A segmented electrode, which is placed at the thruster exit, is shown to affect thruster operation in several ways, whether the electrode produce low emission current or no emission current, although there appear to be advantages to the more emissive segmented electrode. Measured by plume divergence, the performance of Hall thruster operation, even with only one power supply, can approach or surpass that of non segmented operation over a range of parameter regimes, including the low gas rate regime. This allows the flexibility in operation of segmented electrode thrusters in variable thrust regimes. 2

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First experimental measurements are presented for the kink instability in a linear plasma column which is insulated from an axial boundary by finite sheath resistivity. Instability threshold below the classical Kruskal-Shafranov threshold, axially asymmetric mode structure and rotation are observed. These are accurately reproduced by a recent kink theory, which includes axial plasma flow and one end of the plasma column that is free to move due to a non-line-tied boundary condition. The current driven kink instability is a magnetohydrodynamic (MHD) instability which affects current carrying plasmas in Nature and laboratory. The kink mode structure and stability condition are strongly dependent on the system geometry and the boundary conditions (BCs). Kruskal[1] and Shafranov [2] (hereafter referred to as KS) considered first the ideal MHD stability of a cylindrical plasma column with magnetic field components (0, B θ , B z ) using cylindrical coordinates (r, θ, z). For an infinitely long (equivalent to periodic axial BCs) column, they obtained a linearly unstable helical kink mode of structure ξ = e i(θ+2πz/L) when the plasma current I p exceeds the Kruskal-Shafranov limitwhere a and L are, respectively, the radius and length of the current channel, and ξ is the displacement of the plasma column from the equilibrium position. The KS theory has been quite successful in predicting the behavior of toroidal plasmas for which the periodic BCs yield a proper accounting for the finite length of the system. In linear systems, however, substantial deviations from KS predictions can result from different axial BCs. The importance of the BCs has long been recognized [3,4] and is of particular relevance to the stability of linetied flux ropes in space physics (c.f. Ref. [5] and survey Ref.[6]), and astrophysical jets [7]. In recent years, there has been a renewed interest in the stability of a line-tied plasma column in laboratory devices (see Refs. [8,9,10] and references therein). The kink stability of a plasma column with line-tied ends has been investigated in linear devices, where line-tying is attributed to the presence of highly conducting end plates [11], and in open systems to a local discontinuity for the Alfvén velocity that forms a virtual boundary around the system [12,13].In this Letter, we experimentally investigate the external kink instability in conditions where one end of the plasma column is line-tied to the plasma source, and the other end is not line-tied and therefore free to slide over the surface of the end-plate. The latter BC is a result of plasma sheath resistance that insulates, at least partially, the plasma from the end-plate. Compared to the (6) external anode. On the right, an axial cut near the external anode. The fast camera (7) is located at the midplane and views the plasma column along the x direction. The bi-dimensional magnetic probe (8) measures (δBx, δBy) at the edge. Also schematically shown is the plasma column whose end rotates at the external anode.line-tied case, we find signi...

Variable plasma jet velocity with low beam divergence over a range of mass flow rates can be achieved through segmented electrode operation of Hall plasma accelerator. With the use of just a cathode side electrode at the cathode potential, the beam divergence can be decreased substantially, at some cost in efficiency. However, the additional use of an anode side electrode retains the same reduced plume divergence, but at efficiencies comparable to the non-segmented operation. The high efficiency persists also when the anode side electrode is biased at an intermediate potential, thus producing two-stage Hall accelerator operation.2