Advanced Collimator Prototype Results for the NLC

The Next Linear Collider accelerator will include a large (kilometer scale) and complex collimation system. The size and complexity of this system could be reduced if the collimator jaws could be made immune to damage from the electron beam. We describe two collimation systems currently under development. Construction of a prototype system which uses wheels that can be moved to a new position after damage is underway. The second system, currently in the R+D phase, uses rotors which are continuously reformed from a bath of liquid metal as the collimation surface. This second system allows operation at beam intensities which damage the jaws on every pulse. We describe results of tests on both systems. NLC COLLIMATION SYSTEM REQUIREMENTS The NLC requires collimation both before and after the main linacs. The post linac collimators in particular are technically challenging as the beam intensities and small spot sizes are sufficient to damage any solid material. Designs to avoid damage by increasing the beam size in the collimation section result in very long system lengths, and impracticably sensitive optical designs. We base the engineering designs here on a beam optics design which provides reasonable tolerances and TABLE 1. NLC beam parameters in the Collimation Section (maximum values) Parameter Value Beam Energy 500GeV Bunch Charge 1.4×10 Beam Emittance (normalized) 300 × 3 10m Spot Size (in x, x’ collimation system) 50 × 5 μm Bunch Length 145 μm Number of Bunches 95 Peak Current ~1200A Required Edge Stability ~5 μm Required surface roughness <1 μm RMS Collimator Gap (half gap) ~100 μm DAMAGE MECHANISMS Beam damage in materials results from melting and vaporization due to energy deposition. In addition, if the thermal stress caused by the temperature rise exceeds the yield strength (or fatigue strength for multiple shots) of the material, damage can also occur. Electromagnetic shower damage This is the most serious source of damage in most high energy accelerators. For each incident particle which intercepts the collimator surface, an electromagnetic cascade of particles is created over a depth of several radiation lengths. For the NLC beam parameters, peak materials temperatures of >10 C would be created at the peak of such a shower. We plan to avoid this damage mechanism by using a combination “spoiler” and “absorber” collimation scheme. The spoiler is a thin (~1/4 radiation length) object which produces a large transverse momentum spread in intercepted particles, but which does not absorb any significant fraction of the beam. Downstream optics are used to convert the transverse momentum spread to a large radial spread where a larger radius absorber can be used. Spoiler 1/4 radiation length Absorber ~20 radiation lengths Input beam with halo Halo with increased transverse momentum Reduced Halo FIGURE 1. Spoiler / Absorber combination Direct ionization damage Even without showering, the direct ionization loss of high energy particles is sufficient to destroy any material at full beam intensity and nominal beam size. Under normal operating conditions, only the beam halo will be intercepted, and the collimator should not be damaged. A Machine Protection System is used to dump errant pulses, however some percentage will still reach the collimation system. Collective Damage The high peak currents in the NLC can produce damage from collective beam effects, even without beam interception. Image current heating and direct electric field ionization are both significant damage sources for the NLC collimators. For small collimator jaw gaps (<100μm full gap) these effects can cause damage on every pulse. DAMAGE RESISTANT COLLIMATOR TECHNOLOGY Consumable Collimators These are collimators where the jaws can be moved to a new position a finite number of times (~1000) after being damaged by the beam. They are suitable for use under conditions where damage will occur on occasional errant pulses. We have considered schemes which involve rotating wheels, bars or tapes which can be moved after damage. Of these, the rotating wheels seem the most promising technology. Tapes, which can allow a larger number of damaging shots, but are considerably more complex mechanically, are being considered as a possible next generation solution.