Bunch stability during high-gradient wakefield generation in a dielectric-lined waveguide

Abstract

A recently developed analytic theory for wakefields generated when a charge bunch, or train of bunches, passes along a dielectric-lined waveguide is applied to examine stability issues for this system. Such a configuration could be the key element in a two-beam accelerator to create colliding TeV-range electron and positron beams. It is shown that a 587 MV/m peak acceleration gradient can be achieved when a train of five 2 nC, 1.0 GeV, 0.20 mm long drive bunches passes along the axis of an alumina-lined waveguide (ε=9.5) with inner and outer radii 0.50 and 5.0 mm. The energy gain of a 0.2 nC test bunch in a 4 m long stage under these conditions is shown to be 1.48 GeV, for an average acceleration gradient of 370 MV/m. However, when a drive bunch is injected parallel to, but displaced from, the waveguide axis, transverse (mainly dipole) forces cause the tail of the bunch to swerve sharply toward the waveguide wall (head-to-tail instability), and to intersect the wall after traversing a relatively short distance. These results suggest that successful exploitation of the appealing strong wakefields generated by a train of drive bunches will require some means of transverse stabilization, such as an array of externally applied focusing and defocusing quadrupoles.

Dielectric Wakefield Acceleration of a Relativistic Electron Beam in a Slab-Symmetric Dielectric Lined Waveguide

“…These recent advances have occurred in the context of cylindrical dielectric-lined waveguides, as this simple geometry maximizes the coupling of the beam,which must be well-focused, have high total charge Q, and short longitudinal extent σ z . Under the condition that the inner radius of the structure, a, be similar in size to σ z , the wake mode wavenumber scales as k m ≃ ω m /c ∝ a −1 , and the single bunch collective wakefield is proportional to (in cgs units) Qk 2 m ≃ Q/σ −2 z [5,6]. This scaling illustrates the connection between small dimensions in both the beam and the structure, and achieving high fields.…”

Theory of wakefields in a dielectric-filled cavity

“…In a dielectric-lined waveguide, an analytical theory has been developed for the enhancement of acceleration gradients by Park et al [17] using a train of beam bunches. However, it is unrealistic because of the infinite waveguide.…”

Femtosecond planar electron beam source for micron-scale dielectric wake field accelerator

“…The gradient produced may not exceed what can be achieved with a single bunch having a total charge equal to the sum of the charges of all the drive bunches; however, it may well be easier to create a train of bunches of moderate charge and high quality than to do so with a single bunch of large charge. A comprehensive theory of wake fields in cylindrical dielectric structures [3] and analysis of stability for this geometry [11] have both been recently published. It turns out that the use of the planar geometry (in 3D geometry rather than the 2D example of Fig.…”