Coherent THz radiation at NewSUBARU NewSUBARU, LASTI, Y. Shoji - PowerPoint PPT Presentation

Coherent THz radiation at NewSUBARU NewSUBARU, LASTI, Y. Shoji University of Hyogo

Introduction to NewSUBARU SPring-8 SR 1 GeV linac Circumference 118.7 m Injection Energy 1.0 GeV Electron Energy 0.5 - 1.5 GeV Type of Bending cell DBA with Inv.B RF Frequency 499.956 MHz Natural Emittance 38 nm (1GeV) Natural Energy Spread 0.047% (1GeV) New Material(1998) Photo-chemical(1998) NewSUBARU Inteferometry(1998) Microscope(2001) Long Undulator Short Undulator LIGA(1997) EUVL(1997) LIGA(1997) LIGA(2004) Optical Klystron R & D (1997)

Special design: Invert Bend Momentum compaction factor Change η in the invert bends --> change α 1 ∆ L / L 0 ≡ α 1 δ + α 2 δ 2 + α 3 δ 3 + . . . . keeping achromatic condition δ ≡ ( Ε−Ε 0 )/Ε 0 34 o -8 o +34 o =60 o 2 α 1 =0 1.5 dispersion function (m) α 1 =0.0014 1 0.5 Q3 Q4 Q3 Q4 0 BEND INV.BEND BEND -0.5 -5 -4 -3 -2 -1 0 1 2 3 4 5 s (m) Normal Bend Invert Bend

Control of momentum compaction factor Momentum compaction factor ∆ L / L 0 ≡ α 1 δ + α 2 δ 2 + α 3 δ 3 + . . . . δ ≡ ( Ε−Ε 0 )/Ε 0 α 1 =1.3 × 10 -3 → ≈ 0 Q3 & Q4 α 2 = 0 SF, SD, (SB) α 3 ≈ 0.5 no control knob α 4 ≈ -20 no control knob

Types of CSR Normal radiation 10 1 CSR burst multi-bunch; normal a1 normal α 1 , high peak current single-bunch; normal a1 10 0 relative radiation power / bunch multi-bunch; low a1, high Vrf --> WIRMS2007 10 -1 10 -2 Steady state CSR small α 1 (quasi-isochronous ring) 10 -3 Beam physics ---> Instability threshold 10 -4 Acc. technology ---> stability problem 10 -5 Linac Pulse CSR --> morning session 10 -6 No laser induced CSR 10 -7 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2 stored beam current per bunch (mA) NewSUBARU has bunch shortening limit What limit the bunch shortenning?

Bunch shortenning at NewSUBARU Deviation from the scaling law 10 0.24 uA/bunch; 120kV σ T ∝ α 1 is valid for α 1 > 2 × 10 − 5 0.24 uA/bunch; 300kV 1.8 uA/bunch; 120kV 8 bunch length ( σ ; ps) 1.8 uA/bunch; 300kV σ T − MIN ≈ 1.4ps at α 1 ≈ 1 × 10 − 5 6 Instability ? No V RF =120kV no I B dependence at I B <2 µ A 4 V RF =300kV 2 Problem of monitor ? No Measured by a streak camera α 1 does not reduce σ T 0 V RF does reduce σ T 0 0.005 0.01 0.015 0.02 /α 1 BESSY & ANKA do not have that problem!

Comparison of NewSUBARU with BESSY BESSY II NewSUBARU Natural Energy Spread 0.08% 0.047% 30 π 30 π Natural Emittance (nm rad) α 1 -1.4x10 -6 5x10 -6 α 3 -0.01 0.5 Damping time 8ms 12ms Lattice non-achromatic DB DBA+IB

Ripple & stability of magnets Magnet Power supply Effect resolution ripple resolution ripple ∆ E/E = 1E-6 B < 1E-6 ∆ B/B = 1E-6 * Inv Bend < 1E-5 Q4 1.5E-5 (16bit) < 2E-6 ∆α 1 = 4E-6* ∆α 1 = 6E-7 ∆α 2 = 2.6E-3 * ∆α 2 = 2E-4 Sext-F 2.4E-4 (12bit) < 2E-5 ∆τ = 0.1 ps RF with FB < 0.02 deg * Imbalance between B and IB main power supply * α 1 =4X10 -6 --> σ T =0.6 ps ( E 0 =1GeV, V RF =300 keV) 270A Eastern Western arc IB arc IB * stability condition; α 1 +2 α 2 δ + 3 α 3 δ 2 >0 2.36A 2.36A −−> α 1 > 4.5 X 10 -6

