4GLS Beam Loss initial brainstorm - 4th May 2007

Machine protection system has to be designed - we need to consider all likely eventualities.

General considerations: some diagnostics will cause loss (e.g. running into an FCup), so we will need different operating modes to allow this.
As with other facilities, we will need an interlock system
We need to obtain a time constant for the cryomodules - how long will they ring if the power goes off? About 0.1ms
Machine protection scales:
30-300 ns is as fast as signals can travel through the 4GLS; machine protection can't go as fast as this.
Typical switch off in machine protection is 10s of us (check).

Vacuum faults:
Slow leak - Gas bremsstrahlung (radiation monitors), ion trapping change (how much?), linac ices up,
Fast leak - triggers gate valve - need to turn off electrons before valve shuts. What happens if a gate valve shuts by itself? This is probably standard accelerator technology, but the beam power is very high in our case. e.g. 5 kJ can hit a faulty valve in 0.1 ms if the electrons are not turned off! Same goes for diagnostics, e.g. screens.

HACL:

Cathode:
Long time constant change (QE change)
Laser shut-off - probably not a problem as there are no electrons
Laser loses sync with RF system (outside bucket) - bunch is at different phase, so is bunched differently and comes out with different energy from injector. How fast can that happen?
VUV-FEL mode: low rep rate selected. Can the laser go CW (chopper fails)? How will the chopper work?
other failure modes of the laser/cathode?

Gun:
HV PSU trip-off (low SF6, PS fault) - injector energy will reduce over a time until we can trip off the laser. Particles will go in the wrong direction in BC1 - need to collimate?
HV PSU set wrong - what happens if we are mis-tuned, and how do protect against it?

Buncher:
Fails, beam is not bunched - what happens? Energy spread and length growth - will this be beyond
Buncher works at the wrong phase? Can the acceleration produced be significant

HACL Modules:
Modules at wrong phase
Wrong voltage - probably be to reduce the voltage
Everything here has a time constant
RF control fault - e.g. missing bunches or pulses - gives transient beam-loading

BTS:

Magnet failure:
Wrong setting/Trip - may for example have to arrange that inductance of magnets is high to make their intrinsic turn-off slow. May get this for free...
Some magnets are more critical than others (e.g. the ones upstream of the main linac)
Collimators are in the HACL arcs to take a short pulse of errant beam, and also to catch halo; main linac can easily change in 0.1ms.
Magnet failure can cause path length change - need to check how big this change can be.
Magnet failure can cause beam movement which can hit the IDs - need to do something similar to what we did for the SRS and DIAMOND, e.g. ray-tracing of possible beam routes through the IDs.

Main Linac:
Check - ILC linac failure modes.

VUV-FEL:
Lasing goes on and off - should be no problem as unlased energy range lies within lased energy range.
Steering from misaligned/faulty magnet arrays - can't go too fast, should be able to be

PLC:
PLC moves or there is a magnet failure - energy recovery is lost. Linac will pick this up.

Dump:
Energy too high - power load too high, irradiation. Beam can also hit crotch and spreader.
Energy too low - absorption depth in dump will be less.
Raster magnets fail - need to independently sense the raster scan failure, as there will be lots of radiation/signal in this part of the machine.

XUV-FEL:

Gun/Injector:
Missed pulses can occur - probably doesn't matter if there are no pulses, e.g. in main linac.
Overpulses (e.g. extra laser pulses) - then get extra electrons that are not being accelerated. Depends on the length of the RF pulse.

Linac:
Wrong relative phase of XUV and HACL bunches. Can the HACL bunches 'push' the linac phase around enough to change the XUV gun phase.
HHG laser can melt something directly?