Performance Characteristics of an Ablative Gallium Electromagnetic Accelerator

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Performance Characteristics of an Ablative Gallium Electromagnetic Accelerator

Thomas, Robert E. / Burton, Rodney L. / Polzin, Kurt A.

Experimental performance measurements are presented for an ablative gallium electromagnetic accelerator driven by a $50 μ s$ flat-top current pulse, with gallium supplied to the discharge by evaporation of the central cathode. The arc impedance, exhaust velocity, and electron temperature are measured for discharge currents in the range of $6 - 24 kA$ and an electrode radius ratio of $r_{a} / r_{c} = 3.4$ . The arc voltage is found to vary linearly with the discharge current, giving an arc impedance of $6 - 7 m Ω$ . An exhaust velocity of $u_{e} = 16 km / s$ is found by using the mass bit and computing the electromagnetic thrust from the discharge current data. This value is invariant with the discharge current and is within the experimental error of the velocities measured using electrostatic probes. Triple probe measurements yield on-axis electron temperatures in the range of 0.8–3.8 eV and electron densities in the range of $0.16 - 2.1 \times 10^{22} m^{- 3}$ . Magnetic induction probe measurements in the interelectrode region yield a peak magnetic field of 0.8 T at 24 kA and no evidence of the spoking instability. An efficiency of 25% at 1600 s is calculated for $r_{a} / r_{c} = 3.4$ using the mass bit and discharge current data assuming the cathode has a flat tip. The efficiency scaling predicts that changing to a conical tip cathode and increasing the radius ratio $r_{a} / r_{c}$ to 10 will result in an efficiency of 52% at an $I_{sp}$ of 3200 s.