Effect of IB field ripple -- path-length ripple Dipole field error produces longitudinal oscillation β S β ( s ) [ ] Deflection θ S ∆ x ∆ x ( s ) = 2sin πν θ S cos ψ ( s ) − ψ S − πν ∆ E E = − η S x ( s ) ∫ L 0 ∆ L 0 = = η S θ S θ S ρ ( s ) ds ∆ L --> ∆ E α L 0 0 Evidence of path-length ripple (at normal operation) 1.5 COD drift with harmonic freq. horizontal displacement ( µ m ) 60Hz sine component 1 (60Hz, 120Hz, 180Hz, . . . .) 0.5 ⎧ ⎫ ⎪ ⎪ β S β ( s ) ] − η S η ( s ) 0 ⎨ ⎬ [ x ( s ) = 2sin πν cos ψ ( s ) − ψ S − πν θ S ⎪ ⎪ ⎩ α L 0 ⎭ -0.5 -1 agreed with expected COD cosine component -1.5 for IB ripple ; ∆ B/B <4X10 -7 4 6 8 10 12 BPM number

Pathlength ripple & RF ripple Forced Oscillation pathlength ripple d τ dt = − α 1 ε + ∆ C e j ω t RF ripple 2 d ε dt = ω S ( τ + ∆ P e j ω t ) − 2 α E ε α 1 2 ∆ P τ = (2 α E + j ω ) ∆ C − ω S j ω t e 2 − ω 2 + 2 j ωα E ω S 2 ε = ( ω S ∆ C + j ω∆ P e j ω t ) 2 − ω 2 + 2 j ωα E α 1 ω S

Pathlength ripple & RF ripple (180 Hz) 10 -1 10 2 (a) (b) natural spread ( σ ) natural spread ( σ ) 10 -2 relative energy spread 10 1 bunch length (ps) 10 -3 pathlength 10 0 10 -4 ε l ) fluctuation ( l pathlength 10 -5 τ l ) fluctuation ( l 10 -1 10 -6 10 -2 rf phase 10 -7 fluctuation ( l ε l ) rf phase fluctuation ( l τ l ) 10 -8 10 -3 10 -6 10 -5 10 -4 10 -3 10 -6 10 -5 10 -4 10 -3 momentum compaction factor momentum compaction factor

Measured energy fluctuation (1) 20Hz (dBm) 20 60Hz (dBm) 60 120Hz (dBm) 120 180Hz (dBm) 180 -30 240Hz (dBm) 240 360Hz (dBm) 360 -40 ∆ X at BPM8 (dBm) -50 -60 -70 The oscillation is harmful -80 BESSY & ANKA have -90 2 3 4 5 0.5 1 no Invert Bend ! fs (kHz)

Feed-back control Energy feed-back S t o r a g e R i n g S i n g l e - P a s s B P M Feed-back OFF ON C i r c u i t y B P M x F e e d - B a c k C o n t r o l l e r b i - p o l a r A C p o w e r s u p p l y (below; with LPF; x2) D i p o l e M a g n e t FB works at normal α 1 but not at small α 1 because of non-linearity?

Non-symmetry for + ∆ E and - ∆ E The bunch length was not always shortest at where the f S was the smallest. It strongly depended on δ (or ∆ f RF ) at small α 1 ( α 1 = 1 × 10 -5 ). 499955.480 kHz 499955.500 kHz 499955.520 kHz 900 5 800 4 700 f S (kHz) relative strenghth +15Hz 3 600 500 2 400 1 300 -30 -20-10 0 10 20 30 0 ∆ f RF (Hz) 0 10 20 30 40 time (ps) bunch shape. FFT spectrum of the beam signal Synchrotron Oscillation is enhanced at δ >0

Non-symmetry for + ∆ E and - ∆ E Non-symmetry has stored current dependence 499955530 Hz 499955550 Hz 0 0 30Hz 0.47mA 0.47mA 50Hz 50Hz 0.02mA 0.02mA 70Hz -20 -20 FFT power (dB) FFT power (dB) -40 -40 -60 -60 -80 -80 -100 -100 -1000 -500 0 500 1000 -1000 -500 0 500 1000 f - f RF (Hz) f - f RF (Hz) Current dependence of fs side-band. f RF peak is normalized to 0dB. White noise is subtracted from the data at 0.02mA

Conclusion ・ Some problems in QI operation are enhanced at NewSUBARU probably because of the Invert Bend. ・ Ring stability is a main technical point of QI operation ・ Higher order α would be the other problem ・ Still there is a phenomenon not understood.