PULSE LASER WIRE Laser pulse storage in an optical cavity as a beam monitor & an X-ray source Kaori Takezawa Kyoto Univ. 2nd Mini-Workshop on Nano Project at ATF 12 th Dec. / 2004 Kaori Takezawa Kyoto Univ. 2nd Mini-Workshop on Nano Project at ATF 12 th Dec. / 2004

1.OVERVIEW 2.PRINCIPLE 3.OPTICAL CAVITY 4.EXPERIMENTAL SETUP 5.CONCLUSION 6.FUTURE PLAN 1.OVERVIEW 2.PRINCIPLE 3.OPTICAL CAVITY 4.EXPERIMENTAL SETUP 5.CONCLUSION 6.FUTURE PLAN 2 2

1.OVERVIEW 3 3 Pulse Laser Wire (Storage laser pulses in optical cavity ) : The systems for New X-ray source & New bunch length monitor at a storage ring

1.OVERVIEW MHz Cavity Electron repetition rate : 357MHz Electron bunches Laser Repetition rate : 357MHz laser pulses Scattered Gamma beam Compton Scattering in every 357MHz As an X-ray source : An optical cavity store higher peak power and get higher flux X-ray with pulse CW laser than CW laser. As Beam monitor : By scanning the laser pulse ’ s phase in the cavity and measuring the Compton signal count rate ; an electron bunch length profile is obtained. Phase Scan

2.PRINCIPLE : Storage of laser pulse 4 4 Resonance condition : Perfect resonance : L = L Not resonance : L ≠ L Imperfect Resonance : L ～ L laser cavity The relationship with laser and cavity : The ext. cavity ’ s parameters ; enhancement factor, reflectivity, …. is the function of Δl and laser pulse width.

2.PRINCIPLE : Enhancement factor 5 5 The reflectivity is bigger at resonance point with Δl ≠0,.this means that enhancement factor is smaller. And data says laser pulse width.

2.PRINCIPLE :Count rate& Measurement 6 6 σ = σ +σ +σ +σ ～ σ Laser pulse width - Laser beamwai st e bunch length e h-beamsize Phase σ = σ +σ Vertical position 2 22 Laser beamwai st e v-beamwaist - Signal flux Laser pulse width 2 The electron bunch length is 20 ～ 40 psec (10mm) > Laser pulse width ( FWHM =7 psec ; 1 mm) >> Laserwire beamwaist( 120um ), electron ’ s horizontal beamsize ( 100um ) Suppose both electron bunch and laser pulses have a Gaussian intensity distribution, The measured profile is also a Gaussian shape.

3.OPTICAL CAVITY 7 7

3.OPTICAL CAVITY : The design 8 8 Cavity length : 714 MHz +/- 2 kHz ( from PZT dynamic range ) Mirrors The radius of curvature : 250 mm The reflectivity : / Beamwaist > 200 um Cavity length is 210mm. It is easy to adjust cavity length with short cavity. For cavity ’ s dynamic range, long PZT is used ( 10um ). Finesse is ～ But Maximum finesse is ～ 500,when the cavity, the length is 714MHz, stores laser pulses, which repetition rate is 357MHz. For enhanced laser pulses in the cavity, Next pulse come after 4times reflection It is difficult to make thin laserwires at long cavity length. Adjustment with PZT 714MHz ρ= 250mm R = 0.997

3.OPTICAL CAVITY : feedback circuit 9 9 Transmission Mode locked Laser Laser Rep.rate feedback Signal Generator Ring RF standard 10MHz 357MHz PZT voltage PI circuit DC Shoulder feedback system ( OFF : background) By a phase detector, the signal is synchronized with Ring 357MHz A trombone for a signal delay ← Feedback ON/OFF

4.EXPRIMENTAL SETUP : Layout 10

4.EXPERIMENTAL SETUP : Optics 11 Isolator Cavity Transmission Reflection Injection mirrors Laser head λ/2 λ/4

5.CONCLUSION : Timing scan nsec Bunch length ← Compton signal In timing scan, we detect Compton signals and the laser pulse ’ s phase in cavity when signals come. Enhanced laser pulses 2times go and back in the cavity, and are scattered 4times every 2.8 nsec.

5.CONCLUSION : count rate 14 Calculated maximum count rate is ～ 2500 [Hz/mA]. Actual count rate is ～ 1500 because of imperfectly adjustment cavity length with shoulder feedback system. σ=laser beamwaist

5.CONCLUSION : VS. RF voltage 15 Data at 19th Nov. had a sharp peak, but There was no peak at 01st Dec. ( Because of the change Ring tune ? )

5.CONCLUSION : VS. the beam current 16 When the number of electron per bunch is bigger, The Gauusian shape of scattered photons count rate change. (The life time of low current electron beam at ring is long ) Count rate [Hz/mA] Phase [psec]

6. FUTURE PLAN 17 For high count rate : Problems as a bunch length monitor : And now We plan the higher finesse cavity (mirror R=0.999) and thin laserwire with beamwaist 50um. In the plan, the enhancement factor is 10 times as much as current systems. New feedback system, Tilt locking can adjust cavity length at resonance peak. We will test those systems in this winter. In current systems, It takes about 20 seconds for data taking. Data can be taken at low current electron beam, but at high current beam cannot. Next week, we will measure bunch length with pulse laser wire timing scan and streak camera. If I can, we will test tiltlocking feedback system.

Kyoto Univ. NIRS Jaeri KEK ISSP And you !! Thanks ! 2nd Mini-Workshop on Nano Project at ATF 12 th Dec. / 2004

企画書の目的と概要

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4.EXPERIMENTAL SETUP : Data taking 12