Proton Driver Developments and the Neutrino Factory Christopher R Prior ASTeC Intense Beams Group STFC Rutherford Appleton Laboratory, U.K.

Past studies of High Intensity Proton Accelerators TRIUMF Kaon Factory European Hadron Facility (EHF) ISIS 0.16  0.24 MW European Spallation Source (ESS) 5+5 MW CONCERT (CEA) 30 MW US Spallation Neutron Source (SNS) 1.4 MW J-PARC 0.75 MW Chinese and Indian Spallation Neutron Source Projects ~0.2 MW Neutrino Factory Drivers at 5, 8, 10, 15, 30 GeV Fermilab proton accelerators at 16 and 8 GeV

IDS Baseline Mean beam power 4 MW Pulse repetition rate50 Hz Proton kinetic energy GeV Bunch duration at target1-3 ns rms Number of bunches per pulse1-3 Separated bunch extraction delay  17 µs Pulse duration: –liquid mercury target≤ 40 µs –Solid metal target> 70 µs

4 MW 50 Hz 10 GeV 5  Peak current in a single bunch, 1 ns rms: ~2500 Amps

Requirements Low level of uncontrolled beam loss throughout the machine – ~1 W/m –Advanced collimation systems High current Hˉ linac (40-70 mA) Accumulate beam –Very low loss beam injection –Chopped beam –Achromat –Phase space painting –Emittance control (halo) –Clean extraction Bunch compression ESS_simulation.swf

Possible “show stoppers” beam chopper Halo control/preparation for injection Injection Bunch compression

Possible Schemes Hˉ linac with pairs of 50 Hz booster and 25 Hz driver synchrotrons (RCS) Hˉ linac with 50 Hz booster RCS and 50 Hz non- scaling FFAG Hˉ linac with chain of 3 FFAGs in series Hˉ linac with 2 slower cycling synchrotrons and 2 holding rings Full energy Hˉ linac with accumulator and compressor rings - CERN, Fermilab

Linac Front-End EU/FP6/CARE/HIPPI –Linac architectures (nc + sc) to 200 MeV –Diagnostics/measurements (UNILAC) and simulation –Code comparison CERN and RAL test stands –Ion source, RFQ and chopper development SNS, J-PARC, Spiral2 facilities chopper RFQ LEBT ion source

RAL 3.0 MeV MEBT Chopper, 324 MHz Chopper 1 (fast transition) Chopper 2 (slower transition) ‘CCL’ type re-buncher cavities 4.6 m Beam dump 1 Beam dump 2

World’s first high-energy superconducting proton linac Design: 60 Hz rep.rate Design energy: 1 GeV Normal Conducting Linac Superconducting Linac SNS Linac

Transport Lines and Beam Accumulation Achromat for transverse and longitudinal collimation. Beam injected over turns via charge exchange injection. Pulse ms compressed to ~500 ns. HEBT RTBT Injection Collimation RF Extraction Target

Injection Charge exchange injection foil, laser stripping Foil issues: –Stripping efficiencies –Multiple Coulomb scattering –Large angle and Nuclear scattering –Energy straggling –Heating –Stress and buckling –Lifetime –Radiation –Stripped Electrons –Emittance Growth Painting –Transverse and longitudinal coupling –option for transverse painting –option for longitudinal painting Unstripped Beam (H 0, H - )

Lifetime of Stark States (Project-X) Missed H - will be stripped in 5 mm N=5 and higher strip immediately N=4, 10 mm,  =30  r N=3, 80 mm,  =150  r N=1,2 will be stripped by second foil Peak field of Chicane T Foil

150 Figures in red are equivalent emittances in  mm.mrad; those in blue are equivalent  p/p values (x ) e - collector mm 15.8 mm Vertical closed orbit Dispersion painting x y

Normalised dispersion D x /√β x ≈1.6 in injection dipole Optics Requirements

Hˉ Injection/Accumulation Injection into longitudinal phase space for RAL booster synchrotron: 0.2 ms, ~150 turns, 180 MeV

Injction Modelling for FNAL 8 GeV Synchrotron

Foil Heating Peak temperature strong function of linac beam size Minimum temp reached only 1 cycle even for 10 Hz (297  K) 9 Secondary Hit (270 turn) Project X intensity 3 x lower Peak temp ~ 1000 K Minimum equilibrium temp at 5 Hz < 600 K  =1 mm Black, 1.5 mm, 2 mm (HINS, 270 turn) Temp due to linac beam only, included secondary hit Temp vs beam size

Steerer Vertical painting mag. Injection septum mag. Stripping foil drivers Septum mag for H0 dump Steerer CT Painting bump mag.Bump mag. Q-mag for H0 dump Painting bump mag. H0 dump J-PARC RCS Injection System

Space charge & halo formation (SNS) Correlated painting with/without SC Anti-correlated painting with/without sc

Laser Stripping (ESS) Ring lattice designed for laser stripping injection for ESS Problems in controlling emittance Promising new work at SNS - may be solution for the future.

