1. Advances in Proton Linac Design and Simulations P.N. Ostroumov January 13, 2012 Proton accelerators for science and innovation Workshop – January 2012

2. 2 Content  This talk about accelerators with energies above tens of MeV to 8 GeV –High current, above several mA to hundreds of mA  General linac layout  Pulsed and CW linacs  Superconducting linacs  Available accelerating gradients  focusing structures  Linac lattice design requirements  Main steps for the lattice design  Beam dynamics simulations –TRACK features  Error studies  H-minus linac features –Additional losses  New tasks –Transient analysis in pulsed linacs

3. 3 Linac Layout High-energy section Medium-energy section Front end Typical RF Linac structure Ion source RFQ DTL – 60 years SDTL IH-structure SC cavities Coupled Cavity Linac (Side coupled structure Disk-and Washer Structure Annular Coupled Structure) SC Cavities (Elliptical Spoke-loaded TEM-class) Frequency jump Lattice transition

4. 4 RF linacs RF Linacs CW NC*SC Pulsed NCSC ATLAS ISAC-II INFN SARAF ReA-MSU SPIRAL-2 FRIB PROJECT X ADS projects ISAC-I RIKEN inj. LEDA RFQ SARAF RFQ LANSCE Synchrotron Injectors (FNAL,KEK, CERN, IHEP….) MMF (Moscow) SNS CERN SPL ESS *Low-energy, several MeV/u Heavy-ions Protons only H-minus only Simultaneous P and H-minus Ions

5. 5 Pulsed linacs with high duty factor  Usually Normal Conducting, now SNS is an exception  Can provide up to ~200 mA  LAMPF, 17 mA –Reduction of the peak heat load of the targets; –Operation in a well-controlled beam dynamics regime with avoidance of particle losses; –Better control of accelerating field stability under beam loading.  High-power linacs are operated with duty factors close to 10%; –Maximizing shunt impedance –DTL –low energy, CCL – high energy  Front End –RFQ –Capability to form a compact longitudinal emittance with substantially lower halo compared to the front end based on external bunchers –Maintaining the transverse emittances unchanged in spite of the bunched structure of the beam exiting the RFQ.  Examples for recent innovative pulsed linacs –SNS: PMQs, SC section –J-PARC: SDTL, ACS

6. 6 From DTL to SCRF Linac 1960- now 2009-Future

7. 7 Continuous Wave Linacs: NC or SC ?  Required wall plug power to create accelerating field  Typical example: 1 GeV CW linac  Superconducting CW linac is much more economic than NC  Both pulsed or CW SC linacs require NC front end for ~0.1 to 10 MeV/u depending on q/A and duty factor

8. 8 Superconducting linacs  Significant difference between protons and electrons –  G =1 elliptical cavities above ~0.8c –Suite of SC cavities for low velocities <0.8c  SC structures are short and designed for fixed velocities: 1)fabrication cost is significantly reduced; 2)focusing elements are required between SC cavities; 3)Limited by stored RF field energy.  CW SC Linacs –No high power CW SC accelerator exists –Low power: ATLAS, ISAC, Legnaro (Italy), New Delhi, ReA3 (MSU)  Pulsed SC proton (H-minus) linacs a)lower construction and operation cost, b)higher accelerating gradients, c)higher feasibility for energy upgrade, d)larger bore diameter

9. 9 Real-estate accelerating gradient along the 1-GeV SNS linac (red dots) and 8-GeV Proton Driver (blue dots). SNS Proton Driver

10. 10 Accelerating gradient and cryogenics load  Cryogenic load: static and dynamic (pulsed and cw)  Dynamic heat load in the LHe system is proportional to  CW linac –there is an optimum accelerating gradient that can minimize the combined capital and operation costs –The optimum value of the accelerating gradient can be further increased by providing a better intrinsic quality factor, Q 0, of the cavity  Pulsed SC linac –the accelerating gradient can be higher than in c.w. linacs because the dynamic cryogenic load is lower –SC cavities require fast frequency tuning to compensate for the Lorentz detuning.

