A study of wall effects on hall thruster operation
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An electron emission from the wall facing the plasma can reduce the sheath potential drop increasing the electron energy flux to the wall. As the flux of cold emitted electrons becomes comparable to the flux of incident plasma electrons the sheath saturates due to the space charge of slow emitted electrons. These effects were studied both theoretically and experimentally in laboratory plasmas and in applications to divertors of fusion devices. Recent theoretical studies have suggested that a space-charge saturated sheath regime can be also realized in Hall thrusters. Typical electron temperatures in these crossed-field discharge devices are on the order of cross-over energy of the secondary electron emission yield (SEE) from ceramic channel walls. Therefore, the walls can act as an effective energy sink limiting the electron temperature. We report here first direct experimental validation of these theoretical predictions by measuring plasma potential, electron temperature and density with emissive and biased probes. A 2 kW Hall thruster with a boron nitride ceramic channel was operated in the discharge voltage range of 200-800 V. A discharge voltage threshold (∼400 V) was found to separate two different plasma regimes of the thruster. Below this threshold, electron temperature and potential distributions yield a linear fit T e(z) ≈ 0.1 · φ(z) along the acceleration region. The maximum electron temperature, T emax, increases linearly with the voltage, but always exceeds (almost twice at 400 V) the cross-over energy of SEE (∼18 eV) expected for Maxwellian electron energy distribution (EDF). This result appears consistent with predictions of Ref. [6] that the EDF is depleted at high energy due to electron wall losses. Above 400 V, T emax saturates (∼50 eV), the acceleration region expands, but the linear fit is still valid outside the thruster. At the discharge voltages of larger than 500 V, T emax tends to increase again, but its placement appears outside the thruster. This result indicates that the wall sheaths are space-charge saturated.