Lessons Learned from Recent SSRL Operations Shanjie Xiao, James Liu - - PowerPoint PPT Presentation

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Lessons Learned from Recent SSRL Operations Shanjie Xiao, James Liu - - PowerPoint PPT Presentation

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Lessons Learned from Recent SSRL Operations Shanjie Xiao, James Liu and Sayed Rokni SLAC National Accelerator Laboratory Radsynch 2017, Hsinchu, Taiwan Overview Lessons learned from three incidents: Excessive dose from gun chopper current

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SLIDE 1

Lessons Learned from Recent SSRL Operations

Shanjie Xiao, James Liu and Sayed Rokni

SLAC National Accelerator Laboratory Radsynch 2017, Hsinchu, Taiwan

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SLIDE 2

2

Overview

Lessons learned from three incidents:

  • Excessive dose from gun chopper current leakage
  • Burning of BL12 burn through monitor
  • Alignment error of BL13 movable mask

RadSynch 2017, NSRRC, Hsinchu, April 19-21, 2017

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SLIDE 3

3

Introduction: SSRL Layout

SPEAR3: 3GeV 500mA Linac & Booster

RadSynch 2017, NSRRC, Hsinchu, April 19-21, 2017

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SLIDE 4

4

Excessive Doses from Area TLDs

130+ area TLDs for SSRL In April 2016, found quite a number of area TLDs read higher than before

  • Registered radiation Aug. 2015

– Jan. 2016

Booster Linac

Lab RadSynch 2017, NSRRC, Hsinchu, April 19-21, 2017

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SLIDE 5

Excessive Doses from Area TLDs

2nd Floor of Bldg. 130

Dose in mrem

103 (16) 0 (0) 80 (19) 0 (0) 87 (19) 0 (0) 0 (0) 0 (0) 13 (0) 0 (0) NA (NA) 0 (0) Elevated dose Normal dose ## (##): Total (Neutron) Dose 1st row for Aug. 2015–Jan., 2016 2nd row for Aug. 2014–Feb., 2015

  • Worst in Linac: 1.6 mSv  37 mSv
  • 0.8-1.0 mSv in some 2nd floor offices (w/

neutrons)

  • Limit: 1 mSv for 2000 hours
  • Equivalent to 0.6-0.8 mSv for 2000 hours
  • ccupancy
  • Occupants in the offices were relocated

immediately

5

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SLIDE 6

6

Reasons of the Excessive Dose

SSRL uses a thermionic gun and a chopper to select 10 Hz beams

COR1 COR2 COR3 COR4 Gun Chopper PM D Slit GT1 GT2 GT3

thermionic gun

RadSynch 2017, NSRRC, Hsinchu, April 19-21, 2017

  • Permanent magnet in chopper bends beam down as

default status

  • Trigger pulses select 5-10 bunches from total of 3000
  • Bunches that arrive early before the trigger are not kicked and

strike below the slit

  • Bunches that arrive late after the trigger are kicked too hard

and are lost above the slit

  • The chopper does not exhibit perfect contrast as some

fraction of the electrons before and after the chopper trigger can be transmitted through the chopper, and then be partially accelerated.

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SLIDE 7

Leak current when chopper is “off”

7

Reasons of the Excessive Dose

RadSynch 2017, NSRRC, Hsinchu, April 19-21, 2017

  • Typically the number of unwanted bunches leaking through the chopper is

minimized during machine startup every year while at the same time

  • ptimizing the desired beam performance.
  • In the 2015 shutdown the gun cathode was replaced and in the process the

alignment was unexpectedly changed.

  • When the desired beam

was optimized, a small amount of leakage current was transmitted through the chopper.

  • Leak after trigger is much

less since the chopper is

  • nly fired 60 times every 5

minutes.

