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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 TM-like 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.

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An electron beam passing through a metamaterial structure is predicted to generate reversed Cherenkov radiation, an unusual and potentially very useful property. We present an experimental test of this phenomenon using an intense electron beam passing through a metamaterial loaded waveguide. Power levels of up to 5 MW are observed in backward wave modes at a frequency of 2.40 GHz using a one microsecond pulsed electron beam of 490 keV, 84 A in a 400 G magnetic field. Contrary to expectations, the output power is not generated in the Cherenkov mode. Instead, the presence of the magnetic field, which is required to transport the electron beam, induces a Cherenkov-cyclotron (or anomalous Doppler) instability at a frequency equal to the Cherenkov frequency minus the cyclotron frequency. Nonlinear simulations indicate that the Cherenkov-cyclotron mode should dominate over the Cherenkov instability at a lower magnetic field where the highest output power is obtained. DOI: 10.1103/PhysRevLett.117.237701 An electron beam interacting with a metamaterial (MTM) structure generates Cherenkov radiation in the backward direction, which is the reverse of the forward traveling Cherenkov radiation generated in conventional media [1][2][3][4]. This property is of great fundamental interest, but it may also prove to be of value in applications such as detectors in particle physics [5] or in microwave generation [6][7][8][9][10][11][12][13]. There have been very few experiments on the generation of reversed Cherenkov radiation. One experimental observation was carried out using electron bunches from an accelerator passing through a metamaterial waveguide, producing emission in the left-handed frequency band of the MTM at 10 GHz [14]. A second experiment used a phased electromagnetic dipole array to simulate a moving charged particle beam, producing reversed Cherenkov radiation in the 8.1 to 9.5 GHz range [15]. These previous experiments, however, have not investigated the interaction of a continuous electron beam with a metamaterial structure, which is of great interest for sources of microwave generation. Our experiments show that when a magnetic field is used to transport the electron beam, the reverse Cherenkov instability is not the dominant effect observed. Instead, the Cherenkov-cyclotron instability dominates over the Cherenkov instability. This result has significant implications for any practical application of metamaterials in microwave generation.High power microwave (HPM) sources are widely used in radar, defense, accelerator, and industrial applications; many examples are given in Refs. [16][17][18]. Modern particle in cell (PIC) codes have been developed that allow the design of HPM devices with full 3D treatment of both the electron beam and the electromagnetic wave [19,20]. Another important development are the advances in electromagnetics, including extensive research on novel photonic and metamaterial structures. These structures open up new possibilities for the design of HPM devices.Metamaterials are artifici...