1Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 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 2009 Linear Collider Workshop of the Americas September 29, 2009

2Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 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 completed on-time and within budget in 2006 at a cost of 1.4 B$ We have spent three busy years in a “ramp-up” phase, increasing beam power from ~5 kW to 1 MW, availability from ~60% to ~85%, beam energy from 840 MeV to 930 MeV SNS now operates ~5000 hrs/year, supporting a 13- instrument user- program

3Managed by UT-Battelle for the U.S. Department of Energy 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 nA protons at 1 GeV, a 1 Watt beam, activates stainless steel to 80 mrem/hr at 1 ft after 4 hrs

4Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 SNS Accelerator Complex Front-End: Produce a 1-msec long, chopped, H- beam 1 GeV LINAC Accumulator Ring: Compress 1 msec long pulse to 700 nsec 2.5 MeV LINAC Front-End Accumulator Ring RTBT HEBT InjectionExtraction RF Collimators 945 ns 1 ms macropulse Current mini-pulse Chopper system makes gaps Current 1ms Liquid Hg Target 1000 MeV

5Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 SNS Linear Accelerator 2.5 MeV1000 MeV87 MeV CCL SRF,  =0.61 SRF,  = MeV386 MeV DTL RFQ Reserve  Front-end system –H- volume production source –4-vane MHz RFQ – two-stage chopper system  Front-end design parameters: –38 mA peak current –68% beam-on chopping –1.0 msec, 60 Hz, 6% duty H-H- H-H-

6Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 SNS Linear Accelerator 2.5 MeV1000 MeV87 MeV CCL SRF,  =0.61 SRF,  = MeV386 MeV DTL RFQ Reserve  SNS linac architecture consists of –Conventional normal conducting structures to 186 MeV –Superconducting structures to 1 GeV  MHz Drift Tube Linac to 87 MeV  805 MHz Coupled Cavity Linac to 186 MeV H-H- H-H-

7Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 SNS Linear Accelerator 2.5 MeV1000 MeV87 MeV CCL SRF,  =0.61 SRF,  = MeV386 MeV DTL RFQ Reserve  World’s first high-energy superconducting linac for protons  81 independently-powered 805 MHz SC cavities, in 23 cryomodules  Space is reserved for additional cryomodules to give 1.3 GeV H-H- H-H- Medium beta cavity High beta cavity

8Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 Linac RF Systems 81 SCL Klystrons DTL Klystrons High Voltage Converter Modulators  All systems 8% duty factor: 1.3 ms, 60 Hz  7 DTL Klystrons: 2.5 MW MHz  4 CCL Klystrons: 5 MW 805 MHz  81 SCL Klystrons: 550 kW, 805 MHz  15 solid-state modulators each providing 1 MW average power  Digital RF controls with feedback and adaptive feedforward CCL Klystrons

9Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 Accumulator Ring and Transport Lines Circum 248 m Energy 1 GeV f rev 1 MHz Q x, Q y 6.23, 6.20 Accum turns1060 Final Intensity1.5x10 14 Current26 A HEBT RTBT Injection Collimation RF Extraction Target  Accumulates 1-msec long H- beam pulse by multi-turn charge exchange injection via a stripper foil

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

11Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 ParametersDesign Individually achieved Highest production beam Beam Energy (GeV) Peak Beam current (mA) Average Beam Current (mA)26 24 Beam Pulse Length (  s) Repetition Rate (Hz)60 Beam Power on Target (kW) Linac Beam Duty Factor (%) Beam intensity on Target (protons per pulse)1.5 x x x SCL Cavities in Service8180 Major Parameters Achieved vs. Designed

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

13Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 Downtime by System SNS Availability FY07: 66% FY08:72% FY09:80%

14Managed 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 MeV) is lower than design (1000 MeV) 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 Proactive and aggressive maintenance strategy (fix annoyances/problems before they limit performance)

15Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 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 How is the SNS Linac Relevant to the ILC?

16Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 SNS Cavities and Cryomodules Fundamental Power Coupler HOM Coupler Field Probe  =0.61 Specifications: E a =10.1 MV/m, Q o > 5E9 at 2.1 K Medium beta (  =0.61) cavity High beta (  =0.81) cavity Slow Tuner Helium Vessel Fast Tuner  =0.81 Specifications: E a =15.8 MV/m, Q o > 5E9 at 2.1 K 11 CMs 12 CMs

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

18Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, HOMB Additional HVCM; enough RF power for design current H01 repaired and put in the slot of CM19 H06 back to service H01 out of service for repair Irregular dynamic detuning Noisy FPHOMB

19Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 Cavity Gradient Limiting Factors (60 Hz Operation) -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 E lim One does not reach steady state mechanical vibration 1 cavity is disabled CM19 removed and repaired CM12 removed and found vacuum leaks at 3 HOM feedthroughs (fixed)

20Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 End group heating/beam pipe heating + quenching/gas burst Electron Loading and Heating (Due to Field Emission and Multipacting) ● 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 – But poor surface condition  processing is very difficult in an operating cryomodule Source of electrons Result

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

22Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 Linac 08, Victoria Canada Gradient Limitations from “Collective Effects” a b c d Beam pipe Temperature individual limits; 19.5, 15, 17, 14.5 MV/m collective limits; 14.5, 15, 15, 10.5 MV/m Flange T Coupler or Outer T Electrons from Field Emission and Multipacting – Steady state electron activity and sudden bursts affects other cavities Leads to gas activity and heating with subsequent end-group quench and/or reaches intermediate temperature region (5-20k); H 2 evaporation and redistribution of gas which changes cavity and coupler conditions Example for CM13: Electron impact location depends on relative phase and amplitude of adjacent cavities

23Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 Large fundamental power through HOM coupler CM19; removed Field probe and/or internal cable (control is difficult at rep. rate >30 Hz) Design gradient Average limiting gradient (individual) Average limiting gradient (collective) Individual; powering one cavity at a time Collective; powering all cavities in a CM at the same time Individual and Collective Cavity Limits

24Managed 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 Electron activity  Destroys HOM-filter notching characteristic  Leads to large fundamental power coupling  Damages feedthrough, HOM signal path  Irreversible Electric Field 10~14 MV/m HOM Signal

25Managed 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 open-loop cavity filling and feed-back and does not effect beam Field Pfwd Pref Amplitude Error (%) Phase Error (deg) RF System Number (SCL is 15-95)

26Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 We have repaired ~10 cryomodules to regain operation of 80 out of 81 cavities – CM19 removed: had one inoperable cavity (excessive power through HOM); removed both HOM feedthroughs – CM12 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 re-worked klystron topology in order to provide higher klystron voltage (for beam loading and faster cavity filling) Further increases in beam energy require increasing the installed cavity gradients to design values Increasing the Beam Energy

27Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 A program is underway to develop and apply plasma cleaning methods to installed accelerator RF components Plasma Processing Development 27 – If successful this 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 – TM020 test cavity – Full RF structures (3-cell test cavity) for procedure development

28Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, 2009 As an urgent matter, we are constructing two spare high-beta cryomodules – These will be 10CFR851-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 GeV – 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 Ongoing and Future Activities

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

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

31Managed by UT-Battelle for the U.S. Department of 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

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

33Managed 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 < beam loss per cryomodule Measured residual activation throughout the SCL at 1 ft Some beam is lost everywhere !

34Managed by UT-Battelle for the U.S. Department of Energy LCWA 2009, September 29, Low-level RF Adaptive Feedforward (Ma, THP005) 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 MeV, extrema are +/- 1.3 MeV; meets specificaton of +/- 1.5 MeV Without Feed-forward With Feed-forward

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