Slotted Waveguide Slow-Wave Stochastic Cooling Arrays
- Fermilab
The slotted waveguide slow-wave stochastic cooling arrays are an integral part of the 4-8 GHz Debuncher Upgrade at FNAL. Unlike the standard array of stripline electrodes, these structures are designed to work when the beam pipe can support many microwave modes. The design theory and beam measurement results of this new type of pickup structure will be presented in this paper. In previous collider runs at Fermilab, all of the stochastic cooling pickup and kicker arrays consisted of stripline or planar loop electrodes. The signals from these electrodes are combined with a binary combiner tree formed by microstrip or stripline transmission lines. With a binary combining scheme, there must be no waveguide modes traveling down the beam pipe that would provide an alternate signal path in parallel to the binary combiner tree. The nominal Debuncher transverse aperture is 30 p-mm-mrad (95% un-normalized). To account for closed-orbit variations, the design aperture of the cooling arrays was set at 40 p-mm-mrad. With lattice beta functions on the order of 10 meters, the transverse dimensions of the beam pipe will be about 40 mm which will propagate waveguide modes above 4 GHz. The presence of travelling waveguide modes in the beam pipe will limit the workable fractional bandwidth of the cooling arrays. The solution was to divide the 4-8 GHz bandwidth into 4 narrower bands with each band having a bandwidth of about 1 GHz. The cooling arrays are built with slot coupled 'slow-wave' waveguide structures as shown in Figure 1. The structure consists of two rectangular waveguides that are coupled to a rectangular beam pipe by a series of slots. The transverse signal is derived from the difference between the two waveguides and the momentum signal is derived from the sum of the two waveguides. The image current that flows along the walls of the beam pipe due to a charged particle beam travelling in the center of the beam pipe excites electromagnetic magnetic waves in the slots which in turn excite travelling waveguide modes in the side waveguides and beampipe. Since the phase velocity of the unperturbed waveguide modes is faster than the beam velocity, the slots also act to 'slow down' the waveguide modes by multiple reflections so that the phase velocity of the waveguide modes along the structure matches the beam velocity.
- Research Organization:
- Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), High Energy Physics (HEP)
- DOE Contract Number:
- AC02-07CH11359
- OSTI ID:
- 984608
- Report Number(s):
- FERMILAB-PBAR-NOTE-626; oai:inspirehep.net:863482; TRN: US201016%%1521
- Country of Publication:
- United States
- Language:
- English
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