L aser Chopper for Project X Dave Johnson Jinhao Rao Arun Saini and Vic Scarpine APC Seminar June 2 2011 Motivation H Potoneutralization Laser Chopping Concept Outline of Conceptual System ID: 1031162
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1. Investigation on the Feasibility of a Laser Chopper for Project XDave Johnson, Jinhao Rao, Arun Saini, and Vic ScarpineAPC SeminarJune 2, 2011
2. Motivation H- Potoneutralization Laser Chopping Concept Outline of Conceptual System MEBT Front-end with Laser Chopping Creating Seed Laser Pulses Laser Pulse Amplification Laser zig-zag cavity Issues and R&D ConclusionTopicsJune 2, 20112APC Thursday Seminar
3. Why look at this?Project X has identified a low-energy H- beam chopper as a critical componentRequirement of a broadband, arbitrary-pattern beam chopper is difficult at 325 MHz and maybe also at 162.5 MHzStandard E/M-field choppers push limits and require absorber blades close in the beam aperture – There has been significant progress in both power supply and kicker plate design !Are there other options?Lasers have been used for laser profile monitors; SNS, BNL… and as a notcher test here at FNALH- + g -> Ho + e-Can a laser system be used as a broadband chopper?June 2, 2011APC Thursday Seminar3
4. Laser Stripping vs Laser ChoppingJune 2, 2011APC Thursday Seminar4Laser Stripping (3 step process)Laser chopping (2 step process)Usually done at high energy:~ few GeVChopping done at low energy:~ few MeVH-H- H0 + e-H-H0gSame Process used for Laser Wire Diagnostics
5. Photoneutralization of H-June 2, 2011APC Thursday Seminar5Photoneutralization cross section of H- by incident photons of energy E is given by:where smax is 4.2E-17 cm2 and E0 is 0.7543 eV. At 2.1 MeV , b = 0.0668 and g = 1.00224, with ion velocity of 20 mm/ns.Is it technically feasible to consider a laser chopping system for Project X?H- beam requirements -> How small can you make beam?Laser beam requirements -> How much power is needed?
6. Assume the following geometry of the H- and laser beam. [From: Shafer LA-UR 98-2643]The photon energy seen by the H- electron in it’s rest frame isThe lab frame energy of a photon from Nd:YAG laser (wavelength of1064 nm) is 1.165 eV is essentially the same in the CM frame for ~90 deg. angle So, for Nd:YAG (1064 nm) Yb:YAG (1030 nm) laser interacting with 2.1 MeV H- @ ~90 deg the photoneutralization cross section is 3.67x10-17 cm2 and 3.80x10-17 cm2 , respectively.Q = 180 : head on interactionGeometry for H- PhotoneutralizationJune 2, 20116APC Thursday Seminar
7. H- NeutralizationJune 2, 2011APC Thursday Seminar7When the probability of interaction is high, and the interaction mechanism is not dependent on H- beam intensity , the single interaction detachment rate, F1, can be expressed aswhere fluence is the number of photons per cm2 at the interaction point, tlaser is the laser pulse length, s is the photoneutralization cross section [cm2] and tcrossing is the transit time of the H- across the laser beam [sec].Assume the laser beam has a uniform irradiance and fully encompass’ the H- beam (top hat transverse profile).Assume a uniform temporal profile with laser pulse length > bunch length.The photon flux is transformed like the photon frequency (energy) which, which like the energy is ~ the same in the lab and CM.where
8. Single Pass H- PhotodetachmentJune 2, 2011APC Thursday Seminar8Assume a laser pulse length , tlaser , of 500 ps, the single pass neutralization fraction as a function of laser pulse energy for several laser beam radii may be calculated. For ~100% neutralization need to have ~10’s of mJ per laser pulseAt 325 MHz rep rate this corresponds to a laser with an average power of 3 to 10 MW !At this point everyone says it can’t be done.But can these numbers be beat down ?Laser Beam RadiusQ-switch10’s to 100’s mJ10-100’s Hz
9. 9Average Power ChallengeRepetition Rate (Hz)1 106Pulse Energy (J) 1 mW1 W1 KW1 MW10310910-31 10310-310-610-9Q-switch Lasers1 GWMode-Lock LasersCPA LasersSNS Laser StrippingSlide from Yun Liu (2nd Laser Stripping Workshop)Laser chopper Few hundred mJoulesFew hundred Mhz> 10’s to 100’s kW
