Coherent Synchrotron Radiation and Short Bunches in Electron Storage Rings. G. Wüstefeld, BESSY, Berlin (Germany)

Transcription

1 Coherent Synchrotron Radiation and Short Bunches in Electron Storage Rings G. Wüstefeld, BESSY, Berlin (Germany)

2 Content content 1. Introduction 2. Low alpha optics for short bunches 3. Coherent radiation 4. More on short bunches

3 Motivation and introduction two example electron rings: BESSY II MLS E = 1.7 GeV 2πR = 240 m 16 cell DBA E = 0.1 GeV to 0.63 GeV 2πR = 48 m 4 cell DBA just started operation MLS= Metrology Light Source, owned by German PTB

4 Motivation and introduction superposition of radiation incoherent emission coherent emission long bunch σ >λ * short bunch σ <λ Power P Power P P ~ p N λ P ~ p N 2 λ number of electrons N number of electrons N * σ always rms!

5 Motivation and introduction superposition of radiation log (photon flux) N detector window coherent SR incoherent SR chamber cut off log (photon energy) cut off wave length λ co=2h h/ρ bunch form factor 2πiz/λ g = n(z)e dz λ 2 Power(λ) = p (N+ N g ) λ 2 λ g λ depends sensitive on the bunch density distribution n(z) total / incoherent power = Power(λ) / (p N) = 1 + N g, N 10 8 λ λ

6 Motivation and introduction Scheme of CSR-bunch interaction phase space distribution CSR from the tail of the bunch affects the head of the bunch p p p CSR simple interaction model bunch-csr on curved orbit charge density q Τ 1.2 Τ q q Τ 3.2 Τ q q Τ 9.6 Τ q M. Venturini & R. Warnock et al. bunch density distribution CSR power bunch shape Gaussian stable deformed unstable, bursting

7 BESSY II: Low alpha optics BESSY II low alpha optics the machine optics relation: (f, Δrf) (α, Δp/p) s 2 α ~ f and σ ~ f 0 s s BESSY II main parameters α = <D / ρ> 0 optics parameter reg.user optics nat. emitt / nmrad 6 30 low alpha optics synchr. freq. / khz mom. com. factor α 7.2E-4 7.2E-4 4E-5 1.6E-6 nat. bunch length rms /ps low alpha at fs=1.75khz : very stable machine operation, good life time 20 ma and 20 hours

8 Low alpha optics Tuning of non. lin. synchrotron frequency & α expansion of α: 2 α=α 0 + α Δp/p + α Δp/p. 1 2 tuning of α quad α 0 sext α 1 octu α 2 BESSY II, 1.7GeV 4 chrom. sextupole families, limited flexibility α = 4E-5 0 α > 0 0 α 1 = 0 α < 0 2 MLS, 630 MeV 3 chrom. sextupole families, & octupole family α = 15E-5 0 α > 0 0 α 1 = 0 α > 0 2 synchrotron frequency fs as a function of Δrf frequency

9 MLS: Low alpha optics Observation of Simultaneous Alpha Buckets fixed points: sinϕ =0, α Δp/p =0 ^= f s =0 syn. frequency / khz rf detuning / khz quadrupole tuning MLS measurement: MLS

10 MLS: Coherent radiation IR beam line at MLS acceptance 64x43 mrad 2 THz detector THz beam spot Si-bolometer rise time ~1 ms frequency THz -13 NEP ~10 W/ Hz InSb-detector rise time ~1 μs frequency THz -12 NEP ~10 W/ Hz

11 MLS: Coherent radiation Coherent radiation detector signal / a.u. THz detector signal versus ring current multi bunch current / ma THz signal versus beam energy MeV, 55mA at 250 kv THz detector voltage / a.u. natural bunch length / mm Low alpha, 630 MeV: the THz signal growth stronger than the ring current, a clear indication of coherent radiation beam energy / MeV less THz power than expected - intra beam scattering - ion trapping - CSR beam excitation, slow damping

12 MLS: Coherent radiation chopped THz signals at MLS signals signal voltage cw THz signal chopper ac THz signal THz detector ac-coupled oscilloscope MLS measurement: THz power, 30 ma by low alpha tuning MLS

13 MLS: Coherent radiation Stable THz Signals at MLS E=630MeV, I=19mA, HV=200 kv, fs=10khz, InSb-detector 100mV/div 100mV/div 50ms/div THz signal, chopper=on 10ms/div THz signal, chopper=off signal amplitude is constant & stable

14 N e BESSY II: Coherent radiation power spectrum analysis power spectra N e form factor power spectra by Fourier transform spectroscopy detector signal / a.u. Pcoherent / Pincoherent gain 10 gain wave number cm wave number cm -1 BESSY II user optics, single bunch 15 ma

15 BESSY II: Coherent radiation power spectrum analysis brilliance of the BESSY II THz spectrum N form factor, sub-ps bunches e Pcoh/Pincoh fs, 140 na fs, 300 na ps, 140 na - Gaussian fit wave number / cm sub-ps beam diagnostics at low currents -1 BRILLIANCE W/mm/sr/(0.1% bdw) THz user optics burstig CSR, SB 15 ma low alpha stable CSR, 18 ma 1THz incoherent radiation 250 ma, user optics black body, 1200 K, 10 mm^ wave number cm -1

16 BESSY II: More on short bunches bursting threshold current dependent bursting in time domain / user optics bunch length - current scaling single bunch current / ma bursting frequency / khz bunch length / ps (rms) 10 1 bursting instability Stupakov & Heifets σ~i 3/ single bunch current / ma user optics 13 ps THz optics 3 ps sub-ps optics 700 fs streak camera data bursting data THz data, FT

17 BESSY II: More on short bunches option for short bunches & more currents upgraded rf-gradient 1.5 GHz, 50MV, superconducting rf-structure rms bunch length / ps 100 x more current 1 ps 40 ma 500 MHz multi bunch rf-module in one of the ID-straights bunch current / ma single bunch current / ma bunch length-current diagram

18 More on short bunches Limits of ultra short bunches: small / low energy rings - ion trapping - slow damping of CSR / impedance heating intra beam scattering - power supply noise - coupling of long. - trans. planes - quantum emission

19 Conclusion Conclusion: the low alpha optics extends the usage of storage rings to intense THz and short, Pico second X-ray pulses coherent THz radiation as a diagnostics tool delivers sensitive and new information on beam dynamics presently achieved results without any larger hardware investment

20 Acknowledgment Acknowledgment Thanks to all cooperators on this subject, in particular to the MLS team for making recent, unpublished results available the MLS & BESSY colleagues for fruitful cooperation and discussion and many results presented here: M. Abo-Bakr, J. Feikes, K. Holldack, M. v Hartrott, P. Kuske, U. Schade A. Hoehl, R. Klein, R. Müller, G. Ulm (PTB) H.-W. Hübers (DLR) and colleagues from ALS, ANKA, BNL, DAΦNE, DESY, KSR, NewSUBARU, SLAC, UVSOR