RAL Booster RCS: complete cycle

Theoretical Models 5 GeV 50 Hz 4 MW 15 GeV 25 Hz 4 MW 8 GeV 50 Hz 4 MW 30 GeV 8.33 Hz

CERN SPL: linac + accumulator & compressor rings

SPL Proposal - M. Aiba Detailed feasibility study for the accumulator and compressor for the 6-bunch SPL option Includes lattices, injection, foil heating, compression. Instabilities in the accumulator ring still under investigation. Studies of 3 and 1 bunch options are underway. See poster on Tuesday

Proton Driver for a Neutrino Factory (ISS) High intensity ~10 14 protons –Achieved with phase space painting in RCS booster 50 Hz rep rate Booster circumference 400 m Bunch area (h=3) 1.1 eV.sec ns bunch compression –Achieved with FFAG accelerator –0.885 MV per turn for 3.3 ns rms, improved by higher harmonic component to 1.9 ns –FFAG circumference ~800 m Delayed extraction of bunches to meet requirements of muon accelerators and decay rings –May be easier with FFAG than synchrotrons

10 GeV Synchrotron Proton Driver (IDS) High intensity ~10 14 protons –Achieved with phase space painting in RCS booster 50 Hz rep rate Booster circumference 400 m Bunch area (h=3) 1.1 eV.sec ns bunch compression –Achieved with RCS accelerator (h=24) –1.3 MV per turn for 3 ns rms compression –RCS circumference same as for FFAG (~800 m) Finemet, rather than ferrite, used for rf cavities. 3 GeV RCS Booster 0.2 GeV H  linac RCS Main synchrotron

Techniques for Generating Short Bunches High RF voltage High RF frequency Bunch rotation in phase space Use of unstable fixed point Use of transition energy (incl. FMC lattices, isochronous rings etc) (Review by M. Brennan, Workshop on Instabilities, BNL, June 1999)

Bunch Compression ~1-2 ns rms High RF Voltage –Bunch shortens as RF voltage increases during adiabatic compression, but only as fourth root of V. –Required voltage rapidly becomes very large. High RF frequency –Leads naturally to shorter bunches but with small longitudinal emittance and high charge per unit phase space area. –Experience of high intensity machines suggests an empirical limit of 1-2 x /eV.s for machines which pass through transition. However, no theoretical basis exists.

Bunch Rotation Slow adiabatic decrease of RF voltage to reduce momentum spread without emittance increase. Rapid increase of RF voltage so that the mis-matched bunch rotates in ¼ synchrotron period into upright configuration. Factors of in compression, but emittance growth unless bunch is “used” immediately. Susceptible to instabilities (e.g. microwave) and beam loading. High frequency instabilities increase  L and limit compression. Large transverse space charge tune spread, orbit distortions and path-length variations. Volts reduced adiabatically from 300kV to 25 kV, then increased rapidly to 500 kV. Bunch compression factor = 2.2 ISIS model

Switch phase of RF voltage to unstable fixed point to stretch bunch. Then switch RF phase back to stable fixed point so that bunch rotates to upright position with minimum length. Compression factor may be distorted by non-linear voltage regions. Bunch length: ±12.5 ns (5.6 ns rms) Momentum spread: ±5.0 x Longitudinal emittance:  L ~ 0.7 eV.s Number of particles per bunch: 2.5 x Bunch Compression using Unstable Fixed Point

Unstable Fixed Point + Separate Compressor Ring Stretch bunch at unstable fixed point (  <  t, volts on) Transfer to separate compressor ring with  >  t for phase space rotation. No volts needed. Non- linearity problems exist but are reduced.

Flexible Momentum Compaction Lattice Lattice gymnastics involving frequency slip parameter  =1/  t 2 -1/  2. Set  =0 and impose RF voltage to create large  p/p without change in bunch length. Switch  back on and bunch will rotate to short length. Experiment at Brookhaven AGS with transition jumping quadrupoles achieved 2.5 ns for 5 x protons (factor ~3 in compression)  =0:  p/p increase without bunch lengthening  <0, phase space rotation to upright (compressed) position

Compression using transition Work with  close to, and just above,  t. Space charge assists compression. High RF voltage (1.5 MeV at h=8) based on  s =180 o Simulation gives the required 1 ns bunches but sensitive to space charge depression of  t.

Bunch Compression in the RAL 5 GeV Synchrotron Driver During final 500 revs of accelerating cycle, an additional h=24 voltage system swings in, rising to a peak of 500 MV During final 500 revs of accelerating cycle, an additional h=24 voltage system swings in, rising to a peak of 500 MV Final bunch length =1ns rms,  L  eV.s  p/p  Final bunch length =1ns rms,  L  eV.s  p/p  Space charge depresses  t which may be a problem for machines working near transition. Longitudinal space charge can be reduced using an inductive impedance Z // /n (~5 , say). As bunch length decreases,  p/p increases and higher order momentum effects need to be taken into account. Acceleration to 5 GeV and final compression (  t =6.5,  = 6.33 at top energy,  = ) Voltages chosen to avoid instabilities (microwave: V sc < 0.4 V applied ). Lattice resistant to space charge effects

IDS Baseline Mean beam power 4 MW Pulse repetition rate50 Hz Proton kinetic energy GeV Bunch duration at target1-3 ns rms Number of bunches per pulse1-3 Separated bunch extraction delay  17 µs Pulse duration: –liquid mercury target≤ 40 µs –Solid metal target> 70 µs ~150 µs