LARP Phase II Secondary Rotatable Collimator Evolution of design changes

1. LARP Phase II Secondary Rotatable CollimatorEvolution of design changes 3rd March, 2010

2. Evolution of chamber design • The collimator vacuum chamber and transition regions have gone through numerous revisions. Keeping track of each can be daunting. This short presentation is to aid in understanding the key changes that have occurred. • The evolution being rather organic means the trapped modes studies have occurred at different points in the design resulting in simulations of different chamber geometries.

3. Initially… • Initially we intended to use a rectangular chamber similar to the CERN Phase I graphite collimators. • We intended to completely deal the back half of the chamber from the beam by use of longitudinal “sheaths” along the top and bottom of the jaw. Transition regions foils and plates would have also been used to completely seal the beam cavity. • Difficulties in sealing the jaw against the sheaths led to the investigation of removing the sheaths altogether. Trapped mode studies suggested that there would be no serious implications to removing the sheaths. • With no need to install sheaths a rectangular chamber was no longer needed which opened the possibility of using a cylindrical chamber design.

4. Cylindrical chamber • The cylindrical chamber offer several advantages: • A round removable flanged end cap to the chamber can facilitate easy access to the jaws after impact tests in TT66 without the need to cut metal (that may be radioactive). • Easier assembly (just a baseplate and top cover) • Greater rigidity inherent in cylinder and thinner walls making a lighter device that will heat up less due to beam shower.

5. Final Design with thin foils • It became evident that the Phase II collimator should have integrated BPMs to aid in aligning the jaws to the beam. • Such an addition no longer allows for room for the removable end cap. • But the other advantages to the cylindrical chamber still applicable • The transition foils were also greatly simplified to a mere single sheet of copper. Bench measurement and theoretical analysis concluded that the geometrical component to the impedance would be small compared to the resistive wall and therefore the transition regions details were not important (other than trapped modes to be studied). This minimalist RF foil was just large enough to “hide” the rotation mechanism from the image foil.

6. Final design with thick foils“Modified Current Design” • Trapped modes studies revealed that the thin foils resulted in greatly increased R/Q for longitudinal modes creating rather large beam heating of hundreds of Watts. • The solution is simply to widen the foils. This simple change results in over two orders of magnitude smaller longitudinal R/Q. (studies still ongoing) • No other changes to the design. Wide RF Foil

7. To clear up the nomenclature in Liling Xiao’s talk Previous Design Transition regions and ends similar to initial rectangular chamber but with cylindrical top. Current Design What is shown on slide 4 above. Using thin foils Modified Current Design Same as “Current Design” just with wider foils Same as “Current Design” just with wider foils.