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246 Chapter 6 current densities are achievable and sometimes used. Third, the cathode insert can be heat shielded well in this geometry, which greatly reduces the radiation losses of the cathode at operating temperatures. This decreases the amount of power that must be deposited in the cathode to maintain the required temperature for electron emission. This reduces the cathode heating losses to a small fraction of the discharge power, significantly reducing the discharge loss of the plasma generator. Since nearly the entire discharge current runs through the orifice, the current density there is highest in the system, and a sufficient plasma density must be generated locally to carry the current. For the 25-A discharge case shown in Fig. 6-3, the plasma density in the orifice is on the order of 1014 cm–3. The discharge current flowing through the 2.5-mm-diameter orifice is described by I = neevA , (6.1-1) where ne is the plasma density, e is the electron charge, v is the electron drift velocity, and A is the cross-sectional area of the orifice. Solving for the drift velocity gives v= I =7.7 104 m/s <