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SLIDE 8

8

Lessons Learned and Mitigations

Careful study and control on radiation sources Mitigations

  • Create a tuning procedure to reduce the leak:

perform at the beginning of each run

  • Add shielding to shadow offices
  • Add a radiation monitor beside Linac
  • Doses are reduced to normal

Chopper 1st accelerate section New shielding on this side

RadSynch 2017, NSRRC, Hsinchu, April 19-21, 2017

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SLIDE 9

9

Introduction: SSRL Beam Line Front End

Synchrotron Beams Fixed Mask Movable Mask Injection Stoppers Hutch

  • For insertion, movable mask moves in first and then injection stoppers
  • For withdraw, injection stoppers move out first and then movable mask

Grazing incidence, water-cooled mask for SR Burn-through monitor: bladder connected to ambient air Tungsten block for bremsstrahlung Concrete Wall

RadSynch 2017, NSRRC, Hsinchu, April 19-21, 2017

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SLIDE 10

10

BL12 Burn Through Monitor

In November 2015, the BTM of the 1st BL12 injection stopper was burned, vented SPEAR3 ring, and dumped stored beams

  • BTM system operated as designed
  • Burn while inserting the stopper moved into beams
  • Burn rapidly relative to insertion time
  • Beam was terminated before damaging stoppers
  • No personnel radiation exposure
  • Cause of failure investigated
  • Insertion command logic ok
  • Insertion timing tested
  • No root cause found

Normal beam center

RadSynch 2017, NSRRC, Hsinchu, April 19-21, 2017

  • Horizontally displaced as this front end serves a 10mrad

chicane and only one IVU was installed.

  • Vertically displaced because burn through rapid relative

to insertion time resulting in melt as stopper first intercepted beam at the bottom of the stopper.

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SLIDE 11

11

BL12 Burn Through Monitor: Lessons Learned

Uncommanded state faults should be trapped

  • Introduce state fault trap logic
  • If Movable mask not closed and injection stoppers not open  fault, dump beam

Avoid challenging BTM via introduction of MPS if feasible

  • A refractory metal plate attached to the leading edge of the upstream

injection stopper featuring MPS monitored temperature sensors

RadSynch 2017, NSRRC, Hsinchu, April 19-21, 2017

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SLIDE 12

12

BL13 Degraded Vacuum

During Fall 2015 startup, vacuum at BL13 degraded when the line was closed (movable mask and injections stoppers inserted)

  • MM was in an angle larger than design since 2007
  • MM was reduced to its design angle 1.2° in summer 2015
  • Isolation valve after MM could be slowly heated (125°C over 2 hours)
  • Increase MM angle to 1.7° resolved the heating issue
  • Vented the line in summer 2016
  • Isolation valve damaged
  • Injection stopper BTM locally damaged (not burned)

Isolation Valve Injection Stopper

RadSynch 2017, NSRRC, Hsinchu, April 19-21, 2017

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SLIDE 13

13

Cause: Movable Mask Alignment Error

  • MM was not fiducialized in 2007 as expected by design engineer such that

when aligned the MM did not completely occlude the fixed mask aperture

  • Mask center should be aligned to vertical beam center
  • Aligned to mask tip
  • Larger pitch angle before 2015 coincidently compensated for fiducialization
  • ffset
  • Other beam lines are checked to be ok

BL13 Movable Mask

RadSynch 2017, NSRRC, Hsinchu, April 19-21, 2017

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SLIDE 14

14

Lessons Learned

High power undulator beam line masks operate at small grazing angles calls more frequent alignment verification

  • Annual start up alignment checks for high power density undulator BL

Improve alignment communication

  • More explicit documentation of feature positions rather than overall position

information to ensure clear and complete communication of alignment results

Increasing undulator power has pushed mask designs to operate at low intercept angles to maintain cyclic thermal stress at the endurance limit

  • The alignment tolerance risk is more significant than the cyclic stress risk.
  • Similar to other light sources, accept sub-endurance cycle lifetimes in

exchange for better mask mis-alignment tolerance.

RadSynch 2017, NSRRC, Hsinchu, April 19-21, 2017

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SLIDE 15

15

Summary

Excessive dose due to gun chopper current leak

  • Need better understanding and control on radiation sources (Chopper OFF)

BL12 burn through monitor

  • Need state trap to catch uncommanded faults

BL13 movable mask alignment error

  • Need more frequent check on beam lines with small alignment tolerance
  • Need to improve alignment communication
  • Need to balance the alignment tolerance risk and cyclic stress risk

This work is supported by the U.S. DOE, Office of Science, Office of Basic Energy Sciences, under Contract No. DE- AC02-76SF00515

RadSynch 2017, NSRRC, Hsinchu, April 19-21, 2017