11. 11 Linac lattice design  The primary focus of the physics design is to deliver the linac lattice for the following engineering analysis and the development of fabrication drawings.  The physics design and engineering development of the linac must be iterated several times to assure final linac performance and cost-efficiency  Must satisfy the major requirement of high-power accelerators –Avoid excessive uncontrolled beam losses, choice of apertures –Minimize emittance growth –Avoid beam halo formation  The accelerator lattice must be integrated with other systems –chopper, beam collimation, diagnostics system, beam corrective steering, vacuum system, cooling system, etc.  “Hands-on maintenance” of the accelerator components –radioactivation limit of 20 mrem/hour at a distance of 1 m from the component surface after extended operation of the linac (~100 days) and 1 hour of downtime –This results in losses less than 1 W/m, especially above 100 MeV.

12. 12 Accelerating structures  RFQ –Pulsed – many options –CW, limited number of RFQs operates in CW mode LEDA  DTL, SDTL for pulsed linacs  CCL –SCL, DAW, ACS – pulsed linacs  Low-beta SC structures –TEM-class: QWR, HWR, spoke-loaded  High-beta SC structures –elliptical  Apertures –>12  rms beam size

13. 13 SC structures  A transition energy from NC to SC structures is one of the optimization parameters for a new generation of pulsed high-power proton and H¯ linacs.  New class of spoke-loaded SC cavities have been developed to cover the velocity range 0.1<  <0.8. –These cavities are competitive with reduced-  elliptical cavities due to higher shunt impedance, reduced microphonics and Lorentz detuning. –Spoke-loaded multi-cell cavities have a very high coupling coefficient, and the spectrum and impedance of higher-order modes are greatly reduced. –The aperture of spoke cavities can be increased without a significant impact on the shunt impedance.  Reduced-  elliptical cavities can be effectively applied  The use of SC accelerating structures is proven to be cost-effective in the high-energy sections of pulsed proton or H¯ linacs for beam energies above ~180 MeV - SNS.

14. 14 Focusing structure  RFQ – electric focusing  Electromagnetic quadrupole (EMQ) focusing –Commonly used in NC accelerators (DTL, CCL,..) –High-energy section based on SC cryomodules  PMQs –Effectively applied in the SNS DTL  Focusing structure in SC Linacs –Warm EMQ - protons –Cold EMQ – electron accelerators –SC solenoids Short focusing period enables higher accelerating gradients

15. 15 Design requirements for high-intensity proton linacs  Avoid parametrically excited envelope instabilities at high currents  Adiabatic change of along the linac –minimizes the potential for mismatches –helps to assure a current-independent lattice  Adiabatic change of real-estate accelerating gradients and focusing fields are required to fulfill these conditions.  Avoid the n=1 parametric resonance between the transverse and longitudinal motion The strongest resonance is for n=1 and can occur particularly in SC linacs due to the availability of high accelerating gradients and relatively long focusing periods.

16. 16 Stability chart for zero current, betatron oscillations  Example, 8-GeV proton linac

17. 17 Design requirements for high-intensity proton linacs (cont’d)  The coupling space charge resonances can be conveniently analyzed using I. Hofmann’s chart. Emittance ratios 0.5<  L /  T <2 provide larger stable areas in the stability chart  Exact equipartitioning conditions are not a strict requirement, the location of operating parameters near the equipartitioning condition provides a larger area for stability even for very low tune depressions such as 0.3. –equipartitioning conditions corresponds to equal temperatures in phase space planes  Provide proper matching in the lattice transitions to avoid appreciable halo formation. In the perfect “current-independent” design, matching in the transitions is provided automatically if the beam emittance does not grow for higher currents.  To avoid halo formation, the tune depression due to the beam space charge should be above 0.5. The tune depression is considered a boundary between the “emittance and space-charge dominated” beams

18. 18 Examples of NC linac lattice design  RFQ-CCDTL-CCL, 100 mA 211 MeV  Length – 210 m, low accelerating gradient  No rms emittance growth Courtesy L.M. Young

