Vision for the Future: BESSY VSR A Variable Bunch Length Storage Ring

Transcription

1 Vision for the Future: BESSY VSR A Variable Bunch Length Storage Ring Gode Wüstefeld, HZB ESLS, Aarhus, Nov , 211 presented by P. Kuske

2 Outline BESSY VSR - Motivation - Limits of short bunches: measurements & scaling laws - Bunch focusing by sc-cavities - Double beam option - Expected results 2

3 Motivation why short e - bunches: - time resolved, picoseconds X-ray experiments - CSR for THz experiments present situation: - dedicated low-a shifts at BESSY, s=3ps 4 blocks of 3 days per year, two operation modes: 4 ma (bursting) and 15 ma (stable) future goal: - simultaneously 15 ps & 1.5 ps bunch mode up to 1x more current in short bunches ( 1 x more THz power) 3

4 BESSY II synchrotron radiation source BESSY II ring 1.7 GeV storage ring parameters 24/p m nat. bunch length s = 13 ps (rms) number of cells beam current nat. emittance nat. moment. spread 4 rf-cavities.5 GHz, 1.5 MV 2x8 <3 ma 6 nmrad.7e-3 5

5 Low-a optics a tool to produce and study short bunches definition of a relation s, a and V : ΔL L a Δp p σ a V' - pioneering work at BESSY II, since short bunch operation, 13 ps 3 ps (rms) 7 fs are proved and analyzed - MLS ring of PTB first ring to control 3 orders of a a a a1 Δp p a2(δp p ) short bunch studies, double-beam 2 stability thresholds of short bunches - subject of present PhD-thesis by Markus Ries, HZB 6

6 bunch length s / ps (rms) Bunch length current relation for the BESSY II ring at fixed rf voltage amplitude of 1.35 MV 1 user optics measurements streak camera Fourier transform spectrometer theory Stupakov & Heifets 1.1 threshold from coasting beam 1E-4 1E-3 1E THz optics s ~ I b 3/7 8 nc 1 single bunch current I b / ma THz bursting threshold - beyond bursting threshold bunches blow up in energy spread - rule of thumb: (s, I b ) (2s, 5I b ) good agreement between measurement and prediction! Are short bunches restricted to low currents?? 7

7 Are short bunches restricted to low currents?? scaling law between a and I predicted by: - bunched beam theory (Sacherer) - Vlasov-Fokker-Planck simulation - and coasting beam (Landau Damping) Keil-Schnell : Δp Δp I Z /n F a E /e p p bunch length s σ I~a a V' I~V for fixed s increasing the rf-gradient V x 1 a needs to be increased x 1 I can be increased x 1 8

8 BESSY II & sc-cavities sc-cavities for bunch shortening bunch length current relation sc-cavities (scheme) 1x enhanced rf-gradient cavity V 1,f 1 cavity V 2,f 2 ~ 5 m straight 9

9 Voltage / MV Simultaneously long & short bunches present nc-cavity (power) long bunch sc-cavity # 1 (focusing) short bunch sc-cavity # 1 & 2 (focusing) short & long bunches sum voltage.5 GHz, 1.5 MV V =Vxf rf =.75 MVGHz rel. long. phase position / ns 1.5 GHz, 25 MV V =Vxf rf = 37.5 MVGHz 1.75 GHz, 21.4 MV V = Vxf rf = 75 MVGHz - flexible fill pattern, I<3 ma - 15 ps & 1.5 ps pulses simultaneous at all beam ports - all IDs available 1

10 Voltage / MV Simultaneously long & short bunches present nc-cavity (power) long bunch.5 GHz, 1.5 MV V =Vxf rf =.75 MVGHz rel. long. phase position / ns 1

11 Voltage / MV Simultaneously long & short bunches sc-cavity # 1 (focusing) short bunch 1.5 GHz, 25 MV V =Vxf rf = 37.5 MVGHz rel. long. phase position / ns 1

12 Voltage / MV Simultaneously long & short bunches sc-cavity # 1 & 2 (focusing) short & long bunches sum voltage - flexible fill pattern, rel. long. I<3 phase ma position / ns - 15 ps & 1.5 ps pulses simultaneous at all beam ports - all IDs available 1.75 GHz, 21.4 MV V = Vxf rf = 75 MVGHz 1

13 rel. momentum /.1 rel. momentum /.1 rel. momentum /.1 rel. momentum /.1 MAD tracking single particle tracking, BESSY II user optics & two sc-cavities short long % 4% 2-2 short & long bunch momentum Dp/p acceptance +/- 4% long. position / ps long. position / ps ps (rms) short & long bunch quantum excitation & damping 1 damping times long. position / ps long. position / ps 11

14 More advanced scheme: double beam chromatic orbit length: L L (1 a Δp p ) orbits of equal length L=L : I) Δp p II) a 2 solutions if a a a 2 a2(δp p ) Δp/p α/α2 F double beam scheme a > a < beam port long puls short puls 2 sc-rf cavities & low a optics - double beam scheme combined with two sc-rf cavities long and short bunches longitudinally and transversely separated 12

15 vertical displacement More advanced scheme: double beam measurements at MLS e - beam source point image photon beam image (beam port exit) beam current: 17 ma lifetime: 1 h 6cm horizontal displacement transverse separation of photon beams double beams can be easily produced at the MLS low-a optics good life time, high currents 13

16 HZB expertise BERLinPro and BESSY VSR : - BERLinPro cavities close to the BESSY VSR, 1.3 GHz to be scaled to 1.5 GHz and 1.75 GHz - high current beam interaction with sc cavities BERLinPro BESSY VSR 14

17 Summary simultaneously long & short bunches: long bunches, 15 ps < 3 ma short bunches, 1.5 ps (rms) - all beam ports supplied - all IDs available - present transverse user optics applied expected results: beam parameter present 35 bunch filling THz optics BESSY VSR 175 bunch filling user optics BESSY VSR 175 bunch filling THz optics bunch length (rms) / ps current (sb) / ma.4 (.3 nc) current (mb) / ma <3 14 (14)