Advanced Development of Particle Acceleration by Stimulated Emission

Advanced Development of Particle Acceleration by Stimulated Emission of Radiation (PASER) W. D. Kimura, L. Schächter, S. Banna ATF Users Meeting April 4 -6, 2007 1

Collaborators Brookhaven National Laboratory (Accelerator Test Facility) - Marcus Babzien - Karl Kusche - Jangho Park - Igor Pavlishin - Igor Pogorelsky - Daniil Stolyarov - Vitaly Yakimenko University of California, Los Angeles - David Cline - Xiaoping Ding - Lei Shao 2

Outline Background Goals of Proposed Experiment Improvements to Experiment Design and Procedure Description of Experimental Apparatus Phase I – High-Gradient Demonstration Phase II – Staged PASER Demonstration Proposed Schedule and Runtime Needs Conclusions 3

Background Particle Acceleration by Stimulated Emission of Radiation (PASER) successfully demonstrated for first time at ATF in Proof-of-Principle (POP) experiment - PASER does not require multi-TW laser driver or subps e-beam bunch - Only requires train of microbunches with spacing equal to active medium transition wavelength - Requires no phase-matching to stage PASER sections POP experiment can be improved upon in many ways - Hardware and operational improvements - Better control and diagnosing of experimental parameters - More extensive measurements and optimization of parameters - More thorough investigation of new physics related to PASER - More data to compare with model and theory 4

Goals of Proposed Program Primary experimental goals of Advanced PASER Development are: - Design and build improved PASER CO 2 discharge system - Demonstrate much higher energy gain and acceleration gradients (target is >50 Me. V/m) - Obtain more extensive data to characterize process, including investigating new physics associated with PASER effect - Demonstrate ease of staging process - Compare with model and theory Primary theoretical goals are: - Investigate alternative active media, such as Ar+ plasma and solidstate media Ar+ PASER operates at very low gas pressures Solid-state PASER may be capable of ~1 Ge. V/m gradients and electrons travel in a vacuum - Develop theoretical basis for follow-on program 5

Proposed Program Divided Into Two Phases Phase I: - Design, build, and test at STI improved PASER gas chamber - Install improved PASER cell, plus diagnostics, on ATF beamline - Using existing IFEL, produce microbunch train to drive PASER - Perform extensive measurements to characterize and optimize system for maximum energy gain and gradient Phase II: - Install second PASER discharge system - Measure characteristics of staged PASER system - Perform any additional measurements as needed 6

Summary of Experiment Improvements Parameter POP Experiment Proposed Program Comments E-beam energy 44. 6 Me. V 70 Me. V Higher beam energy helps reduce spacecharge and scattering effects. Macrobunch duration 4 ps (effective) 2 – 5 ps Will vary in order to change number of microbunches M Microbunch duration 3 fs Dictated by CO 2 laser wavelength. Cannot change valve. Macrobunch charge 0. 1 n. C Up to 5 n. C Will increase until space-charge effects become an issue. Microbunch charge ~0. 2 p. C Varies Charge in each microbunch depends on macrobunch charge and pulse length. CO 2 gas mixture pressure 0. 25 atm Up to 1 atm Pressure affects optimum energy density. Gas scattering and Cerenkov radiation loss are counter-effects. Will find best operating pressure that gives highest energy gain. Applied voltage 30 k. V Varies Voltage depends on electrode gap separation. Will utilize high voltage driver similar to commercial lasers, capable of higher voltages to permit adjusting energy density. 7

Summary of Experiment Improvements (cont. ) Parameter POP Experiment Proposed Program Comments Electrode length 40 cm Most likely 40 cm The electrode length will be chosen during the design phase of the program. Issues such as space constraints on the ATF beamline may affect the length of the PASER cell. Electrode gap spacing 2. 5 cm <2. 5 cm A smaller gap will make construction of the discharge system easier. Pumping efficiency ~1% >10% (goal) Will take advantage of commercial CO 2 laser technology and techniques. Gas scattering compensation None Use solenoid magnet Will surround electrode to help control gas scattering. E-beam windows 2 m thick diamond Will use same window design. Measure gain of medium Not done Incorporated in design Will be used to determine stored energy density. Main discharge trigger Spark-gap Thyratron More reliable trigger with less jitter. Common in commercial lasers. 8

Possible Design for Improved PASER System New PASER gas chamber designed to hold two PASER discharge assemblies Gas scattering traveling through last half of chamber will not affect PASER energy gain – only reduces beam charge slightly 9

Possible PASER Chamber Design 10

Can Use Permanent-Magnet Quadrupoles for Triplets Before and After PASER Cell STI manufactures PM quads Compact design - Magnetic field tunable using motors to move magnets - Permits obtaining tight focus of beam into cell Can also use hybrid focusing configuration - Use existing upstream electromagnet quads - Use single PM quad just before entrance to PASER cell 11

