Beams Document 3592-v1

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Twisted Waveguide Accelerating Structures: Potential and Challenges

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Patrick G. Hurh
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Patrick G. Hurh
Document Created:
12 Apr 2010, 10:48
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12 Apr 2010, 10:48
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12 Apr 2010, 10:48
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Slowing the phase velocity of an electromagnetic wave in a low-loss resonant structure to the point that it matches the particle velocity is the basis for achieving particle acceleration. The properties of twisted waveguide structures were previously investigated and have shown that slow-wave accelerating fields could be excited within the structure in contrast to the straight structure where the phase velocities of electromagnetic waves are faster than the speed of light.

Traditionally, slow wave structures are constructed by employing reactive loading such as a periodic iris or a dielectric load. However, most practical systems use the periodic corrugation of the waveguide wall. On the contrary, the slow-wave nature in the twisted waveguide structures originates from the fact that the wave path could be elongated by twisting the waveguide, so the fast electromagnetic wave is actually travelling along a longer spiral path, while the slow particles are traveling along a straight path. Controlling the spiral path through the twist-rate can force the longitudinal velocity matching between the electromagnetic wave and the particles.

The conventional multi-cell cavity-based accelerator structures (for instance TESLA-type cavity) have a transverse cross-section that is continuously changed along the acceleration path. The non-uniform transverse cross section is the main reason for the expensive cost and the possibility of serious trapped modes phenomena happens in these cavity-based accelerator structures. On the contrary, the twisted waveguide structures have a uniform transverse cross section throughout the acceleration path which potentially eliminates the troubling trapped modes inside the twisted waveguide structures, offering better field uniformity along the acceleration path and so enhancing the beam stability. Unlike the periodic coupled cavities, the dispersion relation of the twisted structures is similar to that of regular hollow waveguides.

To build a practical accelerating cavity structure using the twisted waveguide, more development work is needed: cavity structure tuning, end wall effects, incorporating beam pipes and input power couplers, and HOM damping, etc.

In this talk, the practical aspects of making more complete twisted waveguide accelerating structures are discussed with the results of computer simulations along with the latest experimental results that have been carried out in SNS.

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