Optical spectroscopy of plasma impurities in a megajoule-class Dense Plasma Focus

A Dense Plasma Focus (DPF¹) such as the MegaJOuLe Neutron Imaging Radiography² (MJOLNIR) is a coaxial deuterium plasma railgun that ends in a z-pinch to produce energetic neutrons. Careful consideration must be given to tracking and maintaining plasma temperatures that maximize neutron yield. Plasma contamination is one mechanism for reducing temperature and thereby reducing fusion events. It has been shown that copper from the anode is sputtered into the plasma during a DPF shot³. However, certain dopants may also increase neutron yield given the proper conditions⁴. The spatiotemporal location of impurities can be monitored with a spectrometer and compared with numerical results to explore the effect on neutron yield. We present a multi-point, time-gated optical spectrometer capable of differentiating these various mechanisms. Simulations are used to determine laboratory placement of a linear, 27-fiber bundle plug outside the MJOLNIR target chamber. On the detector end a micro-channel plate (MCP) provides light amplification and can be gated for a minimum of 10 nanosecond, which resolves the microsecond DPF dynamics.