Part 1 (May 8, 2009):
ST1 and ST2 locked IN.
4.3-GeV (830eV FEL),
and electron beam between 4.3 and 15 GeV (FEL up to 8300eV)

Goal: To evaluate the survivability of bulk boron carbide in the LCLS FEL beam.

Personnel:

RP Lead is Joachim Vollaire, James Liu or Stan Mao

LCLS Lead is Stefan Moeller, Rick Iverson, Paul Emma, Joe Frisch or Jim Turner

Photon Beam Parameters / Tasks / Expected
Duration
830 - 8300eV FEL
[electron beam is 4.3GeV to 15GeV, up to 10Hz, up to 0.25nC
Beam up to main electron dump] / Establish FEL beam to YAGXRAY and monitor YAG
OR
to B4C and monitor B4C / 1 month
Safety Task / Signature
by / Date / Time / Signature
1.  Authorization by LCLS / ASD head John
Seeman
or designee
2.  Undulator Complex BAS allows up to 33 undulator magnets to be on the beam line with FEL beam to YAGXRAY/B4C for checkout. / RP Lead

These HOLD POINTS require test to be stopped, RP and ADSO to be notified:

Once each day MCC Operations will record a picture of the B4C sample with and without beam in the LCLS-elog. Stefan Moeller or his designee will review the state of B4C 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 beam tests will not re-start until a B4C damage mitigation plan is approved and in place (such as limiting electron energy). The B4C sample will be taken out for closer inspection, in order to confirm damages. If no damage is observed, the B4C sample is put back into the beam and the test can re-start.

Part I Tests

These tests will take place during the initial period after the undulators are inserted and X-rays begin to be produced, but before the X-ray diagnostics are available in the FEE. During Part I the stoppers ST1 and ST2 will remain locked in the inserted position.

Probable duration of Part I is one month, roughly May 11 – June 5, 2009.

Key hardware elements required for the Part I tests are:

I-h-1) A piece of B4C similar to that used in ST1, located just upstream of ST1 in the Dump area. This B4C will be remotely insertable into the X-ray beam, with a camera positioned to monitor its front surface for damage.

I-h-2) A fluorescent screen (YAG) and monitoring camera also located just upstream of ST1. This screen will give a crude measure of the FEL performance during the initial period when the X-ray diagnostics in the FEE are not available.

I-h-3) An air gap will be introduced into the X-ray beamline downstream of ST1 and ST2. This will serve as a final FEL absorber, should all else fail. (It should be noted that both ST1 and ST2 contain burn-through monitors which will detect catastrophic failure of the B4C absorbers.)

I-h-4) 2” lead is located at the downstream end of the air gap, in air.

In addition to these hardware elements, an attempt is being made to include an additional B4C stop just downstream of the YAGXAY. This is seen as an extra layer of protection, to protect ST1 in the unlikely event of sudden catastrophic failure of the fluorescent screen during a moment when the upstream B4C test piece is removed.

The key procedural elements for Part I are:

I-p-1) During the initial X-ray period, the YAG screen or both the YAG screen and the B4C test piece will always be inserted into the X-ray beam. The YAG screen image should be able to distinguish a strong FEL beam from a weak FEL or spontaneous X-ray beam, though not in a very quantitative way. A weak FEL or spontaneous beam will provide fluence levels several orders of magnitude less than a fully-saturated FEL beam, presenting no danger of damage to the B4C.

I-p-2) During this period the LCLS accelerator commissioning team will attempt to produce FEL radiation, using the YAG screen as a key diagnostic. All initial testing will be done at a pulse repetition rate up to 10 Hz.

I-p-3) If a strong FEL beam is produced during this March-May period, then the B4C test piece will be inserted and monitored for damage. It will probably be necessary to alternate between putting the B4C and the YAG into the beam to confirm that the B4C is actually experiencing a strong FEL beam.

I-p-4) Initial FEL operation will be attempted at a photon energy of about 8300 eV. Then operation will be attempted at successively lower energies, until 830 eV is reached. At each energy where strong FEL beam is achieved, its effect on the B4C will be evaluated. If efforts to achieve FEL operation at all energies are successful, this strategy will expose to the B4C to increasing power densities. Every effort will be made to expose the B4C to strong FEL beam at each energy for at least 24 hours, though it may not be possible to achieve strong beam for such an extended period at each energy.

I-p-5) Prior to commencing Part II (assuming that strong FEL operation was achieved during Part I), 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 Part II tests can commence while the examination of the first test piece is in progress.

1-p-6) A detailed Beam Containment Test plan incorporating these elements, with appropriate approvals, will be prepared and approved before commencing this work.

It is understood that the Part I tests will not be very quantitative, and that there is a good probability that no strong FEL operation will be achieved at each electron energy level during this period (and therefore, no new information gained about B4C survivability). Achievement of strong FEL beam during Part I is not a prerequisite for continuing to Part II. The Part I tests simply insure that if the B4C damage limit is reached, this fact will be recognized right away.

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 (i.e., 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
3.  Part 1 complete / LCLS Lead
4.  Part 1 complete / RP Lead

Daily log of B4C damage assessment:

Date/Time / Signed by Stefan Moeller or his designee.

Daily log of B4C damage assessment:

Date/Time / Signed by Stefan Moeller or his designee.

Daily log of B4C damage assessment:

Date/Time / Signed by Stefan Moeller or his designee.

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