Observation of THz CSR
Burst at UVSOR-II
1Miho SHIMADA, 1,2Masahiro KATOH, 1*Akira MOCHIHASHI,
1Shin-ichi KIMURA, 2Masahito HOSAKA, 2Yoshifumi TAKASHIMA,3Toshiharu TAKAHASHI
1UVSOR Institute for Molecular Science, 2Nagoya University,
3Research Reactor Institute, Kyoto University*Present Affiliation, JASRI SPring-8

OUTLINE• Accelerators, Infrared/THz beam line, detectors…
• Observation of THz CSR burst before / after RFcavity upgrade– Intensity of THz CSR burst– Temporal structure of burst train and of single burst– Condition for bursting• Measurement with THz diode detector with hightemporal resolution• Effect of laser bunch slicing on THz CSR burst

UVSOR-II Accelerators•UVSOR promotes a research ofTHz CSR. There are two topics.
– THz CSR burst due to the beam
instability(This talk)– Laser bunch slicing(by Mochihashi in the next session.)

Specific at the THz CSR burst experiments
Electron Energy
600 MeV
Circumference
53.2 mNatural Emittance
17.5 nm-rad
Natural Energy Spread
3.4 x 10-4
Natural Bunch Length (rms)~105.2 ps
RF Frequency
90.1 MHz
Synchrotron Frequency14.4 kHz
Momentum Compaction Factor
0.028
Damping time
20 ms
(before upgrade of RF cavity at 2005)

Beamline for THz

BL6B

UVSOR-BL6B IR Beamline
S. Kimura et al., AIP Conf. Proc. 705 (2003)

Solid Angle• BL6B• BL6U

Bending #6
Magic mirror

BL6U

215 ×80 mrad2
7×7 mrad2

To BL6U
Diamond /z-cut
Quartzwindow

JASCO THz microscope
FARIS-1••

Bruker
IFS66v/S

Refectionabsorption
spectroscopy
0500 1000
stationThe large solid angle of BL6B
is realized by a magic mirror.
BL6U is a monitor beamline.

THz detectors••

Hot electron bolometer (InSb)

THz diode detector(Schottky)

Time resolution

1.6 µs ( ~ 10 revolution of UVSOR-II)~ 100 psSpectral range

3cm-1～50cm-1(including the effect of the beamline
optics, BL6B)

7.3cm-1～11cm-1

Sensitivity

5×105 V/W

~3×103 V/W

Operation temperature

4K (cooled by Liq. He)

Room temperature

THz diode detector has a hightime resolution and works atthe room temperature.
The sensitivity of thebolometer is much higher
than THz diode detector

Upgrade of RF Cavity (2005)
After upgrade of RFcavity, the characters ofthe THz CSR burst haschanged.

Old Cavity

New Cavity• Threshold• time structure of burst
train and the single pulseetc…

Before Upgrade

After Upgrade

RF Frequency

90.1 MHz90.1 MHzRF Voltage

55 kV

150~200 kV

Synchrotron Frequency

14.4 kHz19.4 kHzRMS Bunch Length105.2 ps

78.1 ps

8000 (unloaded)

23800 (unloaded)

500 kΩ

2.9 MΩ

Quality Factor
Shunt-impedanceOUTLINE• Accelerators, Infrared/THz beam line, detectors…
• Observation of THz CSR burst before / after RFcavity upgrade– Intensity of THz CSR burst– Temporal structure of burst train and single pulse ofbursting– Condition for bursting• Measurement with THz diode detector with hightemporal resolution• Effect of laser bunch slicing on THz CSR burst

Before RF cavity upgradeMeasurement ofintensity of THz burst
Y. Takashima et al. JJAP 44(35), 2005, 1131

•Average intensity is measured byusing lock-in-amp and mechanicalchopper with 100 Hz.
Multi-bunch mode is proportional
to the beam current.

Lock-in-Amp. Output (a.u.)•040628 UVSOR-II

10

Multi-bunch
Single-bunch1• There is two current regions
30 40 50 60 70 8090100
200of THz burst, 80 mA andBeam Current (mA)
140 ~ mA.
• Peak intensity of burst is 105
times larger than normal SR. Average intensity of terahertz radiation(raw data does not show here)with a multi-bunch and single-bunchmodeTime structure of terahertz burst train
Y. Takashima et al. JJAP 44(35), 2005, 1131•••

Intensity of the burst CSR isstronger at the higher beam current.
The burst appear chaotically at
extremely high beam current of 206mA while quasi-periodically at thelower current.
The repetition of the burst at 80 mA
is lower than at the current over thesecond threshold.

Lock-in-Amp. Output (a.u.)

040628 UVSOR-II

10

Multi-bunch
Single-bunch~ 71 Hz130

40 50 60 70 8090100

200

Beam Current (mA)

~ 11 Hz~ 90 HzTime structure of a single pulse of the burst
Y. Takashima et al. JJAP 44(35), 2005, 1131

VRF=55 kV, fs = 14.4 kHz

period ~ 33 kHz~ 2 fs

VRF=28 kV, fs = 10.3 kHz

period ~ 22 kHz~ 2 fs••Time period in the single burst pulse
with VRF of 55 kV is near to thetwice of the synchrotron frequency
fs, 14.4 kHz.
When VRF is 28 kV, fs changes into
10.3 kHz, which is near to half of 22kHz.
Synchrotron frequency seems
to have strong relationship withburst.

