Coherent THz radiation at
NewSUBARU

Y. Shoji

NewSUBARU,LASTI,
University of Hyogo

Introduction to NewSUBARU
SPring-8 SR

1 GeV linac

Circumference
Injection Energy
Electron Energy
Type of Bending cell
RF Frequency
Natural EmittanceNatural Energy Spread

118.7 m
1.0 GeV
0.5 - 1.5 GeV
DBA with Inv.B
499.956 MHz
38 nm (1GeV)
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 KlystronR & D (1997)

Special design: Invert BendMomentum compaction factor

Change η in the invert bends--> change α1keeping achromatic condition∆L/L0 ≡ α1δ +α2δ2 + α3δ3 + . . . .
δ≡ (Ε−Ε0)/Ε02

dispersion function (m)34o -8o +34o =60o

α1=0

1.5

α1=0.0014
10.5
Q3 Q40
BEND-0.5-5

Normal Bend

Invert Bend-4-3

Q4 Q3

INV.BEND-2-10
1s (m)

BEND

2345

Control of momentum compaction factor
Momentum compaction factor∆L/L0 ≡ α1δ +α2δ2 + α3δ3 + . . . .
δ≡ (Ε−Ε0)/Ε0α1 =1.3×10-3 → ≈ 0
Q3 & Q4
α2 = 0SF, SD, (SB)α3 ≈ 0.5
no control knob
α4 ≈ -20
no control knob

Types of CSR

Normal radiation

relative radiation power / bunch

101100

CSR burst
normal α1, high peak current

multi-bunch; normal a1
single-bunch; normal a1
multi-bunch; low a1, high Vrf--> WIRMS2007

-1

10-2

Steady state CSR
small α1 (quasi-isochronous ring)
Beam physics ---> Instability threshold
Acc. technology ---> stability problem

1010-310-4
10-5

Linac Pulse CSR --> morning session-61010-7 -410-3

1010-2-1

101001102No laser induced CSR

10stored beam current per bunch (mA)

NewSUBARU has bunch shortening limit
What limit the bunch shortenning?

Bunch shortenning at NewSUBARU
Deviation from the scaling law

σ T −MIN ≈ 1.4ps at α1 ≈ 1×10−5
Instability ? No
no IB dependence at IB<2µA
Problem of monitor ? No
α1 does not reduce σT
VRF does reduce σT

0.24 uA/bunch; 120kV
0.24 uA/bunch; 300kV
1.8 uA/bunch; 120kV
1.8 uA/bunch; 300kV−5bunch length ( σ; ps)σ T ∝ α1 is valid for α1 > 2 ×1010

BESSY & ANKA do not have that problem!

86VRF=120kV

4VRF=300kV

2Measured by a streak camera
000.005

0.01/α10.015

0.02Comparison of NewSUBARU with BESSY

BESSY IINatural Energy Spread
0.08%
Natural Emittance (nm rad) 30π
α1-1.4x10-6
α3-0.01
Damping time
8ms
Lattice
non-achromatic DB

NewSUBARU
0.047%
30π
5x10-60.512ms
DBA+IBRipple & stability of magnets
MagnetPower supplyresolution

Effectrippleresolution

rippleB< 1E-6∆E/E= 1E-6

Inv Bend< 1E-5∆B/B= 1E-6 *Q4

1.5E-5 (16bit) < 2E-6

∆α1= 4E-6*

Sext-F

2.4E-4 (12bit) < 2E-5

∆α2 = 2.6E-3 * ∆α2 = 2E-4

RF with FB< 0.02 deg∆α1 = 6E-7
∆τ = 0.1 ps* Imbalance between B and IB
main power* α1=4X10-6 --> σT=0.6 ps (E0=1GeV, VRF=300 keV)* stability condition; α1 +2α2δ + 3α3δ2>0−−> α1> 4.5 X 10-6

supply270A
Easternarc IB
2.36A

Westernarc IB
2.36A

Effect of IB field ripple -- path-length ripple
Dipole field error produces longitudinal oscillationβ S β (s)
θ S cos[ψ (s) − ψ S − πν ]2sin πνL0 x(s)∆L0 = ∫ 0ds = ηSθ S -->∆Eρ (s)Deflection θS ∆x∆x(s) =∆L∆E
η= − S θSE
αL 0horizontal displacement ( µm )Evidence of path-length ripple (at normal operation)
1.5

60Hz

sine component

1

COD drift with harmonic freq.
(60Hz, 120Hz, 180Hz, . . . .)0.5⎧⎪ β β(s)ηS η(s)⎫⎪Sx(s) = ⎨cos[ψ (s)− ψ S − πν ]−⎬θ S⎪⎩ 2sin πν
αL0 ⎪⎭

0-0.5-1cosine component

-1.5
468

BPM number

1012

agreed with expected COD
for IB ripple ; ∆B/B <4X10-7

Pathlength ripple & RF ripple
Forced Oscillationdτdtpathlength ripple= −α1ε + ∆C e jωt

RF rippledε ω S=(τ + ∆ P e jωt ) − 2α Eεdt α122τ=

(2αE + j ω )∆C − ω S ∆P2

ω S − ω 2 + 2 jωαEj ωteωS 2∆C + jω∆ Pjωtε=() 2eα1 ω S − ω 2 + 2 jωα EPathlength ripple & RF ripple(180 Hz)102(a)

natural spread (σ )relative energy spread

-2

10(b)

natural spread ( σ )

101

-3

10

pathlength
fluctuation ( εl l)

-4

10-5

10-6

10

rf phase-7

10bunch length (ps)

10-1

100
pathlength
1010 -2

fluctuation ( l ε l )10-8 -610

10-5

fluctuation ( lτ l )-1

rf phase fluctuation ( l τ l )10-4-310

momentum compaction factor

10-3

10-6

10-5

10-4

10-3

momentum compaction factor

Measured energy fluctuation (1)20Hz (dBm)
60Hz (dBm)
120Hz (dBm)
180Hz (dBm)
240Hz (dBm)
360Hz (dBm)

-30

2060
120
180240360∆X at BPM8 (dBm)

-40
-50
-60
-70

The oscillation is harmful

-80
-90
0.5

21fs (kHz)345

BESSY & ANKA haveno Invert Bend !

Feed-back control
Energy feed-back

Storage Ring
Single‑PassBPM
CircuitFeed-back OFF ONyBPMxFeed‑BackControllerbi‑polarAC power supply(below; with LPF; x2)
Dipole Magnet

FB works at normal α1but not at small α1 because of non-linearity?Non-symmetry for +∆E and -∆E
The bunch length was not always shortest at where the fS was thesmallest. It strongly depended on δ (or ∆fRF) at small α1 (α1 = 1×10-5).
499955.480 kHz
499955.500 kHz
499955.520 kHz

900

5800fS (kHz)relative strenghth

4
32700
600

+15Hz

500
400

100300-30 -20 -10 0 10 20 30
102030∆fRF (Hz)40time (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 Hz499955550 Hz

0030Hz0.47mA50Hz
0.02mA-20

FFT power (dB)FFT power (dB)

-20

-40

-60

-80

-100-1000

0.47mA50Hz
0.02mA
70Hz

-40

-60

-80

-500

0f-f

RF500(Hz)

1000-100
-1000

-500

0f-f

RF

500

1000(Hz)

Current dependence of fs side-band. fRF peak is normalized to 0dB. White noise issubtracted 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.