Plans for a Superconducting Proton Linac at CERN Part 1 R. Garoby – 10/06/2010

OUTLINE Introduction (SC linac) Low Power SPL in a new LHC injector complex High power SPL as a proton driver R & D plans

Introduction: Basics of Superconducting RF Linacs* * Material from M. Vretenar and F. Gerigk

Why Linear Accelerators ? Linacs are mostly used for: 1. Low-Energy accelerators (injectors to synchrotrons or stand-alone) for protons and ions, linear accelerators are synchronous with the RF fields in the region where velocity increases with energy. As soon as velocity is ~constant, synchrotrons are more efficient (multi-pass instead of single pass). 2. Production of high-intensity proton beams (in comparison with synchrotrons, linacs can operate at higher repetition rate, they are less affected by resonances and have losses more distributed  more suitable for high intensity beams (but in competition with cyclotrons…). 3. High energy lepton colliders for electrons at high energy, the main advantage being the absence of synchrotron radiation.

Proton and Electron Velocity b2=(v/c)2 as function of kinetic energy T for protons and electrons. electrons Relativistic (Einstein) relation: Classic (Newton) relation: “Einstein” protons “Newton” Protons (rest energy 938.3 MeV): follow “Newton” mechanics up to some tens of MeV (Dv/v < 1% for W < 15 MeV) then slowly become relativistic (“Einstein”). From the GeV range velocity is nearly constant (v~0.95c at 2 GeV)  linacs can cope with the increasing particle velocity, synchrotrons are more efficient for v nearly constant. Electrons (rest energy 511 keV, 1/1836 of protons): relativistic from the keV range (v~0.1c at 2.5 keV) then increasing velocity up to the MeV range (v~0.95c at 1.1 MeV)  v~c after few meters of acceleration in a linac (typical gradient 10 MeV/m). Accelerating gradients that we can provide with a linac is nearly constant (2 MeV/m) --- synchrotron radiation ((beta*gamma)**4)/(r**2)

Synchronism condition The distance between accelerating gaps has to be proportional to particle velocity Example: a linac superconducting 4-cell accelerating structure Synchronism condition: t (travel between centers of cells) = T/2 In an ion linac, cell length has to increase (up to a factor 200 !) and the linac will be made of a sequence of different accelerating structures (changing cell length, frequency, operating mode, etc.) matched to the ion velocity. For electron linacs, b =1, l =l/2  An electron linac will be made of an injector + a series of identical accelerating structures, with cells all the same length Note: in the example above, the increase in beta inside the structure is neglected!

Superconducting Proton Linac The same cavity shape can be adapted for use at different proton velocities (beta’s), just changing the cell length accordingly to beta. … but the real estate accelerating gradient reduces with b Need for different cavity shape/design at lower b b=0.52 b=0.7 b=0.8 Cell length has to be matched to the increasing particle velocity b=1 CERN (old) SPL design, SC linac 120 - 2200 MeV, 680 m length, 230 cavities

Why are sc cavities attractive? Instead of Q values in the range of ~104 for normal conducting cavities, superconducting cavities easily reach 109 - 1010, which drastically reduces the surface losses Pd (basically down to ~0) ➜ large accelerating gradients with minimal peak RF power (= power required for beam acceleration in the structure) However, due to the large stored energy, the cavity filling time tl increases (often into the range of the beam pulse length), and hence the average RF power (only valid for SC cavities)

Low Power SPL in a new LHC injector complex * Detailed during Workshop on “New Opportunities in the Physics Landscape at CERN” CERN, 10-13 May, 2009

Motivation for new injectors 1. Reliability ­ The present accelerators are getting old (PS is 50 years old…) and they operate far beyond their initial design parameters Interest of new accelerators tailored to the LHC and its future needs 2. Performance ­ Luminosity depends directly upon beam brightness N/e* Brightness is limited by space charge at low energy in the injectors Þ Need to increase the injection energy in the synchrotrons New LHC injectors

