LLRF Stability Improvements for the Operation of SACLA T. Ohshima*, H. Maesaka, S. Matsubara, Y. Otake RIKEN, SPring-8 center LLRF workshop 2015, Shanghai, Nov. 3-7

OUTLINE 1.Introduction to XFEL facility, SACLA 2.Stability requirements 3.Present situation of RF stability 4.Cause of drifts (humidity & temperature) 5.Counter measures a.Upgrade Humidifier of klystron gallery b.Precise air conditioner for master oscillator room c.Optical length control of transmission line for reference signals 6.Results of counter measures 7.Summary

1. Introduction to SACLA XFEL facility in Japan – Provide coherent X-rays to users with ~0.6mJ, ~10fs, 4keV~15keV, 60pps – Characteristics: 8GeV, thermionic electron gun, C- band accelerator, in-vacuum undulator Multi beam line operation (BL3 & BL2) Newly installed dedicated accelerator for BL1 (EUV region): First lasing on 15 37eV

1. Introduction to SACLA Timing & LLRF system H. Maesaka, et.al, ICALEPCS2009,Kobe

2. Stability requirements RF parameters – Compress 1ns beam from gun to ~10fs after BC3 – use 238, 476, 1428, 2856, 5712 MHz Requirements – to keep FEL power fluctuation within 10%rms at pulse width of 10fs for short and long terms (calc. *) phase stability : < 100fs rms amplitude stability : < 1E-4 rms – Phase relations among reference rfs should be kept constant *H. Tanaka, et. al., Proc. of the 4th annual meeting of PASJ, 613 (2007).

3. Parameter drifts Short term stability 23fs rms measured at RF deflector*  enough for user operation Long term stability We experience parameter drifts *Y. Otake,et. al, Phys. Rev. ST Accel. Beams 16, (2013) Sep. 19, 2013 example: 238cavity phase & laser power for 24hr

4.Cause of drifts (humidity & temperature) kly gallery Large fluctuation of FEL intensity in spring and autumn The variation of humidity is large at those seasons Correlation between humidity & cavity phase humidity before improvement (2012)

– Replace Humidifier from ON/OFF control to Linear Control increase humidification capacity 5. Counter measures a) Humidity Klystron Gallery before (2012)  ~5K after (2015)  ~1K

5. Counter measures b) Precise Air Master Oscillator Room – We use Five reference RFs – The phase relations should be stable – Humidity & Temperature Change of the master oscillator room might affect to the relations master oscillator optical transmitter 5712MHz E/O 1/2 E/O 1/2 E/O 1/3 E/O 1/2 E/O 2856MHz 1428MHz 476MHz 238MHz WDMEDFA+Divider to Local Units

Introducing precise air conditioner to master oscillator room temperature variation < 0.1 K humidity variation < 0.2% p-p after before 5. Counter measures b) Precise air master oscillator room

temperature variation – We use optical transmission system for reference RFs. – Phase Stabilized Optical Fiber < 5ps/K/km – Temperature stabilized housing for optical fiber – No counter measure for humidity change 5. Counter measures c)Optical transmission system

Humidity and temperature variation – Introducing optical length feedback control system presented by Y. Otake 2013 LLRF 5. Counter measures c)Optical transmission system frequency stabilized laser EO: GHz master oscillator EO: 5.712GHz x8 Fiber Stretcher control Faraday rotator mirror Fiber Stretcher output phase det 5.712GHz control Fine Coarse TXU EO: trigger, 5712, 238MHz inter- ferometer inter- ferometer Master oscillator room Local control unit optical fiber

Without Optical Length FB With Optical Length FB daily drift of ~ 0.1ps p-p was reduced to < 0.05ps p-p 5. Counter measures c) Optical Length Control Correlation between humidity and SB phase Trend of humidity and SB phase

6. Result of counter measures Trend graph of 238 cavity phase upgrade kly gallery introduce precidion air master start beam commissioning

7. Summary Counter measures to RF parameter drifts – Stabilize the master oscillator room & klystron gallery – Introduce optical length feedback control transmission lines for reference signals Situation was improved. But there are still parameter drifts – condition of cathode of gun, drift of high voltage pulser used at beam chopper,,, We continue making further efforts

spare

SB LBLB CBCB With feedback Without optical length feedback