# Presentation: 087971

## Mini Gap Insertion Devices

- Christoph Steier
- ALS Accelerator Physics Group

-  Issues
-  difference fixed gap/variable gap
-  difference warm/cold (20K/4K)
-  Lifetime and Injection efficiency measurements
-  Where to go from here

- Lawrence Berkeley National Laboratory

## Issues

-  Lifetime
- Field Quality
- Gap
-  Injection Efficiency
- Same as Lifetime
-  Resistive Wall or HOM heating
-  Impedance/Instability Thresholds
-  Vacuum Issues
-  Radiation Damage
-  Performance
-  Cost
-  Schedule

- Lawrence Berkeley National Laboratory

## Lifetime

-  Gap dependence measured with scraper  OK down to a full physical aperture of about 5 mm (IDs open/closed) at y = 6.25 m
-  Verified scraper scale, need to simulate impact of dispersion bump
-  Total distance from beam (as determined by fit to data) consistent with mechanical geometry (within 0.5 mm)

- Lawrence Berkeley National Laboratory

## Lifetime II

-  Started simulating effect of field roll-off (superconducting device)
-  very rough/first simulations for Soren’s field data look OK
-  No reason to believe that it should be a problem for permanent magnet design (Spring-8, ESRF and especially SLS experience).
-  Overall both options are probably OK, but more remains to be done for superconducting option and it is a higher risk (impact on whole machine versus impact on one beamline)  Not ready to allow fixed 5 mm gap
-  Movable gap has further advantage that one potentially could profit later from a possibility to go to even smaller gaps than 5 mm

- Lawrence Berkeley National Laboratory

## Injection Efficiency

-  First measurements indicate it is not great, but acceptable
-  Worst case requires use of collimators to protect permanent magnets
-  Risk permanent magnets: Radiation Damage, Superconducting: Quenches
-  Other considerations same as for Lifetime

- Lawrence Berkeley National Laboratory

## Resistive Wall/HOM heating

-  Resistive Wall or HOM heating
-  Has caused (great) trouble at MaxLab and Elettra
-  Extreme anomalous skin effect (LHC, Ruggiero et al., NSLS, Podobedov)
-  NSLS upgrade calculation (5 W/m) for 4 K, 5 mm copper chamber would scale to about 2 W/m with current ALS parameters (lower current 400/500 mA, longer bunches THC)
-  Huge problem at 4K (cryocooler impossible)
-  ALS upgrade parameters (750 mA, potentially shorter bunches) would bring this to 7 W/m (just current increase) or up to 22 W/m (shorter bunches) at 4 K and more at 20 K
-  Even worse in two bunch operation (warm up required)
-  This does not include HOM, SR or normal heat conduction

- Lawrence Berkeley National Laboratory

## Resistive Wall/HOM heating II

-  High risk for cold chambers
-  Solutions (20 K, larger gaps than 5 mm, maybe different material with smaller anomalous skin effect, harmonic cavities, ...)
-  Risk is smaller for permanent magnet structure (resistive wall heating is actually worse, but heat can be effectively cooled). Solutions work at existing accelerators with beam parameters quite similar to ALS
-  Might become a problem with upgraded ALS parameters as well  Maybe thicker copper shield and better cooling connections needed

- Lawrence Berkeley National Laboratory

## Impedance/Instability Thresholds

-  Issue are single bunch instabilities (TMCI, microwave)
-  Can make beam quality bad enough (especially TMCI) to make beam useless (experience at APS, ALS, ...)
-  Issue in principle about equal for permanent magnet/superconducting device (maybe actually better for superconducting – resistive wall)
-  One device no danger for routine multibunch operation, but two-bunch mode might become impossible/severely limited in current
-  Fixed gap device of course is much higher risk, since it cannot be opened during two bunch mode (again have to evaluate risk for one beamline versus whole machine)
-  There are possible solutions (feedbacks, THC), but require lots of R&D

- Lawrence Berkeley National Laboratory

## Vacuum Issues

-  Cold surface acts as cryo-pump
-  Even small amounts of synchrotron radiation (and potentially the RF-fields – resistive wall heating) can scrub the surface
-  Need gas traps ...
-  Effective outside pumping is more difficult in superconducting case than  for permanent magnet Ids which are open to the side
-  Might pose radiation danger (Bremsstrahlung) and lifetime problem
-  Bake-out proves to be no problem for permanent magnet device, no clear answer yet for superconducting device
-  Overall: R&D issue and risk for superconducting device, proven to be no problem for permanent magnet one

- Lawrence Berkeley National Laboratory

## Radiation Damage

-  Probably higher risk for permanent magnet one (demagnetization) than for superconducting (isolation damage, quenches)
-  Lots of measurements exist and experience at Spring-8 (5-8 mm gaps, 8 GeV, NdFeB), SLS (5 –6.5 mm, 2.4 GeV, NdFeB, no collimators) and ESRF (6 GeV, 5 mm, SmCo)
-  Problem so far only with accidents (ESRF, APS) and only for out of vacuum devices !
-  Risk is not so much stored beam operation, but top-up.
-  Needs to be addressed for ALS upgrade.
-  No answer and no experience available for superconducting devices with small gap. Frequent quenches ?
-  Probably collimation and monitoring/interlocks can solve problem for both cases

- Lawrence Berkeley National Laboratory

## Performance

-  Because of the limit on the maximum field (vertical emittance increase) at 1.5 T or below and the minimum photon energy requirement (250 eV), the performance advantage of the superconducting device seems small for this application (can be much bigger in other cases)
-  There is some slight uncertainty about the exact performance of the permanent magnet device (about 10%) -> Steve, Sumitomo
-  Permanent magnet device could profit in performance if at a later time we allow to reduce the gap even below 5 mm, fixed gap superconducting device could not

- Lawrence Berkeley National Laboratory

## Cost/Schedule

-  Cost and schedule seem to be to the advantage of the permanent magnet device. It is available commercially and has significantly less R&D issues. Superconducting devices are available somewhat commercially, but not tested and not for our beam and magnet parameter range.

- Lawrence Berkeley National Laboratory

## Summary

-  Small gap (about 5 mm vacuum gap, or tbd if centered in straight) permanent magnet undulator OK
-  Some R&D issues left (field, radiation damage)
-  Fixed gap cold vacuum chamber needs significantly more R&D
- Resistive wall chamber heating might be too high even at 20K
- Instability thresholds (TMCI, microwave) could make two-bunch running impossible
- Vacuum issues are not solved

- Lawrence Berkeley National Laboratory