| Paper | Title | Page |
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| THPEA045 | Development of a Dielectric-loaded Accelerating Structure with Built-in Tunable Absorption Mechanism for High Order Modes | 3777 |
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As the dimensions of accelerating structures become smaller and beam intensities higher, the transverse wakefields driven by the beam become quite large with even a slight misalignment of the beam. These deflection modes can cause inter-bunch beam breakup and intra-bunch head-tail instabilities along the beam path. We propose a built-in tunable absorption mechanism for damping the parasitic transverse modes without affecting the operational modes in dielectric loaded accelerating (DLA) structures and wakefield power extractors. The new principle for HOM absorption is based on electron paramagnetic resonance. The dielectric tube of the DLA has to be doped with a material exhibiting high EPR, for example ruby, Al2O3 overdoped ~1% with Cr3+. The absorption frequency can be tuned by an external DC magnetic field to match the frequency of the transverse mode. At the resonance imaginary part of permeability becomes significant and the dielectric tube acts as an absorber for the transverse modes. The external DC magnetic field is solenoidal and has to have a magnitude of about 3 kG. This configuration in fact is desirable to focus the beam and provide additional control of beam break up. |
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| THPD062 | Argonne Wakefield Accelerator Facility (AWA) Upgrades | 4425 |
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The AWA Facility is dedicated to the study of advanced accelerator concepts based on electron beam driven wakefields. The facility employs an L-band photocathode RF gun to generate high charge short electron bunches, which are used to drive wakefields in dielectric loaded structures, as well as in metallic structures. Accelerating gradients as high as 100 MV/m have been reached in dielectric structures, and RF pulses of up to 44 MW have been generated at 7.8 GHz. In order to reach higher accelerating gradients and higher RF power levels, several upgrades are underway: (a) a new RF gun with higher QE photocathode will replace the present drive gun; (b) the existing RF gun will generate a witness beam to probe the wakefields; (c) three new 25 MW L-band RF power stations will be added to the facility; (d) five additional linac structures will bring the beam energy up from 15 MeV to 75 MeV. The drive beam will consist of bunch trains of up to 32 bunches, with up to 60 nC per bunch. The goal of future experiments is to reach accelerating gradients of several hundred MV/m and to extract RF pulses with GW power level. |
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| THPD066 | Observation of Wakefields in a Beam-Driven Photonic Band Gap Accelerating Structure | 4431 |
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Wakefield excitation has been experimentally studied in a 3-cell X-band standing wave Photonic Band Gap (PBG) accelerating structure. Major monopole (TM01- and TM02-like) and dipole (TM11- and TM12-like) modes were indentified and characterized by precisely controlling the position of beam injection. The quality factor Q of the dipole modes was measured to be ~10 times smaller than that of the accelerating mode. A charge sweep, up to 80 nC, has been performed, equivalent to ~30 MV/m accelerating field on axis. A variable delay low charge witness bunch following a high charge drive bunch was used to calibrate the gradient in the PBG structure by measuring its maximum energy gain and loss. Experimental results agree well with numerical simulations. |
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| THPD070 | Numerical and Experimental Studies of Dispersive, Active, and Nonlinear Media with Accelerator Applications | 4443 |
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Current advanced accelerator modeling applications require a more sophisticated treatment of dielectric and paramagnetic media properties than simply assuming a constant permittivity or permeability. So far active media have been described by a linear, frequency-dependent, single-frequency, scalar dielectric function. We have been developing algorithms to model the high frequency response of dispersive, active, and nonlinear media. The work described also has applications for modeling of other electromagnetic problems involving realistic dielectric and magnetic media. Results to be reported include treatment of multiple Lorentz resonances based on auxiliary differential equation, Fourier, and hybrid approaches. We will also report on recent measurements of paramagnetic active microwave materials using EPR spectroscopy. Comparison of the results to numerical simulations will be presented. |