Laser Stripping for H - Injection

Laser Stripping for H- Injection Wolfgang Bartmann ATS Seminar, 23-Jan-2014

Outline • Introduction to multiturn injection • Motivation for H- injection • Principles of H- injection with foil • Required hardware • Limits • Machines with different parameters • Laser stripping as alternative • Principles • Laser requirements • Implications for lattice and optics • Challenges • Status • References CERN Seminar, Laser Stripping

Multi-turn injection for hadrons • For hadrons the beam density at injection can be limited either by space charge effects or by the injector capacity • If we cannot increase charge density, we can sometimes fill the horizontal phase space to increase overall injected intensity. • Condition that the acceptance of receiving machine is larger than the delivered beam emittance CERN Seminar, Laser Stripping

Multi-turn injection for hadrons Septum magnet Varying amplitude bump Closed orbit bumpers • No kicker • Bump amplitude decreases and inject a new bunch at each turn • Phase-space “painting” CERN Seminar, Laser Stripping

Multi-turn injection for hadrons Turn 1 On each turn inject a new batch and reduce the bump amplitude • Example: • CERN PSB injection, • fractional tune Qh = 0.25 • Beam rotates p/2 per turn in phase space Septum CERN Seminar, Laser Stripping

Multi-turn injection for hadrons Turn 2 CERN Seminar, Laser Stripping

Multi-turn injection for hadrons Turn 15 Phase space has been “painted” In reality filamentation occurs to produce a quasi-uniform beam CERN Seminar, Laser Stripping

Charge exchange H- injection • Multiturn injection is essential to accumulate high intensity • Disadvantages inherent in using an injection septum • Width of several mm reduces aperture • Beam losses from circulating beam hitting septum • Limits number of injected turns to 10-20 • Charge-exchange injection provides elegant alternative • Possible to circumvent Liouville’s theorem, which says that emittance is conserved • Convert H- to p+ using a thin stripping foil, allowing injection into the same phase space area CERN Seminar, Laser Stripping

H- injection foil stripping 24 m CERN Laser Stripping, WS Fermilab

H- injection painting • Now we “overinject” onto the same phase space area already occupied by the circulating beam CERN Seminar, Laser Stripping

Foil stripping - HW • Chicane and painting bumpers (~4 each) • Chicane: one of these septum like depending on geometry • Stripping foil • Thin foil: Foil holder with exchange possibility • Thick Foil: convert unstripped H- and H0 to protons to guide them into dedicated waste beam channel • Vacuum and radiation compatibility • Waste beam handling • Internal dump (PSB) • Short line to external dump • Electron collector • Careful design of beam trajectories required due to different angles for H-,H0 in chicane magnets • Machines with running H- injection suggest tracking studies of waste beams and a high number of diagnostic possibilities CERN Seminar, Laser Stripping

Problems with foils Foil damage • Present machines on the high beam power frontier are approaching the limits of foils Sarah Cousineau, Laser stripping WS, Fermilab, 26-27 Sept-2013 CERN Seminar, Laser Stripping

Problems with foils • Beam loss and radiation • Beam loss due to foil scattering (foil is highest loss point in SNS accelerator complex) • Emittance growth due to foil scattering • Relevant for multi-purpose machines • Laser stripping avoids a mechanical interaction with the beam CERN Seminar, Laser Stripping

Comparison of machine parameters …where laser stripping is considered (except for ISIS) – likely not complete CERN Seminar, Laser Stripping

Include laser into injection setup Wiggler or laser for neutralisation Dedicated design of D3 Laser for excitation CERN Seminar, Laser Stripping

Laser stripping concept: H-to H0 • Either Lorentz-stripping in a wiggler magnet… • or Photo-dissociation CERN Seminar, Laser Stripping

Lorentz stripping • H- ion moving in magnetic field  Lorentz force tends to break it up • Binding energy of extra electron 0.76 eV • In ion rest-frame electric field E is the Lorentz-transform of the magnetic field B in the lab-frame • The ion’s lifetime can be parametrized as , CERN Seminar, Laser Stripping

H- neutralisation: Wiggler • Magnet with ∫B·dl = 0  Lorentz stripping • Vertical  no extra horizontal angular spread which would increase the laser power • HP-PS: at least 0.6 T to keep emittance growth small • Integration could be an issue CERN Seminar, Laser Stripping

H- neutralisation: Laser • No emittance growth • Easier integration than wiggler? • Numerical calculation of neutralisation degree • Laser micro pulse energy required for 99% neutralisation: • 60mJ per micropulse • with factor 1000 reduction from recycling • fRF=352MHz and Tinj =2ms • gives 42 J per macropulse Demanding for the laser (and vacuum window) CERN Seminar, Laser Stripping

