Return to top for description of conclusions.

Optimization: Pole Gap

Given the challenge in constructing a 3mm thin stainless steel support plate, the superferric wiggler pole gap should be raised by at least 10mm. The observed minimal dynamic aperture degradation at increased pole gap suggests that the gap could be raised even more, as long as the current can be increased to achieve the 25ms ILC damping time target. With a 1.67T peak field, this allows the wiggler pole gap to be raised to 98mm, leaving a suficient safety margin below the superconducting quench limit.

Optimization: Pole Width

The size of the dynamic aperture depends clearly and directly on the wiggler pole width. This allows the reduction of the wiggler width if the benefit of narrower poles outweighs the minimal-to-significant decrease in dynamic aperture that would happen at a reduced pole width. Therefore, as long as a smaller dynamic aperture is acceptable, the wiggler width could be reduced to save money. However, the increased engineering challenges of a narrower wiggler support structure and increased photon density on the vacuum chamber do present a serious barrier to reducing the pole width. Balancing the cost/benefit ratio of narrower poles against these challenges leads to a final recommendation to keep the wiggler pole width at 238mm--the same as the original CESR-c wigglers.

Optimization: Length and Peak Field

Reducing the number of poles in the ILC damping wigglers would produce instant cost-savings in the construction of 160 wigglers; however, a shorter wiggler is unlikely given the increase in horizontal emittance coming from stronger wiggler fields. However, since the radiation equilibrium horizontal emittance also depends on the wiggler period, a 12-pole wiggler with a shorter period could shift the emittance, 0.64nmrad, below the target of 0.60nmrad. Therefore, if the period of a 12-pole wiggler can be reduced to meet the target emittance, then a 12-pole ILC damping wiggler could work, otherwise the current 14-pole design should be used.

Optimization: Period, 12-poles

Given these dynamic aperture results, a 32cm wiggler looks to be above the period where the dynamic aperture starts to degrade. Reducing the period and pole number from 40cm with 14-poles to 32cm with 12-poles corresponds to a 31% reduction in total wiggler length which will greatly reduce the wiggler unit cost.

The damping time from a 32cm, 12-pole wiggler is 3% higher than the target; therefore, in a 200ms storage cycle, the beam will be damped for 7.8 damping times instead of 8 damping times. This will result in very little decrease in the radiation equilibrium emittance and should be an acceptable trade-off given the cost savings. A 31% reduction in total wiggler length corresponds to about a 25% reduction in the cost of this superferric wiggler. Compared to the modified CESR-c wiggler which was already twice the length of the CESR-c wiggler (which were $250,000/wigler), the cost of 160 SIOC wigglers in the ILC will save $20 million. These cost savings are considerable enough to recommend that the ILC damping wiggler be designed with 12-poles and a 32cm period.

Return to top for description of conclusions.

Created November 10, 2006
Modified November 15, 2006