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aAa / Plan for evaluating the survivability of bulk boron carbide in the LCLS FEL beam
Part 2
R. Iverson, S. Moeller,
J. Arthur, J. Vollaire
June26, 2009
All changes or additions to this procedure must be reviewed, approved and documented in the following table by RP Leader and LCLS Leader.
Changes & Additions / Date/Time & InitialItem / Description / LCLS Leader / RP Leader
Part 2:
ST1 and ST2 allowed OUT. Three Hutch shutter pairs locked IN.
Electron beam between 4.3-GeV and 15 GeV (FEL photons from 830 eV up to 8300eV)
Goal: Continue to evaluate the survivability of bulk boron carbide in the LCLS FEL beam.
Personnel:
RP Lead isJoachim Vollaire, James Liu or Stan Mao
LCLS Lead is Stefan Moeller, Rick Iverson, Paul Emma, Joe Frisch or Jim Turner
Photon Beam Parameters / Tasks / ExpectedDuration
827eV photons
[electron beam is4.3GeV,up to 30Hz andup to 0.25nC
Beam to main electron dump] / Establish photon beam todirect imager YAG screen and monitor B4C material for damage. / 1months
Safety Task / Signature
by / Date / Time / Signature
- Authorization by RSO
or designee
- Authorization by LCLS
These HOLD POINTS require test to be stopped. ADSO and RSO are to be notified:
Once each day MCC Operations will record a picture of the B4C sample inside the ST0 can and a picture of the YAG screen inside the direct imager (picture will be taken without beam) in the LCLS-elog. Stefan Moeller or his designee will review the state of B4C and YAG screen sample daily. If at any time the B4C sample appears to be damaged, the beam test stops and no beam will be allowed past the D2 stopper in the BSY. ADSO as well as Joachim Vollaire, James Liu or Stan Mao will be notified. The B4C sample will be taken out for closer inspection, if damages are confirmed, the beam tests will not re-start until a B4C damage mitigation plan is in place (which may include limitations on the electron beam energy). If no damage is observed, the B4C sample is put back into the beam and the test can re-start. If the YAG screen of the direct imager appears to be damaged, the beam is not allowed past the ST1/ST2 stoppers. The screen must be taken out for closer inspection. If damages are confirmed, the B4C protecting the C1 collimator and the B4C protecting the temporary beam stop inside the FEE must be visually inspected. If damages are observed on the collimator or temporary dump, the beam tests will not re-start until a B4C damage mitigation plan is in place (which may include limitations on the electron beam energy). If no damage is observed, the damaged YAG screen must be replaced and operation can resume.
Part II Tests
These tests will take place after the X-ray diagnostics become available in the FEE. During Part II the stoppers ST1 and ST2 will not be locked, and may be removed to permit beam to pass into the FEE. The three pairs of hutch shutter at the downstream end of the FEE will be locked in the inserted position during this period, preventing beam from entering the NEH.
Probable duration of Part II is two months, roughly June 1 – Aug 1, 2009.
Key hardware changes for the Part II tests are:
II-h-1) As mentioned above, the stoppers ST1 and ST2 will be enabled, so that they can be removed from the beam path to allow beam passage into the FEE. The fluorescent screen used in Part I will be removed from the beam path. In addition, the additional B4C which was placed in the beam path downstream of the screenwill be removed.
II-h-2) The air gap downstream of ST2, and associated heavy metal Bremsstrahlung shielding, will be eliminated as the beamline is completed to the FEE. A new air gap will be located in the downstream half of the FEE (just downstream of collimator C1 right after the temporary beam dump), downstream of the X-ray diagnostics and upstream of the hutch shutters.
II-h-3) The three pairs of hutch shutter (located at the downstream end of the FEE) will be locked in the inserted position, precluding beam passage into the NEH.
II-h-4) The B4C test piece located in the Electron Dump area will be logically connected to the stoppers ST1 and ST2, so that it is inserted automatically when ST1 and ST2 are inserted, and removed when the stoppers are removed.
II-h-5) A temporary FEL beam stop will be placed just upstream of the air gap in the FEE, such that it will completely intercept the range of paths of the FEL beam. This beam stop will incorporate a B4C spoiler and heavy metal X-ray stop. This beam stop will be fixed in place during Part II.
II-h-6) The fluorescent screens (Ce:YAG) and monitoring cameras of the Direct Imager located upstream of collimator C1 will be inserted.
The key procedural elements for Part II are:
II-p-1) LCLS must satisfactorily complete a Readiness Review before receiving permission from SLAC management to open shutters ST1 and ST2. One part of the review will address the results of the B4C tests that have been performed, and the survivability of the beam containment system in the FEE.
II-p-2) The FEE diagnostics will give a much better quantitative measure of the FEL pulse intensity. These diagnostics can also help to establish FEL beam production especially at lower energies if that has not already been achieved.
II-p-3) During this period there are two distinct modes of operation: Mode (i) with stoppers ST1 and ST2 closed and the B4C test piece inserted, and mode (ii) stoppers ST1 and ST2 open with FEL beam entering the FEE with the YAG screen of the Direct Imager always inserted.
II-p-4) With ST1 and ST2 inserted during mode (i) and allowed access to the FEE, removal of any hardware between Wall 1 and the temporary beam dump will be controlled by a Radiation Safety Work Control Form (RSWCF) and in this case the beam will be parked upstream of the undulator on stopper D2.