19. 19 SNS  Original design with SC section Courtesy D. Jeon

20. 20 Properties of a SC linac, cost-effective design  The acceleration is provided with several types of cavities designed for fixed beam velocity. For the same SC cavity voltage performance there is a significant variation of real-estate accelerating gradient as a function of the beam velocity.  The length of the focusing period for a given type of cavity is fixed.  There is a sharp change in the focusing period length in the transitions between the linac sections with different types of cavities  The cavities and focusing elements are combined into relatively long cryostats with an inevitable drift space between them. There are several focusing periods within a cryostat.  Higher Order Modes –Additional heat load –Beam instabilities –Becomes noticeable at beam currents >10 mA in elliptical cavities

21. 21 Lattice design steps  Select the type of accelerating structures for the given velocity range.  SC linac: select the geometric beta of the cavities  Define approximate transition energy for the frequency change.  Select the types of cavities that are suitable for the given velocity range. –Optimize the electrodynamics and the mechanical design of the cavities. –Design the cavities to reduce the ratio of peak surface fields to the accelerating field. –Assume experimentally proven peak surface fields in the cavities  Select the focusing lattice  SC linac: select the cryostat length and inter-cryostat spaces working with cryogenic and mechanical engineers  Develop lattice tuning for the beam without space charge –Avoid zero-current resonances –Two types of strong parametric resonances can cause this problem: parametric resonance of the transverse motion and parametric resonance of the longitudinal motion (or “structure resonance”)

22. 22 Lattice design steps (cont’d)  Calculate longitudinal acceptance for the zero-current beam using the Monte Carlo method –phase setting of accelerating cavities, especially in the case of the SC linac, must provide the largest possible acceptance –Large acceptance is important to avoid beam losses in the linac when various errors are present.  Calculate tune depressions using rms parameters, check the lattice tune to verify and avoid strong space charge resonances  Provide matching of the beam for the design peak current in all lattice transitions if the lattice design is not strictly current independent  Simulate beam dynamics using multi-particle codes  Study beam losses in the presence of machine errors using a large number of multi-particles, ~10 6 and multiple seeds, ~500.

23. 23 8 GeV proton SC linac, wavenumbers

24. 24 8 GeV proton SC linac, phase advances  6 different types of cavities

25. 25 Coupling resonances, Hofmann’s chart  45 mA

26. 26 Slide 26 Code benchmarking for the SNS design. Beam dynamics simulations

27. 27 TRACK and PTRACK codes © COPYRIGHT 2007 UChicago Argonne, LLC  Developed for design, simulation and commissioning of hadron linear accelerators and related beam transport systems  Tracks multi-component ion beams  End-to-end simulation from ion source to target  Wide range of electromagnetic elements with 3D fields (pre-calculated RF and static electric and magnetic fields)  Treats interaction of ion beams with matter  Error simulation for all elements  Beam loss analysis with exact location of particle loss  Automatic transverse and longitudinal beam tuning procedures. Realistic transverse correction procedure.  3D space charge in Cartesian and Cylindrical coordinates (PIC or CIC method), 2D space charge for DC beams.  Optimization of beam parameters in realistic 3D fields (including space charge) based on tracking of probe particles  Linear and higher order matrices

28. 28 Unique features and applications of TRACK  Windows version includes on-line graphics on a PC screen  Simulates H-minus stripping due to various mechanisms (passage through the stripper, static and RF magnetic field, residual gas, black- body radiation, intra-beam stripping)  Simulation of electron beams with space charge is available  Serial version (Windows and Linux) are available at the web-site http://www.phy.anl.gov/atlas/TRACK/  Parallel P-TRACK can simulate 10 9 particles on 128000 processors. Available at ANL BG/L. Does not have yet Manual.  We are willing to open the source code for DOE Labs but we need ½ FTE and 1 postdoc effort to collaborate with all Users on modification of the source code per User’s needs

29. 29 Application of TRACK: Heavy-ion linac for the FRIB  Acceptance and 5-charge state uranium beam image at the entrance to the high-energy section of the FRIB driver linac

