The potential of hydrogen armature experiments

Unlike aluminum or copper plasma armatures, hydrogen armatures of the same temperature and electron density are virtually transparent to ultraviolet and are totally transparent to soft x-rays (∼ 20 eV). Thus, it may be feasible to observe significant interior plasma dynamics with an appropriate experimental device. Some possible experiments of this type are discussed. The typical rail gun armature is aluminum or copper and at typical number densities of 10
²⁰
- 10
²¹
/cm
³
and
T\sim 3-5
eV is opaque to radiation well into the kilovolt x-ray region. As a result, spectroscopic diagnostics cannot provide any information about the interior of the plasma. The principal contribution to the short mean-free path for light absorption in metal plasmas is the bound-free transitions. By contrast, at these densities and temperatures, hydrogen is almost completely ionized and thus the only significant absorption mechanism is inverse bremstrahlung. This leads to a mean free path for 10 eV radiation in hydrogen which is in the range of a centimeter. Thus, hydrogen armatures are transparent in the soft x-ray region and if a hydrogen armature rail gun can be developed, soft x-ray spectroscopy can be used to explore the internal physical phenomena. Two phenomena which are conjectured to exist in rail guns are arc spots and MHD or fluid turbulence. In the following, we suggest experiments in hydrogen armatures which should detect these phenomena if they are present.