LCLS-II Beamloss
Update 12/8/11
J. Welch
Overview
Dumps
Beamline sections
Assumed Parameters
Normal
Operation*
Performance
Requirement
MCB
Gun Voltage
Me
V
6
6
6
L0 Energy
Me
V
135
135
135
TD11B Energy
Me
V
250
250
250
Repetition Rate
Hz
120
120
120
Charge per Bunch nC
.25
.25
.5
1
10
10
Bunches per
Pulse
¼ of LCLS-I Charge
*From CDR
Open Issues
• How much beam loss might there be in the dump
line given high energy extraction from deep taper
and self-seeding?
• Need reasonable MCB scenarios
• Extra loss across BC1 or BC2 for higher dark
current or profiling with collimator?
– presently set at 2% of beam, (~50W BC2)
• Should chicane collimators be able to intercept
nearly all the beam?
• Losses from pulsed kicker are not well
understood.
Dark Current Assumptions
Normal
Operation
Performance
Requirement
MCB
Exit of Gun
< 5 nA, 30 mW
30 mW
60 mW
Before
BX01/BXS
< 19 nA, 1 W
1W
2W
At TD11B
< 19 nA, 1 W
1W
2W
Dark current observed from LCLS-I Gun: typically 2 pC to 20 pC, with
maximum of 42 pC ( 0.2 – 2.4 nA, max 5 nA) [F. Zhou 2011]
Dark current from an accelerator section is estimated to be 19 nA
based on Ross (1991). It is assumed not to propagate beyond one
section. Getting new measurements of dark current in Linac
sections.
Beamloss Requirements
Normal
Operation
Performance
Requirement
MCB
FC01B, FCG1B
W
0.2
2
4
SDMP
W
5
50
100
TD11B
W
7.5
100
200
W
405
5,000
10,000
...
DUMPH
• LCLS-II is designed for one bunch at 0.25 nC
• Performance Requirements are set to be
consistent with 10 bunches per pulse, 120 Hz,
0.25 nC.
• MCB has additional conservative factor of 2
above beamloss requirement.
MCB Definition: RSS 2011
• 2.3.3 Maximum Credible Beam
The concept of a maximum credible beam is used to determine the
required shielding and protection devices. It defines the highest beam
power that the accelerator can deliver to a point assuming that the BCS
devices that limit beam power have failed.
In estimating the maximum credible beam, conservative, but reasonable,
assumptions should be used. Assumptions will be reviewed by the RSO and
RSC. For example, it is not necessary to assume that BCS device failure will
coincide with a significant effort to retune the accelerator to deliver higher
beam power than was approved for the facility.
– MCB is not maximum conceivable beam
– Need feedback on interpretation
MCB scenario
• We are commissioning multi-bunch on TD11 and
all BCS fails. Beam goes to high energy and high
power.
– multi-bunch commissioning only makes sense for source
currents such that the final beam poweris less than design
capability of dump - 5 kW. Scaling 13.5 GeV back to 250
MeV, the 5 kW beam would have a power of about 100 W
at TD11.
– make MCB at TD11 2 x 100 W = 200 W to make it more
conservative.
– similarly MCB at main dumps is 10 kW (not 100 kW in
LCLS-I)
LCLS-II MCB
•
Decline to use the explosive emission scenario
– never observed at LCLS
– gun is not designed to run with explosive emission
– Judge that simultaneous continuous operation with explosive emission from the cathode and
simultaneous BCS failure is not credible.
•
Accidents scenarios
–
–
–
–
Iris error and feedback off could generate higher bunch charge.
typo/software bug in the charge setpoint
...
Judge not reasonable to assume such accidents are coincident with both multibunch
commissioning and failure of BCS.
– max charge off cathode ~ 2 nC due to space charge – much less than 10 x 0.5 nC limit assumed
in MCB
•
New Cathode
– even if QE is 10% we would still only want 0.25 nC bunches.
– New multibunch optics would be needed to generate higher currents for operation.
LCLS-I/LCLS-II MCB
Energy
[MeV]
Charge/pulse Current [nA]
[nC]
Power [W]
LCLS-I
Gun Exit
SDMP
4
2,500
300,000
1,200
110
380
45,000
5,000
6
5
600
4
135
5
600
100
LCLS-II
Gun Exit
SDMP
• LCLS-I is based on current from continuous
explosive emission from the cathode
• LCLS-II is based on current limited to 2x
performance requirement.