Abstract
We investigated a corrugated mm-scale capillary as a compact accelerating structure in the driver-witness acceleration scheme, and suggested a methodology to measure the acceleration of the witness bunch. The accelerating fields produced by the driver bunch and the energy spread of the witness bunch in a corrugated capillary and in a capillary with a constant inner radius were measured and simulated for both on-axis and off-axis beam propagation. Our simulations predicted a change in the accelerating field structure for the corrugated capillary. Also, an approximately twofold increase of the witness bunch energy gain on the first accelerating cycle was expected for both capillaries for the off-axis beam propagation. These results were confirmed in the experiment, and the maximum measured acceleration of $170\text{ }\text{ }\mathrm{keV}/\mathrm{m}$ at 20 pC driver beam charge was achieved for off-axis beam propagation. The driver bunch showed an increase in energy spread of up to 11%, depending on the capillary geometry and beam propagation, with a suppression of the longitudinal energy spread in the witness bunch of up to 15%.
Highlights
Dielectric wakefield acceleration (DWA) in planar and circular dielectric structures has seen major developments in understanding the maximum achievable accelerating gradients [1], dielectric breakdown limits [2] as well as applications for beam compression [3], modulation [4] and correlated energy spread compensation [5]
The accelerating fields produced by the driver bunch and the energy spread of the witness bunch in a corrugated capillary and in a capillary with a constant inner radius were measured and simulated for both on-axis and off-axis beam propagation
The driver bunch showed an increase in energy spread of up to 11%, depending on the capillary geometry and beam propagation, with a suppression of the longitudinal energy spread in the witness bunch of up to 15%
Summary
Dielectric wakefield acceleration (DWA) in planar and circular dielectric structures has seen major developments in understanding the maximum achievable accelerating gradients [1], dielectric breakdown limits [2] as well as applications for beam compression [3], modulation [4] and correlated energy spread compensation [5]. The Cherenkov radiation and SPR follow a dispersion relation, cos Θ 1⁄4 2πm þ pffi1ffiffiffiffiffiffiffiffiffiffi ; ð2Þ kd β εdðωÞ where Θ is the azimuthal angle with respect to the bunch velocity, β normalized bunch velocity, k wave number in the dielectric, d groove period, m diffraction order and εdðωÞ dielectric permittivity as a function of frequency. The theory gives a clear indication that high intensity fields with a distinct frequency spectrum can be produced inside a vacuum channel once the radiation is reflected back from the waveguide cladding and diffracted at the corrugation. We compare the measurements and simulations of the witness bunch acceleration and energy spread of both witness and driver bunches in corrugated and conventional dielectric lined capillaries with the same inner radius. The term mm-scale is used to differentiate the cylindrical dielectric structures with typical transverse sizes in the order of a millimeter [16] from capillaries with aperture sizes in the order of hundreds of micrometers [17,18]
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