Miniature Plasma Cathode for High-Power Terahertz Orotron and Clinotron Oscillators

Summary form only given. The development of compact terahertz sources capable of output powers and efficiencies greater than 100 mW and 5%, respectively, will require electron beams of high current density (>50 A/cm²) and very small dimensions (~100 micron-scale). Generating such dense beams using thermionic emitters necessitates compressing the beam by about a factor of 10 which may negatively impact beam quality. Consequently, alignment and heat load on the interaction circuit from beam interception may be problematic. At modest current densities, the beam must be located very close to the interaction structure in order to achieve good beam-wave coupling and high efficiency. We propose development of miniature plasma cathodes capable of generating >100 A/cm² for pulsed Cerenkov and Smith-Purcell oscillators operating at terahertz frequencies. The advantage of this approach is that the beam could be located farther from the interaction circuit and still self-excite the oscillations. In these structures, the current density, j_st, required to start oscillation is given approximately by j_st(A/cm²) = 2middot10⁴(delta_s/H)(1/(L_int ² (cm)))(1+3beta₀)V_b(kV) where delta_s is the skin depth, H is the height of the beam above the structure, L_int is the interaction length, beta₀ is the Lorentz factor of the electrons, and V_b is the accelerating voltage applied to the beam. As an example, a copper circuit operated at 850 GHz and excited by a beam with current density of 100 A/cm² would drive a 5 mm-long structure at 2.5 times the start current. The design and initial test results on a miniature plasma gun with pulsed control grid will be presented. A 250 mA pencil beam with a diameter of 200 microns has been achieved, corresponding to a current density of 800 A/cm². Measurements of the- stability of the beam current will be presented, and fluctuations due to ion oscillations in the grid extraction region will be discussed