The BESSY Low Alpha Optics and the Generation of Coherent Synchrotron Radiation

1. The BESSY Low Alpha Optics and the Generation of Coherent Synchrotron Radiation J. Feikes, K. Holldack, H.-W. Hübers*, P. Kuske, G. Wüstefeld BESSY and * DLR, BERLIN ‘Frontiers of Short Bunches in Storage Rings’ Frascati, INFN, 7-8 November 2005 see contribution in ICFA Beam Dynamics Newsletter No. 35, December 2004 wuestefeld@bessy.de

2. Content content 1. Low alpha optics 2. Coherent radiation 3. Bunch-length current relation 4. Limits of short bunches 5. Upgrade option: short bunches at BESSY II G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

3. single & multi bunch 1.25 MHz to 500 MHz 10 mA < I< 0.1 mA (2mA~10 electrons) 10 The BESSY Low Alpha Optics main parameters the machine optics 4 sextuple families for beam dynamics corrections short bunches by low a manipulation a² ~ fs = control value very stable machine operation, good life time 20 mA and 20 hours application: coherent THz radiation, ICFA No. 35, article by U. Schade et al. short x-ray pulses at BESSY, accepted PRL, A. Krasyuk et al., 2005 G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

4. fs=350Hz -6 α=-3·10 Longitudinal tune synchrotron frequency and alpha • synchrotron frequency fs as a function of rf frequency • fs increases strongly with deviating rf frequency • optics tuned by sextupoles (long. chromaticity) • extracted momentum compaction factora • fit to measured data d =0.1% rms G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

5. InSb-bolometer bursting CSR CSR intensity 1 turn 1 turn stable CSR CSR intensity H.-W. Hübers et al. Applied Phys. Lett. 87,184103(2005) H.-W. Hübers et al., ICFA Newsletter No. 35 time resolution of single turns, t=1ms temporal resolved THz pulse of pulse length of few 100 ps: multiple reflections in THz beam line THz Detectors at BESSY hot-electron bolometer HEB resolution of single bunches, t=30ps Detector parameters G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

6. 1 10 -4 10 -9 10 -14 10 1 10 100 1000 10000 Spectral range of CSR interferogram raw data power spectrum source comparison 0.1THz 1THz user optics burstig CSR, SB 15 mA incoherent radiation 250 mA, user optics /sr/(0.1% bdw)] 7 10 gain 7 10 gain 2 low alpha stable CSR, 18 mA BRILLIANCE [W/mm black body, 1200 K, 10 mm^2 -1 WAVENUMBER [cm ] user optics, single bunch 15 mA Brilliance of the BESSY THz spectrum power spectrum analysis by Martin-Puplett Fourier transform spectrometer G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

7. CSR bunch length measurement • 700 fs, 300 nA • - 870 fs, 140 nA • - 1.2 ps, 140 nA Fourier transform spectroscopy, coherent and incoherent THz radiation interferogram Haïssinski equation and CSR wake bunch shape n(z) - fit Fourier transform of bunch shape form factor g(l) Fourier transform of interferogram power spectrum, raw data includes f(l)-factor incohere. power P = p N coherent power P = p N (N -1)g(l) norm. power spectrum P / P = N g(l) e incoh l coh e e l coh incoh e power spectrum coherent • f(l) power spectrum incohere.• f(l) P / P = coh. incoh. normalized power spectrum Gaussian relation between bunch length and spectral band width shape of THz pulse Kramers-Kronig analysis Power spectrum and bunch shape of stable bunches Theory machine parameters experimental data THz pulses G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

