Generation of High-Power, Reversed-Cherenkov Wakefield Radiation in a Metamaterial Structure

We present the first demonstration of high-power, reversed-Cherenkov wakefield radiation by electron bunches passing through a metamaterial structure. The structure supports a fundamental transverse magnetic mode with a negative group velocity leading to reversed-Cherenkov radiation, which was clearly verified in the experiments. Single 45 nC electron bunches of 65 MeV traversing the structure generated up to 25 MW in 2 ns pulses at 11.4 GHz, in excellent agreement with theory. Two bunches of 85 nC with appropriate temporal spacing generated up to 80 MW by coherent wakefield superposition, the highest rf power that metamaterial structures ever experienced without damage. These results demonstrate the unique features of metamaterial structures that are very attractive for future high-gradient wakefield accelerators, including two-beam and collinear accelerators. Advantages include the high shunt impedance for high-power generation and high-gradient acceleration, the simple and rugged structure, and a large parameter space for optimization.

Figure 1

Schematic diagram of the experimental setup.

Figure 2

MTM structure design. (a) Alternating wagon wheel plates and spacer plates. Forty plates of each type are clamped together to form an 8 cm long structure. (b) Wagon wheel plate geometry.

Figure 3

Dispersion curve of the fundamental transverse magnetic mode intersecting with the beam line $\omega =k_z{v}_z$ at 11.42 GHz, where $k_z$ is the longitudinal wave number, and $v_z$ is the beam longitudinal velocity. The horizontal axis represents the phase advance per period as $\varphi =k_{z}p$, where $p=2\mathrm{mm}$ is the period. A bead pull measurement was done around the design frequency, as shown in the blow-up figure along with the simulation result.

Figure 4

Plot of the longitudinal electric field $E_z$. The two-dimensional field plot on the top shows the normalized $E_z$ field on the middle plane in the linear scale. The MTM structure is represented in grey. The one-dimensional plot on the bottom shows the $E_z$ field on the beam axis for a single 45 nC bunch. The distance is normalized to the longitudinal wavelength ${\lambda }_z$. In both figures, the electron bunch travels to the right. The peak accelerating field available to a trailing bunch (in the blue region) is $43\mathrm{MV}/\mathrm{m}$.

Figure 5

High-power microwave extraction from a single bunch. (a) Output microwave power in the two ports from a single 45 nC bunch. Solid lines: experiment, dashed lines: CST simulations. (b) Frequency spectrum. (c) Comparison of experiment and analytical theory of the extracted microwave power as a function of the transmitted charge.

Figure 6

Experimental measurements of backward power with two bunches. Voltage signal from (a) a single bunch, (b) two bunches with 0 deg phase difference, (c) two bunches with 180 deg phase difference. (d) Highest rf power from two bunches in phase with a total charge of 85 nC in perfect agreement with the CST PIC simulation.