Experimental observations of cyclotron instabilities relevant to the auroral kilometric radiation emission process

Summary form only given. Results are presented of an experimental and numerical investigation of radiation emissions from an electron beam with a horseshoe distribution in velocity space. This process is relevant to the phenomenon of auroral kilometric radiation (AKR) which occurs in the polar regions of the Earth's magnetosphere. In these regions of the auroral zone, particles are accelerated into the increasing magnetic field of the Earth's dipole. In the laboratory experiment an electron beam with an initial spread in velocity was injected into an increasing magnetic field leading to the formation of a horseshoe shaped velocity distribution, in simulation of the auroral process. This distribution function is expected to be unstable to emission of cyclotron radiation, and was therefore investigated as a potential mechanism for the generation of auroral kilometric radiation. Results were obtained from electron beams with energies of 75-80 keV and cyclotron frequencies of 4.45 and 11.7 GHz. The radiation emission was measured using a real time FFT spectrometer and was observed to be close to the cyclotron frequency. Measurements of the electron beam transport characteristics confirm that the horseshoe distribution was obtained in the experiment. Determination of the radiation antenna pattern at the output of the experiment allowed the complex TE mode structures excited by the beam to be analysed and numerical integration provided an estimate of the efficiency. At 11.7 GHz, multimode excitation with an efficiency of ~1% was observed whilst at the lower cyclotron frequency of 4.45 GHz the highest efficiency obtained was 2% corresponding to an output power of 19 kW. This was achieved using a cyclotron detuning of 2.5% below the cut-off frequency of the TE
_0,1
radiation mode in the apparatus. These results are in close agreement with the predictions of the 2D PiC code KARAT. The efficiency is also comparable with estimates for the AKR generation efficiency.