Solenoid Around Electrode Can be Made With Permanent Magnets Permanent magnet (PM) solenoid has advantages over electromagnet solenoid - More compact; does not require water-cooling or high-current power supply; is inherently stable; and can have stronger fields - STI has already performed preliminary magnetic analysis of PM solenoid for photocathode electron gun Magnetic field plot Example of component layout 12

Measuring Gain of CO 2 Discharge Gives Excited-State Energy Density Use CO 2 laser probe beam to measure gain versus discharge parameters Will also use to optimize pumping efficiency PASER theory predicts optimum energy density is a function of other parameters - Gain measurements important for verifying this dependence 13

Optimum Energy Density Dependence Reveals Interesting New Physics Optimum energy density of active medium (wact) shows oscillating dependence on number of microbunches M, but not beam size (Rb) - Implies collective effects of entire ensemble of electrons affects ability to extract energy from medium 14

Summary of Major Phase I Tasks Measure gain as function of CO 2 gas mixture pressure, composition, and high-voltage settings at STI Once installed at ATF, determine optimum tune for e-beam through cell - Adjust quads and solenoid - Maximize delivered charge Use double-period STELLA undulator for operation of IFEL at 70 Me. V - Adjust CO 2 laser power to undulator to achieve desired modulation - May possibly utilize STELLA chicane to reduce drift space Use CTR to monitor bunching efficiency Perform PASER experiments - Vary number of microbunches by varying e-beam pulse length - Systematically scan over other parameters - Measure energy spectrum with and without discharge present 15

Schematic of Staged PASER System Second discharge system would be identical to first one Note, staging requires no special positioning of second discharge with respect to first one 16

Summary of Major Phase II Tasks Use Phase I results to find optimum operating condition for second stage Determine optimum e-beam tune through both stages - May primarily affect downstream e-beam optics, e. g. , exit triplet - Again, aim for maximum charge throughput Measure energy spectrum with and without second discharge on - Should see doubling of energy gain - Vary parameters to determine dependence - Compare with model predictions 17

Advanced Concepts: Ar+ PASER Advanced PASER concepts will be investigated in parallel with experimental effort and will focus on studying alternative active media Argon ion laser active medium - Breakdown of medium less of issue because medium is already a plasma - Argon ion photons are 50 times more energetic than CO 2 photons - Highest gain in pulsed argon lasers is at 476. 5 nm at 20 – 30 m. Torr - Pinch effect has been observed that might help enhance local excited-state density - Low pressure means may be able to use discharges similar to gas-filled capillaries (eliminates windows) Challenge is making microbunch train with 476. 5 nm bunch separation - Possibly use seeded FEL driven by Nd: YAG pumped dye laser Creates microbunch train as by-product of FEL process Approach being pursued at UCLA Neptune Lab in collaboration with STI - Use wire-mesh technique to generate custom microbunch train 18

Advanced Concepts: Solid-State PASER Use Nd: YAG rod as active medium with e-beam traveling through hole in center of rod - All scattering/breakdown effects eliminated because electrons travel through vacuum - Nd: YAG photons are 10 times more energetic than CO 2 photons - Excited-state energy density is ~10 times higher - Hence, energy density may be 100 times larger → ~1 Ge. V/m possible - BUT, field from electrons does not appreciably penetrate into rod unless electrons are highly relativistic, i. e. , 1 Ge. V Challenge is making microbunch train with 1. 06 m bunch separation AND with 1 Ge. V energy - Possibly make train at low energy and accelerate in damping ring Issues such as effects of coherent synchrotron radiation must be studied - In principle, can use wire-mesh technique, but 1 Ge. V beam must have low emittance 19

Proposed Program Schedule and Runtime Needs Estimate for runtime requirements - Phase I: 6 weeks - Phase II: 4 weeks 20

Role of Collaborators ATF staff responsible for - Generating microbunch train using STELLA undulator - Optimizing e-beam tune through system - Operation of CTR diagnostics UCLA (Prof. Dave Cline) responsible for - Graduate and postdoc support - Similar role as during STELLA 21

Conclusions Advanced PASER Development program offers unique opportunity to investigate a new paradigm in advanced acceleration schemes - PASER is potentially a simpler scheme capable of comparable acceleration gradients as other advanced methods - If wire-mesh technique is used to make microbunch train, then this eliminates need for IFEL PASER effect also has interesting physics - Collective e-beam effects on excited molecules affect energy exchange process - Opportunities to test better active media There is a synergism between PASER, wire-mesh technique, STELLA, and inverse Cerenkov acceleration that is unique to the ATF 22