H- neutralisation: LaserFeshbach resonance as alternative? • Feshbach resonance appears at 113.49 nm (10.93 eV) in the H- photodissociation spectrum • HP-PS: • With 1064 nm laser accessible with intersection angle of 37° • To reach 99% neutralisation (crosssection~1.39e-15 cm2) a laser micropulse energy of 190 uJ is required • However: Δλ/λ~5.2e-6 while beam Doppler spread ~2e-3 • Need another factor 5e2 for full beam neutralization, leads to a laser micropulse energy of 73 mJ CERN Seminar, Laser Stripping

H- neutralisation: Lasern=2 shape resonance as alternative? • The resonance at 112.95 nm for the reaction cross-section of 9.8e-17 cm2 • HP-PS • Requires 2.8 mJ micropulse energy but with much larger linewidth of 1.8e-4 resulting in a total micropulse energy of 31 mJ • The H0 is then already in an excited state and can be resonantly excited from n=2 to n=3 • Dual advantage of longer lifetime for spontaneous decay and shorter lifetime for stripping to p+ in magnetic field CERN Seminar, Laser Stripping

H0 --> p+ strippingDoppler shift of laser frequency HP-PS: • 1064 nm laser • n=2 can be reached with 47.5° between ion beam and laser • n=3 with 8.39° CERN Seminar, Laser Stripping

H0 --> p+ strippingdivergent laser beam • Scheme developed and tested at SNS (Danilov et al.): • Resonant excitation of ground-state H0 in field free region • Stripping of excited electron in magnetic field • Large spread of effective resonance frequencies divergent beam H- Divergent laser light CERN Seminar, Laser Stripping

H0 --> p+ strippingdivergent laser beam • Using this scheme for HP-PS: • Excitation to n=2 or n=3: 360 and 92 uJ laser micropulse energy • This leads to > 20 MW laser peak power! • Spontaneous decay reduces the efficiency: • 1.7% of n=3 decay in 25 cm drift between laser interaction and stripping point • How to reduce laser power (Danilov et al., Future prospects for laser stripping): • Locking between laser and beam temporal structure • Dispersion tailoring • Photon recycling • Bunch length minimisation • Vertical beam size minimisation • Beam angular spread minimisation Using a divergent beam to compensate the doppler broadening of the transition affects strongly the required laser power CERN Seminar, Laser Stripping

Dispersion tailoring • For off momentum particles: and • Together with the effective laser frequency in the rest frame – - the dispersion angle can be chosen such that the doppler broadening due to dp/p is compensated • HP-PS • Need a D’ of about 2.0 rad • LSS has zero dispersion • Target emittance should not be significantly affected by injection process • Assuming a target emittance of ~13 um • Injected transv emittance: 0.4 um, emittance growth less than 0.3 - 0.5 um • Assuming a maximum acceptable dispersion of 0.2 m • This leads to a maxiumum D’ of 0.05 rad Interesting option if D’ can be accomodated in optics design of injection region CERN Seminar, Laser Stripping

Dedicated design of stripping magnetFringe field stripping – emittance growth • Lifetime in magnetic field depends on quantum state, B-field and ion momentum • Lifetimes of 4 GeV H0* in dependence of B and a simulated fringe field • numerically integrated to get rms angle error and hence emittance growth • n=3 gives Δε of 2 – 4 um for B = 1 T • Careful fringe field design needed! Normalised emittance growth for different H0* quantum states as a function of peak magnetic field CERN Seminar, Laser Stripping

Stark broadening • Stark effect: charged particle in an electric field  transitions will be broadened • Overcome Doppler broadening by placing interaction region in a magnetic field  large Stark-broadening of transition • Single frequency can excite the resonance for all atoms • Stimulated emission suppressed • HP-PS: • To reach Doppler width of 2e-3 need 0.3 T for n=3 transition • Lifetime of H0* only 1e-10 s • Required laser micropulse energy slightly higher than previous method • Excitation takes place over ~0.5 mwhich introduces a large angle between injected and circulating protons  difficult integration Stark-broadening of quantum levels vs B-field for 4 GeV H0 CERN Seminar, Laser Stripping

Laser characteristics for HP-PS (H0 --> p+ stripping) Relevant parameters as input for laser feasibility discussion CERN Seminar, Laser Stripping

