The experimental results of a novel L-band two-cavity image charge focusing relativistic klystron amplifier (IRKA) are presented. A tungsten mesh array was used in the IRKA to focus the intense relativistic electron beams. The beam transport efficiency was equal to the transparency of the mesh array. A beam modulation coefficient of 85% was obtained with 43 kV modulation voltage at the input cavity gap. The extracted microwaves were 193 6 35 MW with a power conversion efficiency of 20.8%. [S0031-9007(98)06082-7] PACS numbers: 84.40.Fe, 52.75.VaAs one of the most intriguing high power microwave devices, intense beam relativistic klystron amplifier (RKA) attracts great interest from accelerator and directed energy communities [1][2][3][4][5][6][7]. Endeavors for understanding the processes in the device have led to fairly good comprehension of the physics [2][3][4][5][6][7][8][9][10][11]. Efforts for raising the performance of the device resulted in very high energy and/or peak power conversion efficiencies at L band [6,7]. However, a strong axial magnetic field was usually required to confine the intense relativistic electron beams (IREB) in a RKA, although the bunching process does not depend on the field.The magnetic field sets some constraints on a RKA. First, it decreases the overall energy conversion efficiency of the device dramatically because it usually consumes much more energy than that of the electron beam [12]. Second, it adds weight, volume, complexity, and cost to the RKA. An image charge focusing method [13] was suggested to replace the magnetic field system in a RKA to mitigate those restrictions [14].Two key points must be substantiated to support the image charge focusing relativistic klystron amplifier (IRKA) concept. They were the focusing abilities of the mesh array to the intense modulated electron beams and efficient beam bunching evolutions in the presence of a mesh array. A novel two-cavity IRKA was designed to demonstrate the above issues and to extract high power microwaves. The experimental results are reported below.The IRKA configuration is shown in Fig. 1. Power to generate the beam was supplied by an induction linac accelerator (LIA). A solid beam was extracted across a 1.5 cm acceleration gap through an anode mesh from a plasma emission cathode using a 4-cm-diam conical graphite surface. The transport system consisted of 26 meshes that comprised cell boundaries. Table I lists the cell lengths. There were 19 meshes between the center of the input and output cavities. The meshes were fabricated by stretching 0.005-cm-diam tungsten wire on a pattern of slots cut into the supporting rings. The width of the square openings between the wires of the anode mesh was less than 2 mm so that the applied electric field in the diode gap would be more uniform and the emittance of the extracted beam would be smaller. The transparency of the mesh was about 0.960, and that of the second to the 19th mesh was 0.974, and 0.965 for the remaining seven meshes. Geometric constraints of the mesh supports c...
