The BESSY (and MLS) 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 see contribution in ICFA Beam Dynamics Newsletter No. 35, December 2004 G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007
abstract & content Abstract The BESSY II optics is tuned to a low alpha mode for bunch length shortening. About 1mm short bunches emit coherent synchrotron radiation in the THz range. Details of the machine optics and measured THz signals are discussed. Plans for the presently commissioned MLS ring * for short bunch generation are presented. Content 1. Low alpha optics 2. Coherent radiation 3. Bunch-length current relation 4. Limits of short bunches 5. Upgrading idea: short bunches at BESSY II * thanks to the PTB and BESSY commissioning team G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007
BESSY II and MLS storage rings Berlin-Adlershof (south east of Berlin) BESSY II foot print Europe's most modern Technology Park The BESSY II ring: energy 1.7 GeV circumference 240 m number of cells 16 / DBA rf frequency 500 MHz MLS 76 m new : Metrology light source MLS the MLS ring: energy 0.2 – 0.6 GeV circumference 48 m Scientific employes: 6500 (+ 6500 students) number of cells 4/ DBA New media: 1500 employes rf frequency 500 MHz 16 m others: 4300 empoyes G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007
the BESSY Low Alpha Optics Low alpha optics for bunch length manipulation the machine optics tune parameters optics parameter reg.user low alpha optics optics tunes Qx / Qy 17.8 / 6.7 14.7 / 6.2 nat. chrom ξ x / ξ y -53 / -27 -35 / -27 Q D S - 4 sextuple families for beam dynamics corrections - single & multi bunch 1.25 MHz to 500 MHz rep. rate current per bunch 10 μ A < I< 0.1 mA - very stable machine operation, good life time 20 mA and 20 hours Q D S G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007
longitudinal chromaticity condition for stable beam operation: α ≠ 0 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) fs=350Hz α extracted momentum compaction factor α = α + α δ + α δ - fit to measured data 2 0 1 2 − δ rms α = − ⋅ α = α = − 6 =0.1% 3 10 , 0 , 0 . 03 -6 α =-3·10 0 1 2 See also: Control of the bunch length on an electron storage ring H. Hama, S. Takano and G. Isoyama, NIM A329 (1993) G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007
low alpha tuning for the MLS low alpha tuning (simulation) for the presently commissioned MLS ring M etrology L ight S ource ( MLS ) of the Physikalisch-Technische Bundesanstalt (PTB), next to the BESSY II site, expected values: User optics α =0.02 , bunch length σ = 4.5 mm at 600 MeV THz optics α =0.001, bunch length σ = 1.0 mm at 600 MeV applied rf: 500 kV, 500 MHz α -value / 0.01 2 chromatic sextupoles dipole 3 sextupole, one octupole quadrupole (curvature of α ) sextupole octupole 3 chromatic sextupoles circumference / m 48 (slope of α ) energy / MeV 200 - 600 100 MeV microtron momentum deviation / % scheme of MLS ring non. lin. low alpha tuning see also R. Müller et al., Infrared Phys. Technol. 49 (2006) 161 G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007
THz detectors at BESSY CSR signals & fast THz detectors InSb-bolometer H.-W. Hübers et al., hot-electron bolometer HEB bursting CSR CSR intensity ICFA Newsletter No. 35 H.-W. Hübers et al. bunch revolution Applied Phys. Lett. 1 turn 800 ns 87 ,184103 (2005) stable CSR CSR intensity temporal resolved THz pulse of resolution of single bunches, pulse length of few 100 ps: detector τ =30 p s 1 turn multiple reflections in THz beam line time Detector parameters resolution of single turns, detector τ =1 μ s Typical values Si-Bol. InSb HEB NEP (W/Hz 1/2 ) ~10 -13 ~10 -12 ~10 -10 time τ Rise (ns) ~10 6 ~1000 ~0.03 first strong CSR signals Frequency (THz) 0.1 - 15 0.1 -1.5 0.3 - 6 T. Nakazato et al.,Phys. Rev. Lett. 63, 1245 (1989) G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007
spectral range of BESSY CSR from Fourier spectra to power spectra interferogram raw data power spectrum source comparison 0.1THz 1THz 1 10 2 /sr/(0.1% bdw)] incoherent radiation user optics burstig 250 mA, user optics CSR, SB 15 mA 7 10 gain -4 10 7 BRILLIANCE [W/mm 10 gain -9 10 low alpha stable black body, 1200 K, 10 mm^2 CSR, 18 mA -14 10 1 10 100 1000 10000 -1 ] WAVENUMBER [cm user optics, single bunch 15 mA brilliance of the BESSY THz spectrum power spectrum analysis by Fourier transform spectrometer in cooperation with Dr. U. Schade, BESSY BESSY offers 4 low alpha shifts of 3 days / year application: coherent THz radiation, ICFA No. 35, article by U. Schade et al. short x-ray pulses at BESSY, PRL 95 , A. Krasyuk et al., 2005 G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007
