1. Compton based Polarized Positrons Source for ILC V. Yakimenko, I. Pogorelsky BNL Collaboration meeting, Beijing, January 29-February1, 2006

2. Outline: Numbers and issues. Target and conversion efficiency Laser test: –Laser spot size –First results from laser cavity tests Plans

3. Polarized Positrons Source (PPS for ILC) Conventional Non- Polarized Positrons: In the proposal Polarized  -ray beam is generated in Compton backscattering inside optical cavity of CO 2 laser beam and 6 GeV e-beam produced by linac. The required intensities of polarized positrons are obtained due to 10 times increase of the “drive” e-beam charge (compared to non polarized case) and 5 to 10 consecutive IPs. Laser system relies on commercially available lasers but need R&D on a new mode of operation. 5ps, 10J CO 2 laser is operated at BNL/ATF. 6GeV 1A e - beam 60MeV  beam 30MeV e + beam  to e + conv. target ~2 m 5-ps, 1-J CO2 laser

4. Linac Compton Source (LCS): Numbers e- beam energy 6 GeV e- bunch charge 10nC RMS bunch length (laser & e - beams) 3 ps  beam peak energy 60 MeV Number of laser IPS 10(5) Total N  /Ne - yield (in all IPs) 10(5) Ne + /N  capture 2% (4%) Ne + /Ne - yield 0.2 Total e + yield 2nC # of stacking No stacking Proposal numbers are in black, Optimistic numbers are in Red

5. Compton Experiment at Brookhaven ATF (record number of X-rays with 10  m laser) More than 10 8 of x-rays were generated in the experiment N X /N e- ~0.35. 0.35 was limited by laser/electron beams diagnostics Interaction point with high power laser focus of ~30  m was tested. Nonlinear limit (more then one laser photon scattered from electron) was verified. PRL 2005. Real CCD images Nonlinear and linear x-rays

6. LCS: Issues to be checked Conversion target and capture efficiency optimization (nearly done) Laser beam generation and injection into the cavity (very encouraging first results) Laser cavity detailed design and tests at low and nominal repetition rate (no funding yet) Electron beam source and IPs optics (ongoing at BNL for different project) Cost and reliability

7. LCS: Conversion target and capture efficiency optimization The proposal relies on 2% conversion efficiency of the  beam (produced in the Compton backscattering) into captured polarized positrons. Approximate analytical model developed for quick parameter optimization predicts up to 4% efficiency. Detailed computer simulation at ANL confirmed 2% parameter set efficiency. Further work is needed to confirm a case with sicker target and 4% efficiency.

8. LCS: Conversion target and capture layout Target: Ti - 0.3 rl AMD: 60 cm long 10T to 2T AMD pulsed Linac: Pulsed L-band with 30MV/m 2% gamma to captured e + beam efficiency predicted by analytical model and detailed computer modeling with ~70% (?) polarization Photon collimator Target AMD Pre-accelerator and solenoid

9. ~50% of the gammas with energies 30-60MeV can be selected by collimator with  ~1. Energy cross-section (top) and angular dependence (bottom) for the Compton backscattering Step1. Energy filtering of the gamma beam

10. Step2: Gamma to pair conversion in target ~30% of the 30-60 MeV gammas will generate e+e- pair in X/3 target. ~25% of them will have combined energy in the 30-60 MeV range. Differential cross-section of the pair production as a function of positron energy

11. Step 3: Positron energy selection ~50% of the positrons will have 30- 60MeV energies. They will loose ~15% of energy in X/3 target on average due to bremsstralung. New energy range 25-60MeV. Total efficiency up to this point: 50% x 30 % x 25% x 50 % = 2%. Computer simulations predict ~2.2%.

12. Step 4: Capturing efficiency All positrons in with the 25-60MeV energies can be captured. Angular acceptance is shown on the left and longitudinal phase slippage is shown on the right graphs

13. Step 5: Polarization The positron longitudinal polarization goes from 0.5 at half the gamma energy to 1 at the full gamma energy for polarized gammas. Integrating from positron energy 50% to 100% of the gamma energy gives a polarization of about 0.8.

