1. Maximum Credible Beam Event Paul Emma et al
Maximum Credible Beam Event Paul Emma et al. March 14, Accelerator Readiness Review Meeting
LCLS Injector Operating/Commissioning & Safety Envelope
Maximum Credible Beam Power
Mitigation (SLAC RP directed controls)

2. LCLS Injector Layout Injector Vault 250-MeV Tune-Up Dump (TD11) – 30 W
Spectrometer
(0.7 W)
Linac Enclosure
135-MeV Spectrometer Sump (SDMP) – 16 W
All power levels are nominal at 120 Hz and 1-nC bunch charge

3. The Process
Physics group estimates maximum credible beam power and nominal beam losses (PRD r4).
Radiation Physics Group determines needed shielding and active protection devices (RP-05-15).
Radiation Safety Committee approves shielding and protection (done).
LCLS installs the required configuration (done).

4. Maximum Credible Beam Power in LCLS
Explosive Electron Emission – assume the total stored energy in the gun is directed into electron acceleration
Acceleration of this current is taken at the highest gradient, including beam loading
Beam losses are calculated based on energy spectrum and physical aperture in bending magnets, all under absolute worst case conditions
From LCLS Injector SAD (SLAC-I R001)

5. Maximum Credible Beam Power in LCLS
The beam-loaded energy gain in one 3-m SLAC RF structure is given by [1]
where P is the structure input power and Ib is the average beam current over the bunch train.
At the acceleration limit (DE = 0) and the highest L0a power, the average current cannot exceed 1.8 A.
With aperture limits, this current produces a maximum MCB of <5.5 kW after the L0b accelerator section and <100 kW in the main dump and end-of-linac.
The established safety envelope for the main linac is 2 MW.
[1] H. Wiedemann, Particle Accelerator Physics, Springer-Verlag 1993, p42.

6. Max. Credible Incident Assumptions
Explosive Electron Emission occurs in the gun
L0a and L0b are set to their highest gradients (well beyond nominal)
Injector beam loss only taken credit for at the DL1 bends with fixed 1-inch aperture (no loss included at quadrupoles)
The DL1 bends are set to their absolute worst case energy (150 MeV – 10% above nominal)
No beam loss taken credit for anywhere in the 1-km long linac, including the LCLS bunch compressor chicanes
All klystrons are at full power and crest phase (not nominal)

7. Estimate for Maximum Credible Beam
Max. stored energy in gun is 10 J at 140 MV/m. Beam loading gives 4 MeV energy, therefore max charge/pulse is (10 J)/(4 MeV) = 2.5 µC at 120 Hz, or 0.30 mA. Studies show 85% lost in GTL, so average power is: (4 MeV)(0.30 mA)(85%)  1.0 kW.
1.2 A accelerated by L0-a. With beam loading, the energy is 21 to 76 MeV, with 48 MeV average. Average beam power at L0-a exit: (48 MeV)(0.38 mC)(120 Hz)  2.2 kW.
Quads between L0-a and L0-b transport 64 MeV electrons so will over-focus MCB electrons causing loss in L0-b structure. Max power deposited in L0-b assumed as full beam power of 2.2 kW.
Quads OFF: Beam may be accelerated by L0-b with range 56 to 160 MeV, for the max 62-MW RF power. Average energy is (160 MeV+56 MeV)/2  110 MeV, average power is (110 MeV)(0.38 mC)(120 Hz)  5.0 kW after L0-b.
Max energy is 160 MeV and DL1 energy acceptance is 5%. Highest energy with max transmission is (1 - 5%)(160 MeV)  150 MeV. Worst case MCB to main linac is 150 MeV with average power of (13%)(0.38 mC)(150 MeV)(120 Hz)  0.9 kW.
Acceleration to 250 MeV, MCB ~ 6mA*250 MeV=1.5 kW after L1
MCB in TD11 ~ 1.5 kW
DL1 bend off: 5.0 kW lost in and after spectrometer dipole.

8. Summary of Maximum Credible Beam Power
Along the Injector and into the Linac
100 16
0.050
6.0

9. Nominal and Dark Current Losses

10. Mitigation: Shielding and Active Protection
Radiation Physics directed controls…
Shielding in penetrations [four in sec-20 alcove and five in sec-21] (0.5 mrem/hr nom. and 175 mrem/hr at MCI)
Local activation ALARA shielding on dumps SDMP & TD11
Two BSOICs in sec-20 alcove (10 mrem/hr)
Two BCS flow switches on SDMP
Two pairs of LIONs (1 & 2) limit linac loss to <100 W
All BCS devices fault both LCLS and LINAC guns
Injector Vault Access with Linac Running…
Backward stopper (RST1) inserted and BX01/02 bends OFF
BSOIC in vault (10 mrem/hr)

11. Sector-20 Penetration Shielding

12. TD11 Dump Penetration and ALARA Shielding

13. Improvements to Sec-21 Penetration Shielding

14. LCLS Injector Vault BSOICs and PICs
Courtesy M. Saleski

15. BCS Long Ion Chambers (LIONs)
Courtesy M. Saleski

16. Conclusions Shielding and protection is in place
Radiation measurements are scheduled (started already)
Every beamline component is being checked (40-page checklist: polarity, connectivity, z-location, motion control, calibration, etc)
Some screens and toroids are delayed (~1 week)
We are ready for an electron beam in late March