ABSTRACT
A novel source for broadband millimeter-wave RF is being designed at Los Alamos National Laboratory utilizing the two-stream instability. This RF source offers the potential for consistent output power over a large bandwidth on a single device. The source is designed to use two co-axial electron beams coupled into solenoidal magnetic fields. This is accomplished using two cathodes independently modulated nested in each other. The electron beam energy from the innermost cathode will fall in the range between 15 and 20 keV with the beam energy from the outer cathode being 75%-95% of the inner beam energy. In order to efficiently match the beam from the outer cathode into the solenoidal magnetic field, shields are used around the cathodes to shape the electric field in the gun region to extend the electrostatic focal length of the beams for better coupling into the magnetic field. We present the design of electron guns and expected performance using Trak simulations.
ABSTRACT
Electron Cyclotron Resonance Ion Source (ECRIS) plasmas contain high-energy electrons and highly charged ions implying that only noninvasive methods such as optical emission spectroscopy are reliable in their characterization. A high-resolution spectrometer (10 pm FWHM at 632 nm) enabling the detection of weak emission lines has been developed at University of Jyväskylä, Department of Physics (JYFL) for this purpose. Diagnostics results probing the densities of ions, neutral atoms, and the temperature of the cold electron population in the JYFL 14 GHz ECRIS are described. For example, it has been observed that the cold electron temperature drops from 40 eV to 20 eV when the extraction voltage of the ion source is switched off, accompanied by two orders of magnitude decrease in Ar9+ optical emission intensity, suggesting that diagnostics results of ECRIS plasmas obtained without the extraction voltage are not depicting the plasma conditions of normal ECRIS operation. The relative changes of the plasma optical emission and the ion beam current have been measured in CW and amplitude modulation operation mode of microwave injection. It is concluded that in the CW mode, the ion currents could be limited by diffusion transport and electrostatic confinement of the ions rather than beam formation in the extraction region and subsequent transport. The high resolution of the spectrometer allows determining the ion temperature by measuring the Doppler broadening of the emission lines and subtracting the wavelength dependent instrumental broadening. The measured ion temperatures in the JYFL 14 GHz ECRIS are between 5 and 28 eV, depending on the plasma species and charge state. Gas mixing is shown to be an effective method to decrease the ion temperature of high charge state argon ions from 20 eV in pure argon discharge to 5 eV when mixed with oxygen.