Laser stripping - HW • Chicane and painting bumpers (~4 each) • Chicane: one of these septum like depending on geometry and specially designed stripping magnet • Stripping foil • Thin foil: Foil holder with exchange possibility • Thick Foil: convert unstripped H- and H0 to protons to guide them into dedicated waste beam channel • Vacuum and radiation compatibility • Waste beam handling • Internal dump (PSB) • Short line to external dump • Electron collector • Careful design of beam trajectories required due to different angles for H-,H0 in chicane magnets • Machines with running H- injection suggest tracking studies of waste beams and a high number of diagnostic possibilities • Laser • Wiggler magnet or laser for H- neutralisation • Laser for excitation + optical resonator • All the equipment to run the laser (optical table, fiber,…) • Vacuum window CERN Seminar, Laser Stripping

Implications for beam, lattice and optics • General • A combined foil/laser stripping system for beam energies of several GeV requires (eg. HP-PS and FNAL) about 20-25 m drift space • Matching of curvature of ring phase space and incoming turn implies: • Foil • Minimum βi given by foil heating • βi /βr: Smaller ratio helps to reduce foil hits but increases foil temperature • αi = αr = 0 • Place foil in fringe field of chicane dipole such that H0*are stripped into the machine acceptance CERN Seminar, Laser Stripping

Implications for beam, lattice and optics • Laser • Laser peak power is proportional to vertical injected beam size if the beam-laser interaction is horizontal • Minimize as much as possible vertical beam size in IR  triplet structure in LSS for FNAL and CERN studies • D’ at interaction point to eliminate Doppler broadening • Longitudinal painting: • large momentum range increases tremendously the required laser power • Bunch length: • increases laser average power • Energy jitter: • increases frequencies to be swept • should be an order of magnitude lower than initial dp/p • Trajectory jitter

Injection area optics • Contradicting requirements for foil and laser  two different optics settings Foil optics Laser optics CERN Seminar, Laser Stripping

Combined chicane for foil and laser H- Stripping of first e- Waste beam Foil 2 H0 Foil 1 p+ Laser B=1.6 T B ≤0.13 T CERN Seminar, Laser Stripping

Items which deserve attention • Laser • Locking to beam temporal structure • Optical resonator • Transport of laser light, radiation hardness • 300 J through a vacuum window • Integration of wiggler and laser with variable interaction angle • Magnet design of wiggler and stripping chicane magnet (fringe field) • Combination of foil and laser system, 3- and 4- magnet bumps with dispersion closed CERN Seminar, Laser Stripping

Summary • High-intensity/brightness  H- injection • H- injection for high beam power • Challenges with foils: damage, losses, radiation, emittance growth • Laser stripping avoids mechanical interaction with beam • H- neutralisation: Lorentz-stripping in wiggler magnet or Photo-dissociation with laser (resonances) • H0 to p+: excite ions on resonance and use the therefore higher probability of Lorentz-stripping in a chicane magnet • Challenges: • Commercial lasers don’t fully match yet the requirements from accelerators but this community is extremely fast evolving • Wiggler design/integration • Fringe field design of stripping chicane magnet • Vacuum windows withstanding laser power CERN Seminar, Laser Stripping

Status quo of laser stripping • At present • R&D phase • Proof of principle at SNS demonstrated 90% stripping efficiency for ~7 ns (see V. Danilov et al., PRST) • Midterm: • 3-year experiment at SNS started with the aim of more than 90% stripping for 10 us (see talk S. Cousineau at FNAL workshop) • Requires a photon recycling optical cavity (see talk M. Notcutt at FNAL workshop) • Final experiment foreseen for Jan-2016 • Longterm • Laser stripping to be operational with similar or better performance than foil stripping … 2020-2025 ??? CERN Seminar, Laser Stripping

References • V. Danilov et. al., Physical Review Special Topics – Accelerators and Beams 6, 053501 • V. Danilov, Future prospects for laser stripping in high intensity machines • B. Goddard, Injection and Extraction, CAS slides • B. Goddard et al., Laser Stripping for the PS2 Charge-Exchange Injection System, PAC09 • S. Cousineau, SNS Laser Stripping, FNAL workshop, see below • I. Yamane et al., POP experiment of Laser Stripping via a Broad Stark State Using BNL 200 MeV H- beam, ICFA-HB2004 • D. Johnson, Conceptual Design Report of 8 GeV H- Transport and Injection for the Fermilab Proton Driver • W. Bartmann et al., Laser stripping for CERN HP-PS, FNAL workshop, see below • W. Bartmann, B. Goddard, H-Injection into PS2 and Laser Stripping, SNS workshop, see below • H. Schönauer, ESS accumulator parameters • Laser stripping workshop 2013 at FNAL: https://indico.fnal.gov/conferenceOtherViews.py?view=standard&confId=6855 • Laser stripping workshop 2009 at SNS:http://wiki.ornl.gov/events/lahbsa/default.aspx Thank you! CERN Seminar, Laser Stripping

Laser Stripping for H - Injection