Description Present Future Linac2 Linac4 PSB LP-SPL New LHC injectors Proton flux / Beam power LP-SPL: Low Power-Superconducting Proton Linac (4 GeV) PS2: High Energy PS (~ 5 to 50 GeV – 0.3 Hz) sLHC: “Super-luminosity” LHC (up to 1035 cm-2s-1) 50 MeV Linac2 Linac4 160 MeV 1.4 GeV PSB LP-SPL 4 GeV New LHC injectors 26 GeV PS PS2 Main requirements of PS2 on its injector: 50 GeV Output energy Requirement Parameter Value 2.2 x ultimate brightness with nominal emittances Injection energy 4 GeV Nb. of protons / cycle for LHC (180 bunches) 6.7 ´ 1013 Single pulse filling of SPS for fixed target physics Nb. of protons / cycle for SPS fixed target 1.1 ´ 1014 450 GeV SPS LHC / sLHC 7 TeV

Comparison LP-SPL / RCS Summary table comparing: RCS (10 Hz) filling PS2 in 14 pulses (1.3 s) + multiple gymnastics in PS2 LP-SPL (2Hz) filling PS2 in 0.6 ms + no gymnastics in PS2 Filling time PS2 Time structure for LHC Relative proton rate Fixed target physics Heavy Ions Upgrade potential Relative Cost1 LP-SPL 0.6ms inherent 2.5 ideal OK high 1.28 RCS 1.3s different 1 low Advantage SPL New LHC injectors 1 The relative cost considers only the items that differ between both options ß The LP-SPL is the best solution for the LHC and it offers a large upgrade potential for the future needs of physics. Ref.: Comparison of Options for the Injectors of PS2, CERN-AB-2007-014 (PAF), http://cdsweb.cern.ch/record/1029954/files/ab-2007-014.pdf

Site layout SPS PS2 ISOLDE New LHC injectors PS SPL Linac4

LHC beams from PS2 (i) Nominal bunch train at PS2 extraction h=180 (40 MHz) with bunch shortening to fit SPS 200 MHz. 168 buckets filled leaving a kicker gap of ~ 300 ns (50 GeV!) Achieved by direct painting into PS2 40 MHz buckets using SPL chopping. No sophisticated RF gymnastics required. Beam parameters Extraction energy: 50 GeV Maximum bunch intensity: 4E11 / protons per LHC bunch (25 ns) Bunch length rms: 1 ns (identical to PS) Transverse emittances norm. rms: 3 mm (identical to PS) Any other bunch train pattern down to 25 ns spacing Straightforward with SPL 40 MHz chopping and 40 MHz system Again without sophisticated RF gymnastics Same brightness per bunch New LHC injectors Chamonix 2010

LHC beam from PS2 (ii) Example 25 ns beam from LP-SPL – PS2: PS2 will provide “twice ultimate” LHC bunches with 25 ns spacing Bunch train for SPS twice as long as from PS Only 2 injections (instead of 4) from PS to fill SPS for LHC PS2 cycle length 2.4 s instead of 3.6 s for PS Reduces SPS LHC cycle length by 8.4 of 21.6 s (3x3.6 – 1x2.4) Reduced LHC filling time New LHC injectors 1 2 Booster SPS injection plateau 3x3.6 s = 10.8 s up to 4 consecutive injections PS LP-SPL SPS plateau ~2.4 s 2 injections PS2 Chamonix 2010 15

Linac4: main characteristics H- Þ charge exchange injection and painting in PSB Ion species H− Output Energy 160 MeV Bunch Frequency 352.2 MHz Max. Rep. Rate 2 Hz Max. Beam Pulse Length 1.2 ms Max. Beam Duty Cycle 0.24 % Chopper Beam-on Factor 65 % Chopping scheme: 222 transmitted /133 empty buckets Source current 80 mA RFQ output current 70 mA Linac pulse current 40 mA N. particles per pulse 1.0 × 1014 Transverse emittance 0.4 p mm mrad Max. rep. rate for accelerating structures: 50 Hz 160/50 MeV Þ factor 2 in bg2)  same tune shift with twice the intensity. Re-use of LEP RF components: klystrons, waveguides, circulators. New LHC injectors Chopping at low energy to reduce beam loss in PSB. Structures and klystrons dimensioned for 50 Hz Power supplies and electronics dimensioned for 2 Hz, 1.2 ms pulse.

