We present the design of an electron velocity map imaging (VMI) spectrometer where the ionizing laser source propagates along the symmetry axis of the spectrometer. The co-axial geometry is useful in a variety of experiments, because it provides a unique 2-dimensional projection of the 3-dimensional electron momentum distribution. Initial simulations show that this co-axial VMI can work with both high energy (more than 100 eV) and low energy (tens of eV) electrons. We demonstrate the performance of this co-axial VMI spectrometer at the Linac Coherent Light Source X-ray Free Electron Laser facility.
An ion gun and detector system have been designed and tested to study energetic ion trajectories and single-particle confinement in the vacuum field and low-density plasma of the Compact Auburn Torsatron (CAT). Thermionic Li+ and Mg+ sources are used. The gun positioning is capable of two degrees of freedom, allowing selection of the pitch angle and initial flux surface. The injection energy of the ions is variable from 5 to 100 eV, allowing investigation of the orbits of particles with large Larmor radii. The ion currents of typical magnitude I=10 nA are detected with a movable paddle probe making use of a boxcar-averaging technique. The detected ion trajectories are compared with the predicted trajectories from a guiding center code that makes use of a detailed model of the CAT magnetic configuration. The ion gun and detector system will be used to study the effect of magnetic islands on energetic ion trajectories.
Torsatron and stellarator plasma devices rely on magnetic surface mapping to verify the detailed vacuum magnetic field structure. Two new techniques of magnetic surface mapping are under development on the compact Auburn torsatron. The first is a fast-rotating fluorescent wire (FRFW) system. Compared with the usual fluorescent screen, fluorescent rod, or the capacitive point probe techniques, the FRFW system has the advantage of greatly increasing the number of detectable toroidal transits and of minimizing surface mapping time. The second technique is a resistive wire system which provides an alternative fast-field mapping method.
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