Relationship between THz Bursts andrms bunch length
The beam current is 177 mA• THz Bursts signals detected by theInSb bolometer• Rms bunch length (white line) andthe THz CSR burst simultaneously
measured by a streak camera(Color image is the raw data of thestreak camera. )The variation of the rms
bunch length is associatedwith the burst occurrence.

Response of a streak camera
synchronized with the signal of thebolometer.

After RF cavity upgradeEffect of beam instabilityon the THz CSR burst• After upgrade of RF cavity, THz CSR burst occurs whenvertical beam instability is monitored.
Unstable electron beam

Horizontal

Stable electron beam

Vertical••The direction of the monitor camera is rotated at 90 degree.
Beam instability to the vertical direction is observed with highcurrent or high RF voltage.

Intensity of THz CSR burst• The intensity of THz CSR burst increase at the higher
RF voltage• High RF voltage is assumed to induce transverse beam
instability and THz CSR burst occurs.

RF voltage 57.9 kVCurrent 65.7mA

RF voltage 89.5 kVCurrent 65.5mA

RF voltage 121 kVCurrent 65.0mA

Time structure of theburst train with various
RF voltage••

The burst occurs periodically when
the RF voltage is low.
When the RF voltage is high, theburst train turn into chaotic.

50 mV65.5mA RF Voltage 89.5 kVThese features are almost thesame with before RF upgrade.
But…

•The single pulse of the burst does notcontain the structure with synchrotron
frequency.

50 mV
65.6mA RF Voltage 78.9 kV

250 mV65.2mA RF Voltage 111 kV

250 mV65.0mA RF Voltage 121 kV

OUTLINE• Accelerators, Infrared/THz beam line, detectors…
• Observation of THz CSR burst before / after RF
cavity upgrade– Intensity of THz CSR burst– Temporal structure of burst train and single pulse ofbursting– Condition for bursting• Measurement with THz diode detector with hightemporal resolution• Effect of laser bunch slicing on THz CSR burst

Response of the bolometer and VDI terahertz
diode detector••

The envelope of the time development of the diode detector areconsistent with the bolometer.
Temporal resolution of VDI detector is shorter than revolution
frequency of 5.6 MHz (170 ns).

Time structure of the THz CSR burst measured
by bolometer and THz diode detector

Scale 20µsec•

Scale 4µsec

The rapid time development can beobserved by the THz diode detector.
At the left image, we can see a sort oftime structure, which could not be
resolved by the bolometer.

Scale 1µsec

CloseUP– Blue ： Bolometer– White ： THz diode detectorOUTLINE• Accelerators, Infrared/THz beam line, detectors…
• Observation of THz CSR burst before / after RFcavity upgrade– Intensity of THz CSR burst– Temporal structure of burst train and single pulse ofbursting– Condition for bursting• Measurement with THz diode detector with hightemporal resolution• Effect of laser bunch slicing on THz CSR burst

Does laser bunch slicing induce bursting ?

Laser bunch slicing system at UVSOR-II

Laser Parameters
Pulse Energy
2.5 mJ/pulse
Pulse Width
130 fs ~2ps
Rep. Rate
1 kHz(synchronized with 90.1 MHz)
Wavelength
790 – 810 nm

Laser bunch slicing with high current• Laser bunch slicing is performed while bursting.

•At UVSOR-II, the laser bunch slicing does not induce burst interahertz region unlike other storage ring.

Conclusion• 2004-2005– We observed two thresholds for the THz CSR bursts.
– Time structure is strongly associated with the synchrotronfrequency.
– Time trains of the burst is quasi-periodic at the lower beam
current but turn into chaotic at the higher beam current– Enhance of the intensity of the THz CSR is strong at the lowerTHz frequency (<10 cm-1)• 2006-2007– THz CSR burst is associated with the vertical beam instability
and RF cavity after the upgrade of RF cavity.
– There is possibility to observe the rapid time structure of theburst by the THz diode detector.
– Laser bunch slicing does not induce the THz CSR burst.

Laser bunch slicing and observation ofterahertz burst (preliminary)••

45.2mA Bunch Slicing

46.7mA Burst

Some example of the bolometer signals in thesame current region. In some cases, there isno burst but signal of laser bunch slicing. Inthe other cases, we observe only THz CSR
burst with around 8 kHz.
At 44.9 mA, THz CSR of both burst and laser
bunch slicing are observed. Repetition of theburst pulse of 8 kHz does not change. Thelaser bunch slicing decrease just after bursting
because of the beam instability.

44.9mA Burst & Bunch Slicing

Intensity of THz CSR burst with low-pass filter
Y. Takashima et al. JJAP 44(35), 2005, 1131

The intensity of burst is expressed as follows,Pburst (k ) = N [1 + ( N − 1) F (k )] p (k )F (k ) = ∫ Q( z )e dz
ikz

Average burst intensity normalized
by incoherent SR (130 mA)

2Q(z) : longitudinal electron density distribution
F(k) : Form factor(Square of Fourier Transformation of Q(z)）

The ratio of the average burst intensity to
the incoherent SR with a current of 130mA is plotted.

Ratio of Intensity ≡ Pburst ( I ) / P (130 mA)•••“Ratio of Intensity” is larger with lower
THz frequency (less than 10 cm-1).
There is no significant difference of thecurrent dependency in terms ofwavelength of the burst CSR.
Average intensity is measured by using
lock-in-amp and mechanical chopperwith 11 Hz.

Normalized at the 130 mA, whereTHz CSR burst does not appear