Linac4: layout New LHC injectors Linac4 is a normal-conducting H− linac at 160 MeV energy, made of 4 types of 352 MHz accelerating structures, matched to the increasing beam energy. A beam chopper at low energy allows modulating the linac beam pulse to minimise losses in the ring. A beam dump at linac end allows setting-up of the beam, will be displaced when connecting to the SPL. The Linac4 project includes important modifications to the PSB injection region (higher injection energy, H- stripping). 160 MeV New LHC injectors 100 MeV 50 MeV Linac4 tunnel and surface equipment building

Linac4: Block diagram H- RFQ CHOPPER DTL CCDTL PIMS New LHC injectors Linac4: 80 m, 18 klystrons 45keV 3MeV 3MeV 50MeV 94MeV 160MeV H- RFQ CHOPPER DTL CCDTL PIMS RF volume source (DESY) 45 kV Extrac. Radio Frequency Quadrupole 3 m 1 Klystron 550 kW Chopper & Bunchers 3.6 m 11 EMquad 3 cavities Drift Tube Linac 18.7 m 3 tanks 3 klystrons 4.7 MW 111 PMQs Cell-Coupled Drift Tube Linac 25 m 21 tanks 7 klystrons 7 MW 21 EMQuads Pi-Mode Structure 22 m 12 tanks 8 klystrons ~12 MW 12 EMQuads New LHC injectors Ion current: 40 mA (avg.), 65 mA (peak) RF accelerating structures: 4 types (RFQ, DTL, CCDTL, PIMS) Frequency: 352.2 MHz Duty cycle: 0.1% phase 1 (Linac4), 3-4% phase 2 (SPL), (design: 10%)

Linac4: Beam dynamics New LHC injectors The Linac4 design (machine architecture, beam optics) allows for high beam power operation  it incorporates modern linac technologies developed for high-power projects (SNS, JPARC, ESS,…) – and has contributed to the development of some of these technologies ! – providing an operational margin for PSB and LP-SPL. Beam optics design to minimize beam loss. Chopping at low energy to reduce longitudinal capture losses in the synchrotron. Charge exchange injection. Remaining losses concentrated on defined spots (collimation) Measures used for keeping beam loss < 1W/m (for hands-on maintenance) at high beam power: Smooth phase advance transitions. Operating point far from resonances. Longitudinal to transverse phase advance ratio 0.5-08 (no emittance exchange). Smooth variation of transverse and longitudinal phase advance. Large apertures (> 7 rms beam size) New LHC injectors

Linac4: Ion source (DESY design) Improved version of the DESY RF volume source (antenna outside vacuum higher reliability). 45 kV extraction 2 MHz excitation at increased power (100 kW for 80mA) First tests in 2009. Plasma chamber (Al2O3) Antenna New LHC injectors Ignition + gas Electron dump

Linac4: RFQ 352 MHz RFQ, 3 MeV energy New LHC injectors Decision (summer 07) to build a taylor-designed CERN RFQ optimised for Linac4, using the experience gained in IPHI and TRASCO: 45 kV injection, 3 m length beam dynamics and RF design jointly by CEA & CERN based on the IPHI (CEA) & TRASCO (INFN) general mechanical design detailed design, construction and brazing by CERN (EN/MME); availability during the first half of 2011 New LHC injectors Main RFQ characteristics