II-p-5) During mode (ii) with ST1 and ST2 open and FEL beam entering the FEE, it will be ensured that the YAG screen of the Direct Imager is always inserted. The YAG screen will be frequently examined for possible damage using the UV and visible illuminator of the Direct Imager.
II-p-6) The goal of the Phase II tests is to discover whether, under full LCLS design pulse intensity conditions at 800 eV FEL energy, the B4C test piece in the Dump area is damaged or not. Therefore, at all times when it is possible to produce high-power FEL pulses (sufficient number of undulator segments installed, etc), the B4C test piece will be frequently inserted into the beam and monitored for damage. It will probably be necessary to alternate between putting the B4C and the FEE diagnostics into the beam to confirm that the B4C is actually experiencing a strong FEL beam
II-p-7) At the finish of Phase II (assuming that strong FEL operation was achieved), the B4C test piece that has been exposed to FEL beam will be removed from the LCLS dump area and tested externally for subtle damage (primarily using microscopy). A new B4C test piece may be installed in place of the first one, so that tests can continue while the examination of the first test piece is in progress.
These HOLD POINTS require test to be stopped. ADSO and RSO are to be notified:
Once each day MCC Operations will record a picture of the B4C sample inside the ST0 can and a picture of the YAG screen inside the direct imager (picture will be taken without beam) in the LCLS-elog. Stefan Moeller or his designee will review the state of B4C and YAG screen sample daily. If at any time the B4C sample appears to be damaged, the beam test stops and no beam will be allowed past the D2 stopper in the BSY. ADSO as well as Joachim Vollaire, James Liu or Stan Mao will be notified. The B4C sample will be taken out for closer inspection, if damages are confirmed, the beam tests will not re-start until a B4C damage mitigation plan is in place (which may include limitations on the electron beam energy). If no damage is observed, the B4C sample is put back into the beam and the test can re-start. If the YAG screen of the direct imager appears to be damaged, the beam is not allowed past the ST1/ST2 stoppers. The screen must be taken out for closer inspection. If damages are confirmed, the B4C protecting the C1 collimator and the B4C protecting the temporary beam stop inside the FEE must be visually inspected. If damages are observed on the collimator or temporary dump, the beam tests will not re-start until a B4C damage mitigation plan is in place (which may include limitations on the electron beam energy). If no damage is observed, the damaged YAG screen must be replaced and operation can resume.
If the test is limited by the inability of the LCLS to produce full design intensity, the performance of the B4C test piece will be tested at the highest pulse intensity obtainable.
Part II will terminate at the time when permission is granted to bring LCLS X-rays into the NEH. At this time, the temporary beam stop and air gap will be removed from the FEE and replaced by beam transport pipe.
Ongoing test capability
As mentioned above, the Part I tests will take place at 10 Hz and probably at less-than-full FEL power. It is possible that full FEL power will not even be achieved during Part II. As long as B4C survivability has been demonstrated under actual LCLS operating conditions, the system will be considered to be provisionally safe, and LCLS commissioning can proceed. However, it is imperative that B4C survivability be re-evaluated, and safe operation demonstrated, every time that FEL power and/or pulse repetition rate is increased significantly. Therefore, the B4C test piece and camera will remain in the Dump area. The test piece will continue to operate in tandem with ST1 and ST2, so that it receives a greater FEL dose than any other piece of B4C beam containment. The integrity of the test piece will be evaluated periodically during LCLS operations. Every significant increase in FEL pulse energy or pulse repetition rate will be immediately followed by a B4C test period.
Detailed Beam Containment Test plans will be developed for assuring B4C survivability each time LCLS operating conditions change in such a way as to raise doubt. The test plans will include not only in-situ monitoring of the test piece by video camera, but also periodic replacement of the test piece and careful external examination of the exposed test piece. Timetables for recording in-situ pictures and for periodic external evaluations will be specified.
The LCLS commissioning plan will incorporate an ongoing program of testing of materials in extreme FEL beams. This program will include exposing B4C to focused FEL pulses, in order to accurately determine the actual damage threshold.
Mitigation plans in case of FEL damage to the B4C test piece
In parallel with the test plan described above, LCLS will develop mitigation plans to employ if FEL damage to the B4C test piece is discovered. These plans may involve replacement of the B4C attenuators with attenuators made of a different material, modifications to the shape of the B4C attenuator pieces (ie, grazing incidence), frequent replacement of the B4C attenuators, etc. As more quantitative information regarding the limits of survivability of B4C in the LCLS FEL beam becomes available, the relative merits of different mitigation plans will be assessed.
* “Mini-workshop to discuss the likelihood of damage to LCLS photon beam containment systems by LCLS FEL X-ray pulses”, held at SLAC on 18 September 2008. See summary by J. Arthur and workshop presentation materials.
Safety Task / Signature by / Date / Time / Signature- Part 2 complete
- Part 2 complete
Date/Time / Signed by Stefan Moeller or his designee.
Daily log of direct imager YAG and B4C sample damage assessment:
Daily log of direct imager YAG and B4C sample damage assessment:
Date/Time / Signed by Stefan Moeller or his designee.Daily log of direct imager YAG and B4C sample damage assessment:
Date/Time / Signed by Stefan Moeller or his designee.1