Energy Recovery Linacs and SRF

Energy Recovery Linacs and SRF Dmitry Kayran

Collider-Accelerator Department Brookhaven National Laboratory

Workshop on High Average Power & High Brightness Beams, UCLA Los Angeles CA November 8 - 10, 2004

OutlineIntroduction ERLs around the worldGoals of R&D ERL at BNLBNL ERL general layout, parameters and SRF componentsConclusion


Main accelerator typesERL Basic idea: Bring the beam through the accelerating structures timed in a way so that the second-pass beam is decelerated, i.e. delivering its energy to the cavity fields. 1) Accelerate in 1st pass of RF cavity2) The return loop is N plus RF wavelengths.3) Recovery energy in 2nd pass RF cavity


Energy recovery* linacs (ERLs) (with the same cavity energy recovery)Problems: a colliding beams, b focusing of two beams with different energies in the RF accelerator. *Energy Recovery - process by which the energy invested in accelerating particles is returned to the RF cavities by decelerating them, and re-used to accelerate others


First Energy Recovery demonstrationSame-cell energy recovery was first demonstrated at Stanford University the SCA/FEL project in July 1986Beam was injected at 5 MeV into a ~50 MeV linac (up to 95 MeV in 2 passes), 150 A average current (12.5 pC per bunch at 11.8 MHz)All energy was recovered. FEL was not in place.

Now the world-record energy recovery demonstration current is 30 mA at the Novosibirsk BudkerINP NovoFEL (2003)


Power Multiplication FactorAn advantage of energy recovered recirculation is nicely quantified by the notion of a power multiplication factor:

where Prf is the RF power needed to accelerate the beam. By the first law of thermodynamics (energy conservation!) k < 1 in any linac not recirculated. Beam recirculation with beam deceleration somewhere is necessary to achieve k > 1.If energy IS very efficiently recycled from the accelerating to the decelerating

ExamplesCEBAF no recovery (matched load) k=0.99; (typical) k=0.8JLAB IR DEMO k=16; JLAB 10 kW Upgrade k=33


JLab demonstration: Energy Recovery WorksGradient modulator drive signal in a linac cavity measured without energy recovery (signal level around 2 V) and with energy recovery (signal level around 0.)With energy recovery the required linac RF power is ~ 16 kW, nearly independent of beam current. It rises to ~ 36 kW with no recovery at 1.1 mA.


Benefits of Energy RecoveryRequired RF power becomes nearly independent of beam current.Increases overall system efficiency. Reduces electron beam power to be disposed of at beam dumps (by ratio of Efin/Einj). More importantly, reduces induced radioactivity (simplify shielding) if beam is dumped below the neutron production threshold.Promises efficiency near that of storage rings, while maintaining beam quality of linacs: superior emittance and energy spread, short bunches.


Operational high power ERLs


Some more uses of ERLsOperational ERLs now are used for FEL applicationThe advantages of ERLs well beyond of that :Average current-carrying capability of storage-ring. Smaller beam emittance and energy spread.Higher photon brilliance and coherence, round sources, and short-pulse-length radiation. 100-fsec pulse width domain.Efficient way to use RF powerMake the ERLs great machines for the future: Light sources High energy electron coolers of ions (previous talk)Electron ion colliders (next talk)


Goals for ERL R&D at BNLR&D ERL will serve as a test-bed for future RHIC projects: ERL-based electron cooling (conventional or coherent). 10-to-20 GeV ERL for lepton-ion collider eRHIC. Test the key components of the High Current ERL based solely on SRF technologySRF Photoinjector (703.5 MHz SRF Gun, photocathode, laser, merger etc.) test with 500 mA. -Preservation of high-charge, low emittance.High current 5-cell SRF linac test with HOM absorbers-Single turn - 500 mA Stability criteria for CW beam current. Attainable ranges of electron beam parameters in SRF ERL.


ERLs beam parameters*) To demonstrate 3 min cooling time for proof pf principal of CeC for single Au bunch at 40GeV/nucl (above transition) with 1e09 particles 66cm length energy recovery may not be necessary. To improve energy spread 3rd harmonic cavity and increase injection energy will help


High Power ERL landscapeCommissionedUnder constructionIn design stage


50 kW 703.75 MHzsystemControl roome- 2.5MeVLaserSC RF GunCryo-modulee- 15-20 MeVSchematic Layout of the BNL R&D ERLMerger systemReturn loop


PARMELA simulations shown: the small emittances can be achieved using bear-can initial distribution Blue 5 nC Red 1.4 nCGreen 0.7 nC4.8/5.3 um 2.2/2.3 um 1.4/1.4umBNL ERL Injector: beam dynamics simulation resultsRMS normilized emittance, mm-mrad30Distance from the cathode, mDistance from the cathode, mRMS normilized emittance, mm-mrad


ERL loop lattice is very flexibleLattice and D functions of the ERL for the different cases longitudinal dispersions (Ds=M56):Positive longitudinal dispersionNegative longitudinal dispersionZero longitudinal dispersion No dispersionDispersion, mb, mTransverse normalized emittances from cathode to dump Q=0.7 nC (PARMELA simulation)b, mb, mDispersion, mDispersion, m


Layout of R&D ERL in Bldg. 912 at BNL


R&D ERL beam parameters (PARMELA simulation result two operational regimes )Operation regimeParameter


Why use SRF Technology?

Since RF power for ERL operation no longer depends on beam current, main losses are wall losses: for SRF cavities high Qs mean small wall lossesQuality Factor Q0:


Linac Cryomodule5 cell SRF cavity703 MHz, 20 MV/m @ Qo=1e10Ferrite Dampers for HOMs


BNL 5 Cell SRF LinacF = 703.75 MHz, E = 20 MeVQ0 ~ 1010, QHOM ~ 103Build: AESProcessed: JLABWill be used: BNLLHe Ballast Tank5 Cell SRF Cavity inside the cryomoduleThe 5-cell cavity was specifically designed for high current, high bunch charge applications such as eRHIC and high energy electron cooling. The loss factor of the cavity was minimized. The number of cells was limited to 5 to avoid HOM trapping. Additionally, HOM power is effectively evacuated from the cavity via an enlarged beam pipe piece 24 cm diameter. The simulated BBU threshold is of the order of 20 AFirst horizontal test in April 2009More details about the 5cell cavity status in the next talk


Energy Recovery Linac 5-Cell Super Conducting RF CavityMagnetic Shield5 Cell RF CavityTuning AssemblyInsulating VacuumFundamental PowerCouplerHelium VesselLHe Ballast TankCollider-Accelerator Department


Cavity VTA testsThe cavity was processed at JLab. After BCP, Rinsing, and low temp. bake (120 C) cavity reached 19 MeV/m at Q0=1010.19 MeV/mEbeam,max 20 MeV


SRF InjectorfRF= 703.75 MHzEnergy=2.5-3 MeV

Average Current: 0.5 ATwo fundamental power couplers: 0.5 MW eachSRF Gun: axial electric field profile for different cathode insertion depth (SUPERFISH result)


Conclusion: A bright future for ERLs ERLs provide a powerful and elegant solution for high average power free electron lasers. The pioneering ERL FELs have established the fundamental principles of ERLs. Operating ERL-FELs reach higher performance Several more are in serious planning stages and will likely be constructed ERLs based on SRF technology will be used for the future: light sources, high energy electron coolers of ions, electron ion colliders


Thank you


Add plot of measured Q vs Eacc*

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Energy Recovery Linacs and SRF