THz cavities and injectors for compact electron acceleration using laser-driven THz sources

We present a design methodology for developing ultrasmall electron injectors and accelerators based on cascaded cavities excited by short multicycle THz pulses obtained from laser-driven THz generation schemes. Based on the developed concept for optimal coupling of the THz pulse, a THz electron injector and two accelerating stages are designed. The designed electron gun consists of a four cell cavity operating at 300 GHz and a door-knob waveguide to coaxial coupler. Moreover, special designs are proposed to mitigate the problem of thermal heat flow and induced mechanical stress to achieve a stable device. We demonstrated a gun based on cascaded cavities that is powered by only 1.1 mJ of THz energy in 300 cycles to accelerate electron bunches up to 250 keV. An additional two linac sections can be added with five and four cell cavities both operating at 300 GHz boosting the bunch energy up to 1.2 MeV using a 4-mJ THz pulse.

Figure 1

The cross-sectional view of the designed three and a half-cell gun.

Figure 2

The schematic view of a cavity. The iris is assumed as a two port network with its forward and backward waves.

Figure 3

The reflected energy from the cavity during the filling process normalized to the stored energy as a function of the reflection coefficient from the iris.

Figure 4

The optimum value for the coupling constant between the cavity and its coupler to minimize the required filling energy.

Figure 5

The door-knob waveguide-coaxial adapter.

Figure 6

The schematic illustration of the THz cavity concept with its coupler and UV laser.

Figure 7

Top: The electric field pattern of the $\pi$-mode and Bottom: the electric field distribution on the axis of the cavity.

Figure 8

The time domain simulation for the on-axis longitudinal electric field at the center of the fourth cell for an input power equal to 1.1 MW.

Figure 9

The final electron beam’s energy and the correlated energy spread as a function of the emission phase.

Figure 10

The particle energy as a function of distance through the ASTRA-designed electron gun system.

Figure 11

The transverse emittance (black) and RMS bunch length (blue) of the bunch at the gun exit are shown for different spot sizes.

Figure 12

The (a) longitudinal and (b) transverse phase-space of the electron bunch at gun’s exit as simulated in ASTRA.

Figure 13

The (a) front view, (b) side view, and (c) temporal profile of the bunch at gun’s exit as simulated in ASTRA.

Figure 14

Two accelerating stages designed as the linac to be installed after the gun.

Figure 15

Acceleration of the electron bunch in the (a) first and (b) second accelerating stages.

Figure 16

The (a) longitudinal and (b) transverse phase-spaces of the bunch exiting from the last accelerating stage as simulated in ASTRA.

Figure 17

Mechanical stress due to Lorentz forces for the gun.

Figure 18

Temperature distribution inside the gun at the end of the filling cycle.

Figure 19

The (a) longitudinal and (b) transverse wake potentials of the gun versus time after the first bunch.

Figure 20

The (a) longitudinal and (b) transverse wake potentials for a bunch flying along the gun.