10. Optimizing Laser PowerJune 2, 2011APC Thursday Seminar10Looking at the equation for the Neutralization Factor (F1), only two quantities are controllable, the photon flux and the crossing time,To enhance the yield, requires increasing laser flux by increasing the pulse energy or reducing the interaction cross section (beam size) or increasing the crossing time of the H- across the laserproportional to horizontal laser beam width and ion velocity by increasing the horizontal dimension of the laser beam while reducing the vertical dimension of the laser beam to keep the cross section constant Another technique would be have the H- ions pass through the laser beam as many times as possible. [From: Shafer LA-UR 98-2643]
11. Front Surface Laser High R Mirror drsWQQ:d:r:2s:Distance between interactionsX_laserPath length of laser between interactionsCavity radiusLLaser angle wrt normalX_laser:Diameter of “top hat” laserFront Surface Laser High R Mirror A Little GeometryJune 2, 201111APC Thursday Seminar
12. Multiple Interaction Neutralization FactorJune 2, 2011APC Thursday Seminar12Assume 100% reflectivity on mirrorsAssume 200 mJ laser pulse on the input of the optical cavityEach curve represents single interaction neutralization factor, F1.If the laser passes through the H- beam N times, the fractional yield FN of H0 is related to the fractional yield F1 of a single crossing by : where R. Shafer , 1998 BIW
13. whereFor a given mirror reflectivity R, the laser fluence available for interaction at each of the Mth interactions is modified by RM such that Assume : 200 uJ laser pulse on the input of the optical cavity 500 ps laser pulse length 6 mm laser beam radiusThen for N reflectionsImpact of Finite ReflectivityJune 2, 201113APC Thursday Seminar
14. Mirror Properties/RequirementsHigh reflectivity dielectric mirror coatings available with reflectivity's > 99.995% with a reasonably high damage threshold .Measured loss for 1 micron beam (absorption) in coating are on the order sub- to few ppm depending on coating.Absorption in substrate ~ few ppm/cmReside in a “clean” high vacuumMust not degrade in a high radiation fieldJune 2, 201114APC Thursday Seminar
15. H-RFQH0 Dump (.6m offset)SSR0Laser Cavity22.5 deg AchromatOptional H+ Leg*Not shown: quads or cavitiesMatch to SSR0Length between RFQ & SSR0 ~ 6mmirrormirrorLaser sourceCartoon of a Laser Chopper LayoutJune 2, 201115APC Thursday Seminar
16. Laser inputDumpDipolesMatched into SSR0Vertical aspect of laser (5mm radius)Laser Cavity*Note: Optics not fully optimized ex,y=0.25 p-mm eZ=0.3 p-mmMEBT Insert Example*June 2, 201116APC Thursday Seminar
17. x/y 0.25 p-mm-mr ez 0.275 p-mm-mrBeam current: 5 mA (space charge included)Energy: 2.1 MeVFrequency: 162.5 MhzEnvelopes: +/- 3sTwo ½ meter laser insertsInput (RFQ out)Output Beam Para.ax 0.193bx 1.04ay 0.06by 1.08az 0.23bz 1.22Maximum ParameterBuncher cavity 90 kV*Quad 14 T/m3sx/y envelope < 6 mm3sz < 600ps *1 Gap @100 kV1.2 cm laserExample of Periodic InsertJune 2, 201117APC Thursday Seminar
18. CW 100 mWNarrow line widthSeed laserPulse patterngeneratorFiber AmpPre-AmpGain ~1010GHz modulatorCryogenicAmplifierCoupling OpticsZig-zag cavity100-200 reflectionsInput pulse to cavity:~200uJ, 0.5-1.0 ns, 162.5 Mhz100 pJ (0.5-1ns) 162.5 Mhz16.5 mW2.5 nJ (0.5-1 ns)162.5 Mhz400 mWCouple to free spaceoptics/top hatconverter25 nJ (0.5-1 ns)162.5 Mhz200 uJ (0.5-1 ns)162.5 Mhz32.5 kW4 W162.5 MHz Conceptual SystemH-June 2, 201118APC Thursday Seminar
19. The Principal Benefits of Cryogenically Cooling Yb:YAG at 77 K19 Increased Crystal Thermal Conductivity, Much Lower Thermal Expansion Coefficient and dn/dT Resulting Smaller Temperature Gradients, Reduced Stress Dramatically Smaller Thermal Focusing and Birefringence Uncorrectable Phase Aberrations Very Small Close to Diffraction-Limited Beam-Quality and Independent of Average Power Much Lower Ground-State Population: Resulting 4-Level Kinetics Rather Than Quasi-Three-Level Increased Efficiency No Absorbing Regions Longitudinally or Radially Significantly Increased Stimulated-Emission Cross-Section For Yb:YAG Low Saturation Fluence and Intensity Moderate Gain Increased Extraction Efficiency Acceptable Absorption Using Commercial Diode Pumps at ~ 940 nmJune 2, 2011APC Thursday Seminar
20. High Average Power Cryogenic Yb:YAG Laser System Distributed Zig-Zag Disk Approach 20CW Oscillator – 7 DiskCW Amplifier – 8 DiskJune 2, 2011APC Thursday Seminar