Linac4: 3 MeV chopper line Chopper = fast electrostatic deflector, removing the fraction of the linac beam pulse which would not be captured in the PSB bucket, in order to reduce beam losses. Compact line design 3.7 m Dynamic range 20 – 60 mA Small e growth 4% long., 8% trans. Tolerant to alignment errors RF bunchers Choppers inside quads Dump New LHC injectors Conical dump: dumping of chopped beam and collimation Chopper structure: double meander strip line, 400mm length, metallized ceramic plate. 2 ns rise/fall time for bunch selectivity (352 MHz beam structure), ±500V between deflecting plates, provided by a special pulse generator (FID technology).

Linac4: Accelerating structures Distributed focusing by PMQs (Permanent Magnet Quadrupoles) at low energy, where space is tight. Use of conventional EMQs above 50 MeV, for flexibility-reliability. 352.2 MHz RF frequency. Maximum RF efficiency (shunt impedance ZT2 )  3 different RF structures (two 0-mode, one p-mode). High accelerating gradients because of space constraints, but still in a safe range (<1.8 Kilpatrick peak surface field). Safety margins: 25% on klystron max. power, 20% on theoretical Q-value. New LHC injectors E0 (MV/m) Max. field (Kilpatrik) E0 (MV/m, cost optimum for 10 yrs. op.) DTL 3.2 1.6 4.3 CCDTL 2.8-3.9 1.7 3.1 PIMS 4.0 1.8 2.7

Linac4: Alvarez DTL (“Drift Tube Linac”) Typical geometry 3 tanks (36-42-30 cells). Overall length 18.7 m. f=352 MHz  l=85 cm W=3…10 MeV  b= 0.08…0.145 for E0=3.3 MV/m, 28 cells Cell length: bl = 6.8cm … 12.3cm PMQ equipped Drift Tube New LHC injectors RF resonator – TM0,1,0 3D view Cell#1 6.8cm Cell#28 12.3cm

Linac4: CCDTL (“Cell Coupled DTL”) 21 short (~1m) 2-drift tube tanks with quadrupoles in between, coupled in 3-tanks modules by coupling cells New LHC injectors CERN development since 2000. Useful above ~40 MeV where the length of the focusing period allows to bring quadrupoles out of the drift tubes. Advantages wrt to DTL: easy use of EMQs (possible adjustment for different currents), relaxed tolerances on machining and alignment, simpler and more economic construction.

Linac4: PIMS (“PI Mode Structure”) 12 tanks (~1m) made of 7 cells operating in Pi-mode, coupled via slots. Overall length 22 m. New LHC injectors Similar to LEP copper cavities. Pi-mode required for efficient acceleration above 100 MeV. Compact size at 352 MHz (no need for 704 MHz).

Linac4: planning 2013 + 2014 2015 New LHC injectors Milestones End CE works: December 2010 Infrastructure: 2011 Installation: 2011-2012 Commissioning: 2012 till 2014 Modifications PSB: shut-down 2014/15 Operation: Spring 2015 New LHC injectors project duration: ~ 7 years

LP-SPL: Main characteristics Required for flexibility and low loss in PS2 Ion species H− Output Energy 4 GeV Bunch Frequency 352.2 MHz Max. Rep. Rate 2 Hz Max. Beam Pulse Length 0.9 ms Max. Beam Duty Cycle 0.2 % Nominal chopping factor 65 % (Flexible chopping scheme) Source current 40 mA Linac pulse current 20 mA Number of ions per pulse 1.1 × 1014 Transverse emittance 0.4 p mm mrad Max. rep. rate for accelerating structures and klystrons: 50 Hz Required by space charge tune spread at the specified beam brightness Re-use of LEP RF components in Front-end (Linac4) Required for flexibility and low loss in PS2 (linac4 chopper with new driver) New LHC injectors Structures and klystrons dimensioned for 50 Hz Power supplies and electronics dimensioned for 2 Hz, 2 ms pulse.

