Multicell accelerating structure driven by a lens-focused THz pulse

Recently, gradients on the order of $1\mathrm{GV}/\mathrm{m}$ have been obtained with single-cycle ($\sim 1\mathrm{ps}$) THz pulses produced by the conversion of high peak power laser radiation, in nonlinear crystals ($\sim 1\mathrm{mJ}$, 1 ps, up to 3% conversion efficiency). Such high intensity radiation can be utilized for charged particle acceleration. For efficient acceleration, a large number of accelerating cells have to be stacked together with delay lines in order to provide proper timing between accelerating fields and moving electrons. Additionally, THz pulses in individual cells need to be focused transversely to maximize the accelerating gradient. This focusing is currently done by the tapering of the waveguide. In this paper, we propose instead to use dielectric lenses for the robust focusing of THz pulses in accelerating cells. Each lens is made from the same material as the delay line in a monolithic unit. We have fabricated prototype units from quartz and silicon. Such an approach simplifies the fabrication and alignment of the multicell accelerating structure. We present a design in which a 100 microJoule THz pulse produces a 600 keV energy gain in a 5-mm long 10-cell accelerating structure for an ultrarelativistic electron. This approach can be extended to nonrelativistic particles.

Figure 1

Single-cycle THz pulse shape (a) and spectrum  (b).

Figure 2

Timing of THz pulses in the structure for acceleration synchronism.

Figure 3

Transverse focusing by tapering of the waveguide. THz pulse entering waveguide (a) THz pulse focused on the accelerating gap (b). Transverse focusing by a lens. THz pulse entering waveguide (c). THz pulse focused on the accelerating gap (d).

Figure 4

E-field distributions at the quartz lens while focusing the short THz pulse. From the time when the THz pulse arrives at the lens, $t=2.6\mathrm{ps}$ (a), focusing begins, $t=5.6\mathrm{ps}$ (b), THz pulse is close to being focused, $t=8.6\mathrm{ps}$ (c), and maximum of focusing, $t=9.6\mathrm{ps}$ (d).

Figure 5

Field enhancement at the focal plane of the lens for different lens materials. Red dashes are for quartz and blue for silicon.

Figure 6

E-field distributions of the THz pulse focusing in a tapered waveguide at consecutive time steps from the time the pulse arrives at the horn (a) to when it is focused (e).

Figure 7

Time structure of the initial THz pulse (black), the electric field at the focus of the lens (blue), and the electric field in the center of the tapered waveguide (red) for optimal focusing parameters, $L=5$, $d=0.4\mathrm{mm}$. Both plots are on the same time scale (20 ps) and are normalized to the amplitude of the incoming THz pulse.

Figure 8

Field enhancement at the focus of the tapered waveguide as a function of the width $d$ and length $L$.

Figure 9

Sketch of a multicell broadband accelerating structure based on dielectric delay sections and lenses.

Figure 10

Time snapshots of thereflection from the structure/vacuum interface.

Figure 11

Bunch acceleration in the structure: (a) Bunch acceleration for sequent time frames. (b) Cumulative energy gain along the beam path.

Figure 12

Parameter optimization scans. (a) Energy gain as a function of injection time. (b) Energy gain as a function of structure periodicity.

Figure 13

(a) 3D scanning microscope metrology of the fabricated structure. THz input and electron trajectory are schematically shown. (b) Optical image of the structure with depth profile lineout shown. (c) Depth profile along the lineout from (b).