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This discussion by M. Sullivan at the Super-B Factory Workshop in Hawaii in January 2004 explores the constraints and desires for the design of the detector beam pipe, including considerations such as material choice, smallest possible radius, SR masking and shielding, and HOM power.
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Detector Beam Pipe Diameter Discussion M. Sullivan Super-B Factory Workshop Hawaii January 19-22, 2004
Detector Beam Pipe Constraints and Desires • Vertex Desires Thinnest, lowest Z material Smallest possible radius • SR masking or shielding Many kWs of SR power in the superB designs • I2R Can the beam pipe absorb all of the power from I2R losses? • HOM
Vertexing Desires • Be is usually the material of choice for the beam pipe • Exotic suggestions: Diamond (probably more multiple scattering) Silicon (detector beam pipe) …? • Double layer (unfortunately) with water to cool the pipe (water layer increases effective thickness – more multiple scattering) • Smallest possible radius (see next talk)
SR Masking and Shielding • Synchrotron radiation fans from upstream magnets need to be kept to a minimum in order to allow for small radius beam pipes • Quadrupole radiation from the final focus quads also sets a limit on how small a beam pipe can be (there is a wall of x-rays down there!) • An accelerator design with many bunches and low beam emittance helps keep the beam size small in the final focus quads (Next slide) Important rule of thumb: 1 Watt of x-rays 1 Mrad/s
SR masking usually concentrates on blocking x-rays from the high sigma particles in the beam tails where the particle density is relatively low. However, as the beam pipe radius shrinks the masking must block x-rays that are coming from the core particles (keep in mind the beam size in the upstream final focus quads is 50-500 times larger than at the IP) Assumed beam-tails for SR background calculations for PEP-II
I2R power PEP-II now Itotal = 1.3+1.9 = 3.2 A r1 = 2.5 cm nb = 1320 sz = 12 mm P = 8 W for a 20 cm long pipe Observed power is about~600 W KEKB observed power is 100 W 21035 PEP-II Itotal = 4.8+11 = 15.8 A (x25) r1 = 2.0 cm? (x1.25) nb = 3400 (x0.39) sz = 4 mm (x5) Power should be about 60 times higher than now or ~480 W 11036 PEP-II Itotal = 10+23 = 33 A (x106) r1 = 1.0 cm? (x2.5) nb = 6800 (x0.19) sz = 1.5 mm (x22.6) Power should be about 1100 times higher than now or ~8800 W Resistive wall pwr ~ I2total/r1/nb/sz3/2
HOM power • The I2R losses grow rapidly with shrinking bunch length and increasing beam currents, but both PEP-II and KEKB see much more power than I2R already. Must be HOM power. • More difficult to predict and quantify. HOM power is also a strong function of the bunch length and beam currents. • Assuming a similar dependence as the I2R losses then the 2x1035 PEP-II machine would see 36 kW of power and the 1x1036 PEP-II would see 660 kW of power WOW!
Summary • The minimum detector beam pipe radius is largely controlled by the size of the beam upstream of the IP. As the beam size at the collision point gets smaller the size of the beam upstream of the IP gets larger. (At the high beam currents of the super B designs even quadrupole radiation has kWs of power). • SR generated by the final focus upstream magnets at some point produces a “wall” of x-rays as the beam pipe radius gets smaller when SR from the core beam (3-5 s) particles starts to be intercepted • I2R losses increase with increasing beam currents and shorter beam bunches but making the beam pipe shorter as the radius decreases can help • More difficult to predict and quantify, HOM power may be a bigger problem. Present machine beam pipes see significantly more power (10-50 times more) than calculated from just resistive wall losses.
