Damping Ring Kickers for the Next Linear Collider

The Next Linear Collider (NLC) uses a damping ring for the electron beam, a pre-damping ring and a main damping ring for the positron beam to reduce the beam emittances. The requirements of the main damping ring kickers are to bend a 2 GeV beam by an angle of 2 mrad over a length of 1.2 m. This results in a required field of 139 G. The magnet aperture is 30×30 mm. The predamping ring kicker requirements are based on a 2 GeV beam with a bend angle of 8 mrad in 1.2 m, or a field of 308 G. The magnet aperture is 62×45 mm (H×V). A pulse width is 130 ns with rise and fall times of less than 60 ns is the same for both the pre-damping ring and main damping ring kickers. The three rings operate at a 180 Hz repetition rate. The kicker magnets being developed to meet these requirements consist of two strip line conductors in the vacuum chamber, for the pre-damping ring kickers they may be loaded with ferrite, to give a matching impedance of 25 Ω. The buses are separated from magnetic flux linkage by a grounded flux excluder, which also serves as a low impedance return for the beam current. Both busses of the magnet are driven in parallel from the same modulator and are grounded at the end opposite the feed. The modulator uses two IGBT stacks which both act as opening switches in order to meet the rise time requirements. 1. Damping Ring Kicker Magnets Several types of magnets were considered to meet the difficult rise time parameters for the damping ring kickers. An air core, strip line, matched impedance magnet was decided upon because of its simplicity, low cost, and it uses no non-linear materials. The magnet is made from a slotted pipe which is housed in the vacuum chamber. A metal strip is brazed onto the top and bottom of the slotted pipe to prevent magnetic coupling between the two busses during the pulse rise time. A drawing of the magnet is shown in Figure 1. This magnet was analyzed with the electro-magnetic field solver MAXWELL. The impedance of the magnet was calculated to be 32 Ω, and the one way transit time to be 3.49 ns per meter. A plot of the B field magnitude with a drive current of one ampere is shown in Figure 2. *Work supported by Department of Energy contract DEAC03-76SF515. Figure 1. Proto type slotted kicker magnet. Figure 2. Calculated B field magnitude. A one meter long, stainless steel magnet was built and tested. An oscillograph of the magnet current, B dot, and the B field are shown in Figure 3. It can be noted from this data that the current in the magnet is constant while the B field rises as time progresses. After eliminating the B dot probe, electric fields, digitization error, and other measurements errors, it was determined that the slope of the B field might be explained by skin depth effects caused by the stainless steel. Because the resistivity of stainless is approximately fifty times greater than copper, the steel magnet structure will allow the currents to penetrate further into the conductors, essentially increasing the width of the slits, and hence the magnetic field. The magnetic filed was recalculated allowing the currents to penetrate further into the magnet busses by increasing the permeability of the conductors. It was found that the magnetic field intensity increased by a 0-7803-5573-3/99/$10.00@1999 IEEE. 1500 Proceedings of the 1999 Particle Accelerator Conference, New York, 1999