21. Design Features of Distributed Zig-Zag Disk Approach 21 Spread Heat Load Over a Plurality Disks Use Readily Available Off-The-Shelf Diode Pump Sources Imaged Into Each Disk Center Pump Through High-Quality Dichroic Mirrors Forced Convection LN2 Cooling Allows Sustained Operation Simple Scalability Increasing Number of Disks or Increasing Pump Power Easy Replacement of Yb:YAG Disks Use Diffusion or Contact-Bonded Crystal Assemblies: Very Low Fresnel Losses and Dual Wavelength AR Coating External Resonator and Extraction Optics Rugged Pump Chambers to Minimize Misalignment and Prevent H2O Condensation on OpticsJune 2, 2011APC Thursday Seminar
22. Input1-stage amp*2-stage amp*Rep. Rate50 MHz50 MHz50 MHzPulse Energy1.1 nJ0.91 uJ15.2 uJPulse Width12 ps12 ps12.4 psPeak power92 W75.83 KW1.23 MWAverage Power55 mW45.4 W758 WEnergy Gain 82716.7*Stage 1 amplifier system: double-pass 7-element cascaded amplifiers*Stage 2 amplifier system: single-pass 8-element cascaded amplifiers200-400 mJ1 ns~30-60 kW2011 Phase I SBIR awarded to Snake Creek Lasers for:“High Average Power Cryogenic Lasers for Laser Stripping Applications”162.5 MhzSummary of Ultrafast Yb:YAG Laser Results reported by Snake Creek Lasers GOALS System Beam-Quality M2 ~ 1.3Initial Amplification of Mode-locked LaserJune 2, 201122APC Thursday Seminar
23. “Typical” H- and Laser ParametersJune 2, 2011APC Thursday Seminar23
24. H- Beam ParametersJune 2, 201124APC Thursday SeminarH- Parameters:Energy 2.100000 [MeV]Linac current 5 [mA]Linac bunch intensity 1.919119e+008 [ppb] Beta 0.066757 Gamma 1.002236 H- velocity 2.002709e+007 [m/s] RFQ frequency 162.500000 [Mhz] Bunch spacing 6.153846 [ns] Bunch length +/-18.500000 [degrees]Bunch length (full) 1.266671 [cm]Bunch length (full) 0.632479 [ns] RMS beam size: H 0.13 [cm] V 0.13 [cm] 100pct beam size: H 0.78 [cm] V 0.78 [cm]
25. Laser Parameter Requirements entering interaction chamber: Laser ParametersJune 2, 201125APC Thursday SeminarLaser Energy 2.000000e-004 [Joules] Pulse length 6.957265e+002 [ps]Laser spot size radius (top hat) 3.900000e-001 [cm]Laser beam area 4.778358e-001 [cm^2]Photon wavelength 1030 nm Photon Energy 1.931068e-019 [Joules] 1.204433e+000 [eV] Photodetachment cross section 3.804687e-017 [cm^2] Photons per pulse 1.035696e+015 Photon rate 1.488654e+024 [photons/sec] Photon fluence 2.167473e+015 [photons/cm^2] Photon flux 3.115410e+024 [photons/cm^2/sec] Laser Peak pulse power 2.874693e-001 [MW]
26. Multi-Pass Cavity and Laser ParametersJune 2, 201126APC Thursday SeminarDistance between crossings 7.800000e-001 [cm]Distance between reflections on mirror 1.560000e+000 [cm]Distance laser travel between crossing 5.842087e+000 [cm]Cavity radius 5.829055e+000 [cm] Cavity diameter 1.165811e+001 [cm]Incident angle 3.827739e+000 [degrees]Crossing angle 86.17 [degrees] Crossing time (per crossing) 3.894725e-010 Number of passes 150 Photon flux in H- rest frame 3.108460e+024 [photons/cm^2/sec]Mirror Reflectivity 9.995000e-001 Average Neutralization factor per interaction 4.182517e-002 Total Neutralization 9.983529e-001 Multipass insertion length 1.162200e+002 [cm] Total laser path length 1.740942e+001 [m]Average power of laser system 32.500000 [kW]
27. R&D ActivitiesJune 2, 2011APC Thursday Seminar27Refine MEBT/Insertion designHINS for testingProject X specificImpact on RFQ ?Optical beam cavityRadiation damage?Coupling optics to amplifier(s) and cavity Laser gain amplifiers – collaboration with industryDemonstrate variable width laser pulse generationBetter matching between H- beam and laser spatial profileSynchronization of laser to beamNeutralization studies at HINSAlready planned for laser diagnostics
28. ConclusionsJune 2, 2011APC Thursday Seminar28We have outlined a conceptual design for a broadband optical chopper based on laser photoneutralization of H- .Generation of required broadband optical pulses based upon optical communication technology.Potential vendors for all components have been identified.Amplification to the required laser pulse energies and frequency has been recently awarded a 2011 SBIR grant for the further development of High Average Power Cryogenic Laser Amplifiers.The HINS facility at MDB can be utilized to test the concepts for the broadband optical chopper concept.
29. Keeping an open mind…“It is difficult to say what is impossible, for the dream of yesterday is the hope of today and the reality of tomorrow.”Dr. Robert H. GoddardJune 2, 2011APC Thursday Seminar29