SC-linac (160 MeV ® 4 GeV) with ejection at intermediate energy LP-SPL: Block diagram SC-linac (160 MeV ® 4 GeV) with ejection at intermediate energy 0 m 0.16 GeV 110 m 0.73 GeV 186 m 1.4 GeV 427 m 4 GeV Medium b cryomodule High b cryomodules High b cryomodules Debunchers From Linac4 Ejection To PS2 New LHC injectors 9 x 6 b=0.65 cavities 5 x 8 b=1 cavities 14 x 8 b=1 cavities TT6 to ISOLDE Length: ~430 m

Elliptical 5 cell bulk Niobium cavities LP-SPL: RF technology Elliptical 5 cell bulk Niobium cavities (e.g.: b=0.47) Auxiliary equipment (e.g.: 1 MW RF coupler) New LHC injectors from G. Devanz (HIPPI meeting Nov. 2007)

LP-SPL: Cryomodules Medium b cryomodule High b cryomodule Energy range: 160 MeV – 732 MeV 5 cell cavities Geometrical b: 0.65 Maximum energy gain: 19.4 MeV/m 54 cavities (9 cryomodules) Length of medium b section: ~110.35 m Energy gain (MeV/m) 1 5 10 15 High b cryomodule Energy range: 732 MeV – 4 GeV 5 cell cavities Geometrical b: 1 Maximum energy gain: 25 MeV/m 152 cavities (19 cryomodules) Length of medium b section: ~286.2 m 100 200 300 400 Position (m)

LP-SPL parameters: RF frequency 704 MHz 1408 MHz Length (5 GeV) 472 m +12% Ncavities 246 +15% Nβ-families 2 3 ε-growth (x/y/z) 5.6/8.2/6.8 6.3/7.8/12.1 Longitudinal beam loss none in simulations lossy runs for realistic RF gradient/phase variations BBU (HOM) IBBU,704 1/(8..128) Trapped modes normal risk 2..4 higher risk RF power density limit (RF distribution) ok problematic Klystrons comfortable: MBK difficult Overall power consumption (RF+cryo, nom. SPL) 28 MW up to -30% Power converter more bulky saves tunnel space Synergy with ESS yes no New LHC injectors

LP-SPL parameters: temperature @ 704 MHz T [K] Eq. capacity @ 4.5 K [kW] Electrical power [MW] HP-SPL, 2% beam d.c. (4% cryo d.c.) 2 19.4 4.48 4.5 104 26.0 LP-SPL, 0.24% beam d.c. (0.32% cryo d.c.) 6.1 1.5 11 2.75 New LHC injectors + not clear that 25 MV/m can be achieved at 4.5 K!

LP-SPL parameters: Summary Frequency/temperature: 704 MHz and 2 K are confirmed, Cavity gradient: 25 MV/m “on average” (= with a high yield) is very challenging and may be costly (in terms of reprocessing), 20 MV/m seems more achievable but will have an impact on linac length (or energy). New LHC injectors ß High-power RF cavity tests of fully equipped cryo-modules are mandatory for realistic SPL layout estimates!! Ref.: Assessment of the basic Parameters of the CERN SPL, CERN-AB-2008-067-BI-RF, http://cdsweb.cern.ch/record/1136901/files/CERN-AB-2008-067.pdf

Max. pulse duration (ms) Max. current during pulse LP-SPL: beam characteristics for ISOLDE Beam energy (GeV) Max. pulse duration (ms) Max. current during pulse (mA) Repetition period (s) Max. protons /pulse (x1013) beam power (kW) 4 0.9 20 0.6 11 120 1.4 ~ 0.6 (3 out of 4 pulses) 31 1 0.35 28 ~ 0.3 (7 out of 8 pulses) 6.1 29 ~ 0.1 (23 out of 24 pulses) 94 PS2 Basic performance New LHC injectors ISOLDE Need phase modulation Needs higher power klystron modulators