1 / 29

Simulation of Positron Production and Capturing.

Simulation of Positron Production and Capturing. W. Gai, W. Liu, H. Wang and K. Kim Working with SLAC & DESY. Start-to-end Model of the Positron Source. Electron Gamma Shower code: EGSnrc. Particle dynamic code: PARMELA. Analytical + Monte carlo. e-. Undulator.

bbolling
Télécharger la présentation

Simulation of Positron Production and Capturing.

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Simulation of Positron Production and Capturing. W. Gai, W. Liu, H. Wang and K. Kim Working with SLAC & DESY

  2. Start-to-end Model of the Positron Source Electron Gamma Shower code: EGSnrc Particle dynamic code: PARMELA Analytical + Monte carlo e- Undulator

  3. AMD profile and PreAccelerator Filed Map AMD field:5T-0.25T in 50cm Accelerating gradient in preaccelerator: 12MV/m

  4. The spectrum of photon generated by helical undulator is known as e- Undulator Photon number spectrum and distribution functions (1) Photon number spectrum in terms of harmonics (2) Harmonics distribution function (3) Energy distribution function (4)

  5. e- Undulator Photons from Helical Undulator– polarization and radiation angle • The amplitude of radiation from helical undulator in terms of harmonic can be obtained as (1) From which the Stokes parameters are obtained as: (2) where x1 describes the linear polarization, x2 describes the linear polarization in coordinate system rotated by 45o with respect to the original one for x1, x3 describes the circular polarization. After knowing its energy and originating harmonic, the radiation angle of photon is also determined as: (3) An uniform distribution on azimuthal direction is assumed

  6. e- Undulator Photons from helical undualtor– originating point and collimating • In addition to distribution functions in previous slides: • Uniform distribution along the length of undulator is assumed • Temporal and transverse distribution is inherited from drive e- beam Collimating: With the originating point ( x0 , y0 , z0 ) and direction vector (dx, dy, dz) assigned, the state of a photon after collimator is determined by its position on the collimator screen as: (1) Where zc is the location of collimator. If r is greater than the collimator iris, the photon is then ignored and will not allowed to strike the target to produce positrons

  7. Undulator Based Positron Source Polarized source: e- Undulator For unpolarized source, the photon collimator and drift between target and undulator can be elliminated

  8. e- Undulator Photon Spectrum and Polarization of ILC baseline undulator Results of photon number spectrum and polarization characteristic of ILC undulator are given here as examples. The parameter of ILC undulator is K=1, lu=1cm and the energy of electron beam is 150GeV. Figure1. Photon Number spectrum and polarization characteristics of ILC undulator up to the 9th harmonic. Only those have energy closed to critical energy of its corresponding harmonics have higher polarization

  9. Positron polarization distribution after target Photon collimator screened out those photon with lower polarization and thus improved the positron polarization. without collimator with collimator The collimator is 2.5mm in radius and is 700m away from the end of a 100m long undulator. Photons are radiated uniformly along the undulator. For the case without collimator, the photon source is treated as a point source.

  10. Comparison of the captured positron polarization spectrum: with/without collimator. Better polarization but lower positron yield due to less photons were used Without collimator With collimator The capturing optics are the same for both with and without collimator. The collimator screened out those photons with lower polarization and thus improved the polarization of resulting positron beam. The polarization of captured positron beam is about 30% without collimator and 63% with the collimator setting used here.

  11. e- Undulator Yield contribution from different harmonics. The cutting angle of collimator is fixed at 3.85mrad • 1st harmonic contributed <15% to the positron yield • 2nd and 3rd harmonic together contributed >45% • Contributions from 4th and 5th harmonic are comparable to 1st harmonic • Positron yield is mostly contributed by high order harmonics

  12. The Effect of Photon Collimator Figure 2. Figure 1. Yield and polarization strongly correlated to collimator configuration. For 150GeV drive beam with K=1 and lu=1cm, 60% polarization required at least 400m drift Figure 3

  13. Optimum yield for 150GeV undulator with lu=1cm and different K Optimum yield dropping from about 1.1 down to <0.9 when K change from 1 to 0.8

  14. Optimum yield for 150GeV undulator with K=1 and different lu Optimum yield decrease from ~1.1 down to ~0.4 when lu increase from 1cm up to 1.6cm

  15. Yield and Polarization for undulator with 250GeV drive beam Yield for polarized undulator based source using 250GeV drive e- beam with K=1.5 and B=1.76T (lu~0.91cm) is about 2.5 per 100m undulator The collimator has a fixed cut angle at 3.85mrad/g, This angle is enough for 150GeV option but still a little bit bigger for 250GeV and results in a smaller polarization