14. IP#1IP#5 2x30mJ CO 2 oscillator 10mJ 5ps from YAG laser 200ps 1 m J 5ps 10mJ 5ps 300mJ 5 ps TFPPC 150ns Ge 1J e-e- LCS: CO 2 laser system pulse length 5 ps energy per pulse1 J period inside pulse train 12 ns total train duration1.5  s train repetition rate150 Hz

15. LCS: LDRD support at BNL LDRD at the level of $110K supports this effort Cavity simulations and tests, injection design are the main goals of the LDRD Available at BNL/ATF CO2 Laser hardware is used to supports this effort. PostDoc with CO2 laser experience will be soon hired to work on this project. Funding is nearly sufficient to complete the main LDRD goals on a two-year scale.

16. LCS: Laser focus characterization mm 75  m100  m150  m Gaussian approximation Transmitted energy 75  m d =75  m 100  m150  m no pinhole High power (~3J, 5ps) CO 2 laser spot size of  =32  m was demonstrated using F#~4 parabola with a hole for e-beam transmission. Proposal assumes  =40  m. Laser is circular polarized as is required for ILC.

17. LDRD – cavity tests Has a potential to increase average intra- cavity power ~100 times at 10.6 microns. Purpose of the test: Demonstration of 100- pulse train inside regenerative amplifier that incorporates Compton interaction point. Demonstration of linear- to-circular polarization inversion inside the laser cavity. Test of the high power injection scheme

18. “~100 times increase of the average intra- cavity power at 10  m” The required laser train format / repetition rate /average acting power at each IP: 100 pulses x 150Hz x 1J = 15 kW. Efficient interaction with electron beam requires short (~5ps) and powerful (~1-2J) laser beam. Such high-pressure laser does not exist. Non-destructive feature of Compton scattering allows putting interaction point inside laser cavity. We can keep and repetitively utilize a circulating laser pulse inside a cavity until nominal laser power is spent into mirror/windows losses. Assuming available 0.5 kW CO2 laser and 3% round-trip loss, 1-J pulse is maintained over 15,000 round trips/interactions (100 pulses x 150 Hz). Thus, 0.5 kW laser effectively acts as a 15 kW laser. Equivalent solid state ( 1  m) laser producing the same number of gamma photons should be 150 kW average power with ~10J, 5ps beam.

19. LCS LDRD: Simplified test setup First observations: Optical gain over 4  s Misbalanced gain/loss regime results in lasing interruption by plasma Single seed pulse amplification continues to the end

20. LCS LDRD: Simplified test setup Balancing gain/losses and plasma threshold results in continuous amplification with two characteristic time constants : –~200 ns due to CO2 inversion depletion –~2  s due to N2-CO2 collisional transfer. Early injection of seed pulse before the gain reaches maximum allows to control train envelope

21. 3% over 1  s The best train uniformity achieved Very encouraging results obtained with simplified cavity test setup: ~200 ps pulse of the order of 100 mJ circulated for >1  s. Further test would require pulse length monitoring and high pressure or isotope mixture based amplifier (to sustain 5 ps beams).

22. LCS: Laser cavity detailed design and tests at low and nominal repetition rate The key point – not funded There is a detailed $250K/2 year proposal from SDI to study amplifier for the cavity at 5 Hz ($100K laser + modifications & R&D ~ $150K) Not modified 150Hz laser price is ~ $450K 1 additional FTE at BNL is needed to support amplifier work and to design, simulate and test laser cavity ~$350K/year – 3 years is needed to test laser cavity at 5 Hz

23. LCS: Electron beam source and IPs optics Photoinjector gun is simulated at CAD/BNL with the required beam parameters Detailed design is needed and estimated 1FTE-year is expected Depend on the number of laser IPs. Design uses 10 IPS. (potentially can be reduced to 3- 5 IPs) Should be delayed till laser cavity parameters (required number of cavities) are verified

24. Acknowledgments: W. Gai (ANL), W. Liu (ANL), V. Litvinenko (BNL), W. Morse (BNL) and I. Pavlishin (BNL) helped with analytical and computational analysis of the conversion process and optical cavity work