ERL07 WG2 Session 2 - Notes

Sakanaka - Ion Trapping

Use kicker to kick out parts of the injector beam - needs fast kicker (1.3 GHz bunch separation), and get ringing. Maybe can avoid ringing by using two kickers Pi out of phase.

Methods to avoid trapping:
1. Gaps - match beam loading in main linac, and use feed-forward RF in injector
2. Beam blow-up - no beam loading in injector, but the fact there is still focusing term from the blown-up bunches mean that the effect on clearing is less good.
3. Moving the beam - 'beam shaking'. Not viewed as a good thing - tends to hits resonances .

Nakamura -Resistive-Wall Beam Breakup

PRST-AB 7, 034402 (2004)

Transverse wakefield can give break-up.

ERL 07 Workshop Notes - Session 1

Michael Borland - Touschek Scattering

Beam-loss mechanisms fall into two classes:
1. Low-energy, unpredictable - collimate at low energy
2. Calculable, e.g. Touschek and Nonlinear Optics

Touschek dominates CW beam loss in low emittance rings. Probably then significant in ERL light source. Touschek is well-studied in rings, but not studied in single-pass accelerators.

Assume gas scattering is smaller than Touschek - at about 1 nTorr.

ERL electron cooler - 5 nC per bunch, 4 um emittance (100 ps bunch length). ERL beam cools both directions. 54 MeV bunches travel 100 m down ERL cooling line - some space charge acting. Will compensate space charge using weak solenoids every 10 m.

3rd-harmonic cavity is used to flatten energy gain of the very long (100 ps) bunch so that energy spread is small enough (4.10^-4 rather than few 10^-3 without 3rd-harmonic)

BBU threshold is high with the small number of cavities and modelled HOMs.

Have a dog-leg which moves the beam H and V at the same time (5 quads for dispersion matching). No perceived issue with space charge at the centre of the dog-leg quad.
300 m turnaround for the beam path.
Accumulation of ions will be only in the turnaround - the cooling region has a big ion beam which will repel the ions in that 100m section.
Path length correction is mechanical - move the arc.
Amplitude stability is 10^-4 in RF - should be good enough.

4GLS Diagnostics Discussion Meeting, 14th May 2007

CDR design has obviously changed, but the diagnostic principles haven't changed. The

HACL Gun:

Gun energy - temp diagnostics line
Gun emittance - temp diagnostics line
Cathode emitted bunch length
Issues of run-up from low current to high current
(separate meeting!)

Assume built gun (check Cornell prototype): current measurements, beam centroid measurements, beam profile measurements, insertable screen/Fcup?
Interlocking issue - make sure screens don't go in at high current.

HACL Injector:

Output energy, chirp, bunch length - phase/voltage measurement of each cavity
Tranverse phase space measurement? May be a 'wish-list item.

Boris/Rob/Julian/Hywel to look at essential measurements

XUV Injector:

Collaboration with PITZ?
Fast amplitude stabilisation of RF amplitude and phase during 20 us RF pulse in XUV injector.

Output energy, chirp, bunch length - phase/voltage measurement of each cavity
Tranverse phase space measurement? May be a 'wish-list' item.
Include BC1 setup

Main Linac:

HACL team - measurements list: (Hywel)/Bruno/Rob/Alex

Need to think about the modes we will be running in:
XUV only: 1nC at 1kHz
HACL only: low current/high current/
XUV/HACL only:

Transfer matrix measurements - are they needed?

Energy Recovery/Dump

Measurement of energy recovery efficiency. Measurement of loss at level required?

HACL and IDs

Modes for commissioning again.
Profile monitors - low current with screens only, high current with alternative monitor - are they needed?
Tomography measurement - SR output resolution vs. resolution of other instruments for measuring of transverse profile, e.g. wavelength, diffraction etc.,

THz measurement in straight 1?
Streak cameras at short bunch length - 100 fs resolution under development, 10 fs
EO does not work at high current, use streak cameras at high current in upstream regions of the HACL ID arc.

How do we get the VUV-FEL to work? Bunch length, time overlap,

Discussion: Hywel/Frances/Neil

XUV-FEL

Emittance measurement is under control (Bruno!)
Seed laser diagnostics

Discussion
Neil/Steve/Frances/Bruno/Brian Sheehy/Graeme

IR-FEL:

Establish group: Boris/Bruno/Neil

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?

Talking to Helen and Sarah

Here we are, talking with Helen and Sarah. Take a look at this website.