  16. Photon collimating • Since the polarization of photon from hellical undulator has strong correlation between it’s radiation angle gq and harmonics index. For undulator based positron source, a collimator with proper configuration is required in order to achieve the required polarization. For 150GeV option, the shortest drift between target and undulator is 400m where the hardedge radius of the collimator iris is about 1.5mm. • For 250GeV option, since the yield is higher and thus the undulator is shorter, the drift could also become shorter. But for 250GeV drive beam energy, collimator iris will be smaller and this could makes it more difficult to align the collimator with the beam

  17. Unpolarized e+ source • For unpolarized e+ source, we don’t need collimator and drift of several hundreds meters. (you still get 30% polarization) • Since we are using all the available photons, the e+ yield will be higher. For 150GeV drive beam, the yield is about 2.3 per 100m undulator. For 250GeV drive beam with the same undulator parameters where K=1, lu=1cm, the yield is about 6.5 per 100m undulator. • For K ~ 1.5 and B=1.76T, for 250GeV drive beam, the yield will be about 12 per 100m undulator

  18. Yield for unpolarized source at different undulator parameters, 250GeV drive beam For yield of 1.5, the required length of undulator using K=1, lu=1cm and drive beam of 250GeV will be only 23m

  19. Yield for unpolarized source at different undulator parameters, 150GeV drive beam ~2.3 For required yield of 1.5, using 150GeV with K=1,lu=1cm, the unpolarized source required a length of about 65m

  20. e- Undulator Error Analysis Sensitivity of Yield to beam jitter Yield and polarization as a function of drive beam transverse position-collimator: location 700m, iris radius 2.5mm Yield and polarization as a function of drive beam transverse position-collimator: location is 300m, iris radius= 1.155mm

  21. Summary • We studied the e+ yield in the immersed target case. Including both polarized (60%) and unoplarized cases. • Many undulator and drive beam parameters investigated. • Also studied the drive beam jitter and found it is not a severe problem. • A lot more cases to be done.

  22. Back-up

  23. Summary on numerical modeling of helical undulator photon source • A complete numerical model of helical undualtor photon source is established with correlations between drive beam profile, undulator length, collimator location and iris included. • Using this model, we find that the majority of captured positrons are produced by photons from high order harmonics. 1st harmonic contributed only less than 15% of the yield. • By comparing the yield and polarization of resulted positron beam, we noticed that the optimum yield decreasing more quickly with undulator period lu than with K. • For the effect of transverse position jittering of drive electron beam, it depends on the location of collimator. The simulation results shows that for collimator with 5mm iris aperture and located at 700m, the yield change is not significant until the offset of the drive beam reaches 0.6mm. But for collimator with aperture of 2.31mm and located at 300m, the yield is decreasing immediately after the offset of drive beam transverse position is over 0.1mm and the polarization of positron beam goes below 0.6 when the offset is over 0.3mm. But in either case, transverse jittering of drive beam is not a problem as long as the collimator is not too close to the undulator

  24. Momentum selection chicane with collimator Bunch compression chicane with collimator Transverse emittance collimator Longitudinal phase space rotation Linac e+ from PPA e+ to SC Linac 28m A possible separation optics To be inserted between preaccelator and SC linac. Only positrons within 150 RF phase capturing window will pass through. Having insertion lost

  25. PARMELA simulation shows after beam pass through the positron separation optics, there are still some particles staying in subsequent RF buckets. So RF deflector should be use to kick off these undesired particles. At exit of pre-accelerator At exit of momentum selection chicane At exit of phase space rotation linacs At exit of bunch compressing chicane

  26. Possible ways to improvement Recently, Yuri has come up with a scheme to enhance the yield by doing phase space shaping of e+ beam while doing spin rotation before inject beam into damping ring. From his results, the capture efficiency enhanced by 50%. If we combine Yuri’s scheme with the positron separation optics presented here, we can overcome the insertion lost and solve the problems associated with dumping 5GeV e+ beam in his original scheme.

  27. Positron separation optics • Since the energy spread e+ beam is too big at beginning it is very difficult to do separation at the beginning. • We also don’t want to separate e+/e- too late because of the problems associated with dumping high energy e+/e- beam

  28. Open questions • Immersed or non-immersed target: Yield changes ~ 35% • Magnetic field distortion and cancellation in immersed target, it is difficult to solve, but will try to work on it.

  29. Multivariate Optimization • Use Start-to-End Model to maximize positron yield delivered to the Damping Ring • Drive beam energy (ILC Baseline:150 GeV) • Collimator Angle (Drift Length and Collimator Radius) • Target Material and Thickness (ILC Baseline: 0.4 rl, Ti) • AMD profile ( 5T-0.25T in 50cm) • Accelerating gradient: • 12MV/m in pre-accelerator • 25MV/m in super conducting linac • Damping ring acceptance: • ex+ey<=0.09m.rad. • DE/E < 1%

More Related