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Bunch compression and the emittance growth due to CSR

Bunch compression and the emittance growth due to CSR. Institute for Molecular Science, UVSOR Miho SHIMADA High Energy Accelerator Research Organization, KEK Kentaro HARADA. Outline. Beam dynamics studies for the 5 GeV ERL Bunch compression down to <100fs

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Bunch compression and the emittance growth due to CSR

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  1. Bunch compression and the emittance growth due to CSR Institute for Molecular Science, UVSOR Miho SHIMADA High Energy Accelerator Research Organization, KEK Kentaro HARADA

  2. Outline • Beam dynamics studies for the 5 GeV ERL • Bunch compression down to <100fs • Optimization of R56, sextupole magnet, RF phase is important. • Preserving emittance with and without bunch compression • Beam envelope optimization is efficient to suppress emittance growth. • Towards user experiment at the test ERL • Short bunch for THz radiation (CSR)

  3. Preservation of low emittance

  4. Layout from merger section to insertion devices section • Test Facility Main Parameter • Injection energy 5 MeV • Full energy 165MeV • Injection bunch length 1 psec～2 psec • Bunch length after compression 0.1 psec • Initial projected emittance 100 nm・rad Generation of 0.1-ps electron bunch with low emittance gives us useful information for the 5GeV ERL

  5. Optics of the achromatic and isochronous arc section(No bunch compression) Twiss parameter bx, by and hx R56=0 Injector+merger Insertion device

  6. Mechanism of emittance growth due to CSR wake (2) Energy change depending on the longitudinal position • CSR wake induces the projected emittance growth (1) CSR emission from the bunch tail catches up with the bunch head Bunch tail Bunch head (3) Displacement of bunch slices Y. S. Derbenev et al , TESLA-FEL

  7. Minimization of the emittance growthdue to the CSR wake qPhase qCSR qPhase=qCSR (x, px) at the arc exit (x, px) at the arc exit Large emittance growth Minimized emittance growth The emittance growth is minimized when qPhasecoincides withqCSR (direction of CSR kick). R. Hajima, Nuclear instruments and Methods in Physics Research A 528 (2004) 335-339 a, b, g ：Twiss parameter, f : bending angle

  8. Evolution of the projected emittance(without bunch compression) • Charge, 77pC/bunch • Emittance growth can be suppressed below 140 nm rad. • The r.m.s bunch length is maintained within the accuracy of a few % (results not shown here) Initial 100nm rad Transverse Phase Space at the end of arc section

  9. Bunch compression at the arc section • Rough estimation of fRF corresponding to R56 • Rs =0.9isintroduced to compressed up to 0.1 ps from 1 ps. • R56 of ARC section was varied from -0.1 to -0.35 m. • RF phase shift, fRF, was determined by following R56.

  10. Bunch length after bunch compression I Longitudinal phase space • CSR wake expands the bunch length. • It is difficult to produce a 0.1ps short bunch with bunch charge of 77 pC • Small R56 is better for bunch compresstion (except R56 = -0.1) 0.77 pC 0.5ps 77 pC 0.5ps

  11. Bunch length after bunch compression II Longitudinal phase space • Electron particles shift to the center of the bunch by CSR effect. • As a result, the bunch length is slightly shorten. 0.77 pC 0.5ps 7.7 pC 0.5ps

  12. Projected emittance growth after bunch compression 7.7 pC • Emittance growth with 77 pC is a few mm-rad. • It is difficult to keep the initial emittance even with 7.7pC • Emittance growth with 0.77 pC at large R56 is less than a few % CSR kick 77 pC

  13. Transverse phase space after the arc section (7.7 pC, ~0.1ps) Not Optimized Optimized • Optimization of the direction of the phase ellipse is effective for compensation of the projected emittance growth. • At the optimized optics --- qCSR ~ qPhase = 1.178 CSR kick CSR kick ex = 191.8 nm rad ex = 151.2 nm rad

  14. Bunch compression

  15. For what the test ERL can be used? • User experiments with Terahertz coherent radiation • SR of terahertz region, from 1THz to 10 THz, is enhanced by coherent radiation, when the rms electron bunch length is shorter than 1ps. • Transverse beam size should be less than the wavelength to keep the coherent enhancement -- (un-normalized ex< 100 mm rad) • User experiments with laser Compton Xrays • It is a challenging task to produce the Xray with sub-pico seconds. So we need the electron bunch with the bunch length less than sub-pico second, and bunch charge as large as possible.