8. CSR spectra at fs=1.75 kHz, s=3 ps different 400-bunch currents raw data, not normalized 1 1 0 0 0 5 0 0 a > 0 intensity / a.u. 1 0 a < 0 intensity / a.u. 2 0 wavenumber 1/cm no significant difference was found in the spectral distribution for a<0 and a>0 3 0 2 3 4 6 8 9 1 5 7 8 9 . . . . . . . . . . . 2 2 5 7 1 0 0 0 0 1 9 0 0 0 2 6 0 0 0 0 0 3 m m m m m m m m m m m A A A A A A A A A A A 4 0 Example of THz power spectra taken at IRIS (BESSY) CSR spectra at fs=1.2 kHz and different currents • higher currents expand • the spectrum to higher • frequencies: • stable bunch deformation • unstable bunch bursting normalized by I² Pcoh/Pincoh Fourier transform measurements at the BESSYIRIS infrared beam line, cooperation with Dr. Ulrich Schade (BESSY), Dr. Jongseok Lee (Seoul National University) G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

9. temporal emission spectrum of CSR bursts (user optics) s~I 0.384 s~I 3/8 bunch length / ps single bunch current / mA single bunch current / mA bursting frequency / kHz streak camera data THz data bursting data s~I 3/7 scaling relation between bunch length s, synchrotron frequency f and current I: bursting instability Stupakov & Heifets Bunch length and current relation bunch length - current relation bursting data user optics 13 ps THz optics 3 ps sub-ps optics 700 fs • results from bursting threshold: • eff. / naturale bunch length s/s = 1.5 • eff. bunch length · unstable mode sk =2ps/l =5 • bunch length ~ current relation s~I ª • a=3/8 from experiments, a=3/7 from theory 0 i i G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

10. x’ D’ H 1/2 e 1/2 j j x D x x H Limits of short bunches I linear coupling matrix, long.-horiz. , the chromatic H-function couples the planes: use transformation properties of dispersive vector (D,D’) to replace m16 and m26 x-z transfer matrix, no energy change similar expression for m55 & m56 - terms, if z’ and a are included! G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

11. quadrupoles ~ a sextupoles ~ da/dd octupoles ~d a/dd 2 2 Limits of short bunches II longitudinale bunch length is chromatic H dependent t z-z’ x-x’ z-z’ x-x’ MAD tracking simulation, 8·10 turns = 10damping times & quantum excitation rf-voltage: 5 0.45 MV s=1.0 mm s=1.0 mm port II H=1.2 rad·m port I H=0.035 rad·m 45.0 MV s=0.5 mm s=0.1 mm bunch length / mm bunch length / mm scheme of presently constructed MLS ring of the PTB, next to BESSY site Emax=600 MeV circumference = 48 m low alpha tuning: G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

12. initial value: no spread in phase space, only natural spread in momentum distribution long. bunch length spread of s=0.4 mm 180° in phase long. space 80 turns around machine s-rms=0.4 mm Limits of short bunches III BESSY II, user optics: MAD-simulation of electron diffusion due to radiation damping conclusion: radiation damping limits the multiple usage of - ‘laser sliced’ electrons for short x-rays - ‘laser sliced’dip as a THz-source G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

13. BESSY II 710 mA user optics & upgrade upgrading by a factor of 20 1000 x more THz power! upgrading by a factor of 100 sub-ps bunches! user optics 13 ps present rf 0.5 GHz & 1.5 MV THz optics 3 ps rms-bunch length / ps user optics 1.5 GHz & 50 MV 1.3 ps upgraded rf THz optics 0.3 fs bursting threshold figure copied from F. Sannibale et al, ICFA Newsletter No. 35 single bunch current / mA bursting threshold applied scaling law for bursting threshold: Upgrading of the rf-gradient option for enhanced THz radiation and short X-ray pulses at BESSY II: upgrading the rf-gradient by a 1.5 GHz, cw superconducting rf-structure placed into one straight ID-section G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005

14. Conclusion Conclusion: the low alpha optics (as one possible scheme) extends the usage of storage rings to intense THz and short X-ray pulses coherent THz radiation as a diagnostics tool delivers sensitive and new information on beam dynamics G. Wüstefeld et al., BESSY Low Alpha Optics, Frascati, 7-8 Nov. 2005