30. 30 45 mA proton beam in the RFQ, longitudinal phase space

31. 31 Phase space plots at the exit of HINS (FNAL) RFQ 45 mA H-minus beam 100 million particles Visualization of 100M particles accelerated in the RFQ Longitudinal phase space plots in the form of density contours. 1M particles (left), 10M particles (middle) and 100M particles (right). HINS (FNAL), 10 MeV section. Beam dynamics of normal and superconducting ion linacs: Large-scale computing for detailed beam dynamics

32. 32 Beam halo  Definition (Wangler, RF Linear accelerators) - beyond ~5 rms beam size  To minimize halo apply design concepts discussed in previous slides  Minimize number of lattice transitions –MEBT, cryomodules, RF jump,…  Design the lattice in emittance dominated regime rather than space charge dominated regime  Space charge dominated regime is more sensitive to mismatches, lattice transitions, machine errors  Apply collimation

33. 33 Emittance and space charge dominated regimes  100 mA, 2.5 GeV fully SC proton Linac

34. 34 Machine Errors  Error simulations –Random, multiple seeds –For the static errors, correction algorithms are applied in each seed –Dynamic errors are not corrected  Type of errors –Misalignments –Dynamic errors of RF and focusing field ErrorValueDistribution Cavity end displacement RT and SC spoke cavity SC elliptical cavity 0.5 mm (max) 1.0 mm (max) Uniform Solenoid end displacement Type 1 (18 cm long) Type 2 (32 cm long) 0.2 mm (max) 0.25 mm (max) Uniform Quadrupole end displacement0.15mm (max)Uniform Quadrupole rotation (z-axis)5 mrad (max)Uniform Cavity field jitter error0.5 % (rms)Gaussian Cavity phase jitter error 0.5° (rms)Gaussian Typical set of errors

35. 35 Beam center correction: 100 random set of errors (seeds), Project X Front-end simulations (FNAL) Beam RMS emittances Beam envelopes Beam Centers: Position & Angle Red: before the correction Blue: after the correction

36. 36 TRACK Application: Design and Simulations of the 8-GeV PD Error simulations: 100 seeds, 1M particles each 0 deg 0 % Beam Emittances: before and after RF errors Beam Envelopes Beam Loss: Different RF errors 1 deg 1 % 1 deg 1 % 0 deg 0 % 1 deg 1 % 2 deg 2 % 0 deg 0 % Fraction: 2E-5 Peak: 0.1 W/m Fraction: 1E-4 Peak: 0.4 W/m Fraction: 3E-2 Peak: 35 W/m

37. 37 H-minus beams – additional losses Courtesy R. Garnett LANSCE, beam losses

38. 38 SNS Linac, TRACK simulation of H-minus intra-beam stripping  Design tune (strong focusing)Modified tune (reduced focusing field) PRELIMINARY

39. 39 LANSCE: Higher transient losses are related to cavity field errors  Time dependence of beam loss in the linac shows higher losses during beam turn-on transient.  All field errors acceptable, i.e. below the “fast protect” threshold of 1  & 1% phase and amplitude error, respectively.  Present feed-forward signal (scaled version of beam current macropulse) not adequate to mitigate error. Courtesy R. Garnett

40. 40 Transient beam loading in multi-cell resonators  Passband modes are excited and produce longitudinal electric field  Can contribute to longitudinal emittance growth  Coupling and decay time can be different for passband modes

41. 41 Additional transient effects  Lorentz force detuning  Microphonics  Variation of beam current along the beam pulse  Algorithm of feedback in LLRF – residual error of the phase and amplitude of the fundamental mode  As far as I know these effects are not yet in simulation codes

42. 42 Conclusion  Design methods for high-power proton accelerators are well advanced  LAMPF and SNS confirm that existing design tools are sufficient to build hands-on maintenance linacs with beam power up to ~1 MW  These tools may be also sufficient for design of ~10 MW machines –Simulations are consistent with beam loss measurements for protons –Significant difference in beam losses for proton and H-minus beams