single bunch CSR-intensity I transition from stable to bursting CSR, user optics CSR from deformed, but temporal emission spectrum stable bunch, fs=7.2 kHz of CSR bursts (user optics) single bunch current / mA start of bursting rel. THz power signal measured CSR signal parabolic fit back ground level 1.6 1.7 1.8 bursting frequency / kHz single bunch current in mA at spectrumsanalyzer G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007
single bunch CSR-intensity II spectrums analyzer records, centered around 1.25 MHz rev. frequency bunch current / mA bunch current / mA fs=7.2 kHz fs=5 kHz Current and temporal 1.25 MHz 1.30 MHz 1.20 MHz 1.20 MHz 1.25 MHz 1.30 MHz emission dependencies of CSR radiation at frequency / MHz frequency / MHz different settings of the bunch current / mA bunch current / mA low alpha parameter fs fs=3 kHz fs=2.1 kHz 1.20 MHz 1.25 MHz 1.30 MHz 1.20 MHz 1.25 MHz 1.30 MHz frequency / MHz frequency / MHz frequency / MHz G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007
bunch length and current relation bunch length - current relation bursting data user optics σ ~ I 0.384 bunch length / ps 13 ps σ ~ I 3/8 THz optics 3 ps σ ~ I 3/7 sub-ps optics single bunch current / mA 700 fs streak camera data THz data bursting instability results at bursting threshold: bursting data Stupakov & Heifets bunch length σ/σ - eff. / naturale = 1.5 0 unstable mode σ k =2πσ/λ =5 - eff. bunch length · i i empirical scaling relation between bunch - bunch length ~ current relation σ ~ I ª length σ, synchrotron frequency f and current I : a=3/8 from experiments, a=3/7 from theory σ σ = + 4 4 3 / 2 ( / ) ( f / f ) ( I / I ) 0 0 0 G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007
limits of short bunches I longitudinal-horizontal couplings effect in the MLS ring longitudinale bunch length is chromatic H dependent. t z-z’ x-x’ z-z’ x-x’ MAD tracking simulation, 5 rf-voltage: 8·10 turns = 10 damping times & quantum excitation 0.45 MV 45.0 MV σ = 1.0 mm σ = 1.0 mm σ = 0.5 mm σ = 0.1 mm H =1.2 rad·m H =0.035 rad·m bunch length / mm bunch length / mm ultra short bunches are only E max =600 MeV possible at small H locations circumference = 48 m see also : Y. Shoji in Phys. Rev. ST Accel. Beams 8 094001 (2005) scheme of MLS ring Phys. Rev. ST Accel. Beams 7 090703 (2004) G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007
limits of short bunches II BESSY II, user optics: MAD-simulation of electron diffusion due to radiation damping initial value: no spread in 180° in phase long. space long. bunch length phase space, only natural spread of σ =0.4 mm 80 turns around machine spread in momentum distribution conclusion: radiation damping limits the multiple usage of - ‘laser sliced’ electrons for short x-rays - ‘laser sliced’ dip as a THz-source ↓ ↓ bunch rotation bunch rotation 180° 9 90 degree quantum excitation ‰ 8 σ -rms=0.4 mm=1.2 ps . u n . o a 7 i t / a i s v n e 6 o d r t m c e 5 u l e t n f e o 4 m r o e b m 3 m e u v n i t 2 a l e r 1 -5 -3.3 -1.6 0 1.6 3.3 5.0 ↓ longitudinal delay / ps ↓ longitudinal delay / ps G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007
upgrading of the rf-gradient idea of enhanced THz radiation and short X-ray pulses at BESSY II rf-upgrading: 1.5 GHz & 50 MV sub-ps bunches! 0.5 GHz & 1.5 MV 13 ps applying the scaling law for bursting threshold: user optics rms-bunch length / ps ∝ σ 8 / 3 present rf I dV / dz z rf 1.5 GHz & 50 MV 3 ps THz optics for upgrading the rf-gradient by a user optics 1.3 ps 1.5 GHz, cw superconducting rf-structure upgraded rf placed into one straight ID-section 0.3 ps THz optics bursting threshold bursting threshold single bunch current / mA G. Wüstefeld et al., BESSY Low Alpha & CSR, UVSOR, Japan, 2007
conclusion Conclusion: the low alpha optics is a scheme to extends the photon spectrum 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 & CSR, UVSOR, Japan, 2007
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