler Tuners out of range Cold-cathode gauge/ turn-on issues 17 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

H 06 back to service Irregular dynamic detuning Noisy FP H 01 out of service for repair HOMB Additional HVCM; enough RF power for design current H 01 repaired and put in the slot of CM 19 HOMB 18 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009 18

Cavity Gradient Limiting Factors (60 Hz Operation) One does not reach steady state mechanical vibration 1 cavity is disabled CM 19 removed and repaired CM 12 removed and found vacuum leaks at 3 HOM feedthroughs (fixed) -Dominated by Electron Loading (Field Emission & Multipacting) -~14 cavities are limited by coupler heating, but close to the limits by radiation heating -Operating gradients are around 85~95% of Elim 19 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

Electron Loading and Heating (Due to Field Emission and Multipacting) Source of electrons ● Field Emission due to high surface electric field · Multipacting; secondary emission – resonant condition (geometry, RF field) – At sweeping region; many combinations are possible for MP · Temporally; filling, decay time · Spatially; tapered region · Non-resonant electrons accelerated radiation/heating – Mild contamination easily processible End group heating/beam pipe heating + quenching/gas burst – But poor surface condition processing is very difficult in an Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009 operating cryomodule Result 20

MP Surface condition Radiation onset FE onset Eacc Radiation (arb. Unit) Measurements of Radiation during RF Pulse Radiation (in log, arb. Unit) SNS Cavity Operating Regime 21 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009 Time

Gradient Limitations from “Collective Effects” • Electrons from Field Emission and Multipacting – Steady state electron activity and sudden bursts affects other cavities Beam pipe Temperature Flange T Coupler or Outer T • Electron impact location depends on relative phase and amplitude of adjacent cavities • Leads to gas activity and heating with subsequent end-group quench and/or reaches intermediate temperature region (520 k); H 2 evaporation and redistribution of gas which changes cavity and coupler conditions individual limits; 19. 5, 17, 14. 5 MV/m • Example for CM 13: collective limits; 14. 5, 15, 10. 5 MV/m a b c Linac 08, Victoria Canada 22 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009 d

Individual and Collective Cavity Limits CM 19; removed Design gradient Large fundamental power through HOM coupler Average limiting gradient (collective) Field probe and/or internal cable (control is difficult at rep. rate >30 Hz) Average limiting gradient (individual) 23 Individual; powering one cavity at a time Collective; powering all cavities in a CM at the same time Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

Gradient Limitations due to HOM Coupler Response to Electron Activity Electric Field 10~14 MV/m Electron activity Destroys HOM-filter notching characteristic Leads to large fundamental power coupling Damages feedthrough, HOM signal path Irreversible HOM Signal 24 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

RF Control • Specification of +/-1%, +/-1 degree • Use of open-loop for cavity filling • Feedback and adaptive feed-forward for control during RF-flattop (beam pulse) • System reports two regulation errors: – Peak error is over entire feedback portion of pulse – Regulation Error is over beam pulse only • Predominant error is at transition of openloop cavity filling and feed-back and does not effect beam Amplitude Error (%) Phase Error (deg) 25 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009 RF System Number (SCL is 15 -95) Field Pfwd Pref

Increasing the Beam Energy • We have repaired ~10 cryomodules to regain operation of 80 out of 81 cavities – CM 19 removed: had one inoperable cavity (excessive power through HOM); removed both HOM feedthroughs – CM 12 removed: removed 4 HOM feedthroughs on 2 cavities – Tuner repairs performed on ~7 CMs – We have warmed up, individually, ~10 CMs in the past 4 years – Individual cryomodules may be warmed up and accessed due to cryogenic feed via transfer line. • We installed an additional modulator and reworked klystron topology in order to provide higher klystron voltage (for beam loading and faster cavity filling) 26 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

Plasma Processing Development • A program is underway to develop and apply plasma cleaning methods to installed accelerator RF components this – If successful should significantly reduce field emission, mulitpacting and increase operating stability of RF structures • First test on SNS cavity allows 2 MV/m increase for same radiation levels • Experimental Program Includes – Witness samples from standard processes 27 Managed by UT-Battelle –for. TM 020 test cavity the U. S. Department of Energy LCWA 2009, September 29, 2009 27

Ongoing and Future Activities • As an urgent matter, we are constructing two spare highbeta cryomodules – These will be 10 CFR 851 -compliant; vacuum vessel envelope was redesigned for pressure vessel compatibility – Cavities for 1 st string have been qualified at Jefferson Lab – Plan is to construct/integrate these spare CMs in-house • The SNS Power Upgrade Project (PUP) has CD-1 approval, and includes the following scope: – 9 additional high-beta CMs to increase energy to 1. 3 Ge. V – Associated RF systems – Ring modifications to support higher energy • We expect to involve industry in PUP CM construction (expect CD-3 approval in 2011) • We are continuing to build SRF support facilities to provide basic repair and testing capabilities in support of long-term maintenance and the Upgrade 28 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

Power Upgrade Cryomodule Design Bayonets remain in original positions “Code” Bolted Flanges 29 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009 29

SRF Maintenance and Test Facility • In place – Cryomodule testcave tied-in to CHL – High-power RF test-stand – Cleanroom facility – Ultrapure Water HPR • High pressure rinse system in fabrication • Vertical Test Area design complete; construction starting • Dedicated Cryogenic Support 30 Managed by UT-Battelle for the U. S. Department of in refrigerator Energy LCWA 2009, September 29, 2009

Conclusion • After a 3 -year “ramp-up” phase, SNS is operating at 1 MW beam power, ~85% availability • SNS SCL provides very reliable and stable operation – Much has been learned about the operation of pulsed SC linac systems – Key gradient-limiting mechanisms have been identified • Plans are in place to increase the installed cavity gradients, and to begin producing Power Upgrade-ready cryomodules • This success is due to a dedicated and hardworking SNS staff 31 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

SNS Integrated Beam Power Performance 32 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

Beamloss in the SNS Linac • Simulation predicts no beamloss in SCL • Measured prompt beam loss in the SCL < 10 -5 beam loss per cryomodule 33 • Measured residual activation throughout the SCL at 1 ft Some beam is lost everywhere ! Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009

Low-level RF Adaptive Feedforward (Ma, THP 005) • Beam turn-on transient gives RF phase and amplitude variation during the pulse, beyond bandwidth of feedback • LLRF Feedforward algorithms are used in operation (Champion, Kasemir, Ma, Crofford) • Plots below show longitudinal distribution during a 50 sec linac beam pulse • LLRF system routinely gives better than 1%/1 degree amplitude/phase stability • RMS energy jitter is 0. 35 Me. V, extrema are +/- 1. 3 Me. V; meets specificaton of +/- 1. 5 Me. V With Feed-forward Without Feed-forward 34 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009 34

E-probe & HOM signals during CM 19 test ; All showed similar behavior 19 b (no feedthrough) showed very aggressive electron activities processing was possible with no feedthrough e-probes while ramping up the gradients HOM 35 Managed by UT-Battelle for the U. S. Department of Energy LCWA 2009, September 29, 2009