  16. For what the test ERL can be used? THz radiation from short bunch THz region 100 fs 1 ps High intensity THz radiation is expected when 0.1 ps bunch is produced

  17. Introducing the sextupole magnets • 2 families of sextupoles were introduced in the arc section. • Optimize numerically the optics to produce the 0.1 ps short bunch with various electron charge by using elegant. • Emittance growth is not critical issue for THz radiaion.

  18. Comparison between with and without sextupole magnets charge 77pC, 1ps (rms), 0.1mm-mrad R56= -0.1m with CSR, without sextupole with CSR, with sextupole T566 < 0 helps the bunch compression With CSR 390 fs (rms), 6.9mm-mrad 59 fs (rms), 5.6mm-mrad distortion by CSR 1ps 100fs optimized by R. Hajima

  19. Bunch compression of a 154pC bunch charge 154pC, 1ps (rms), 0.1mm-mrad re-optimized Acc phase, sext with CSR, with sextupole with CSR, with sextupole T566= -1.23m 235 fs (rms), 30.7mm-mrad 110 fs (rms), 13.5mm-mrad Acc phase : 109.596  108.466

  20. Higher charge per bunch charge 308pC, 1ps (rms), 0.1mm-mrad charge 616pC, 1ps (rms), 0.1mm-mrad with CSR, with sextupole with CSR, with sextupole 224 fs (rms), 32.4mm-mrad 188 fs (rms), 92.6mm-mrad acc phase = 106.754 acc phase =108.287

  21. Summary • No bunch compression • The projected emittance growth due to CSR is not significant with the bunch length of 1 ps, up to 77pC • Bunch Compression • 77pC(100mA) • It is challenging task to compress the bunch length down to 0.1 ps. • 7.7pC(10mA) • The projected emittance growth can be suppressed by the optimization of the optics in the arc • 0.77pC(1mA) • The projected emittance growth is less than a few %

  22. simulation by elegant (25MeV) R56= -0.1m, acc phase (module-1 / 2) = 90 / 126.7 initial 1ps (rms), 0.1mm-mrad 100pC sext-on : T566 = -0.037m sext-off : T566 = 0.38m 355 fs, 9.4 mm-mrad 640fs, 13 mm-mrad

  23. Bunch length after bunch compression I Longitudinal phase space • CSR wake expands the bunch length. • It is difficult to produce a 0.1ps short bunch with charge of 77 pC 0.77 pC 8x10-5 (m) 77 pC 3.5x10-4 (m)

  24. Bunch length after bunch compression II Longitudinal phase space • Electron particles shift to the center of the bunch by CSR effect. • As a result, the bunch length is slightly shorten. 0.77 pC 8x10-5 (m) 7.7 pC 8x10-5 (m)

  25. Projected emittance growth after bunch compression • Emittance growth with 77 pC is very huge. • It is difficult to keep the low emittance even with 7.7pC • Emittance growth at 0.77 pC is less than a few % 7.7 pC CSR kick 77 pC

  26. Transverse phase space after the arc section Not Optimized Optimized • Optimization of the direction of the phase space ellipse is effective for compensation of the projected emittance growth. CSR kick CSR kick ex = 832.6 nm rad ex = 355.8 nm rad (7.7 pC, 0.1pC)

  27. Bunch compression with 0.77 pC R56 = -0.1 m R56 = -0.3 m 8x10-5 (m) 4x10-4 (m)

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