Interaction of an Ultrarelativistic Electron Bunch Train with a $W$-Band Accelerating Structure: High Power and High Gradient

Electron beam interaction with high frequency structures (beyond microwave regime) has a great impact on future high energy frontier machines. We report on the generation of multimegawatt pulsed rf power at 91 GHz in a planar metallic accelerating structure driven by an ultrarelativistic electron bunch train. This slow-wave wakefield device can also be used for high gradient acceleration of electrons with a stable rf phase and amplitude which are controlled by manipulation of the bunch train. To achieve precise control of the rf pulse properties, a two-beam wakefield interferometry method was developed in which the rf pulse, due to the interference of the wakefields from the two bunches, was measured as a function of bunch separation. Measurements of the energy change of a trailing electron bunch as a function of the bunch separation confirmed the interferometry method.

Figure 1

Schematic of the experimental setup.

Figure 2

rf frequency measurement with the interferometer.

Figure 3

Comparison of measurements and simulations of (a) rf energy versus transmitted charge measured with ICT 2. The upper limit of the rf energy is calculated using the incident charge measured before the entrance of W-band structure, and the lower limit is calculated using the charge measured at the exit. (b) The energy distributions with and without the presence of the W-band structure for fixed charge ($\mathrm{ICT}1=3\mathrm{nC}$, $\mathrm{ICT}2=1\mathrm{nC}$).

Figure 4

(a) Measured rf energy vs the change in bunch separation, $\mathrm{\Delta }z\equiv z-z_0$, which fits a 3.3 mm wavelength cosine function ($z_0$ is the nominal bunch separation). Labels (1)–(4) correspond to different relative wakefield phases when the leading bunch and the trailing bunch add. The simulated rf pulse power envelope in cases (1)–(4) are also shown in the figure. (b) Comparison between the simulations and experimental observations of the energy distributions of two bunches after the W-band structure. In case (1), the trailing bunch was decelerated by the wakefield from the leading bunch; in case (3), the trailing bunch was accelerated.

Figure 5

Energy spectrometer measurements of the bunch train passing through the W-band structure, compared with the simulation results on three bunches of a 2 nC (effective charge) drive beam passing through the structure, with the wakefields adding up in phase.