MPD Arcjet Research Articles

Recombination lasing in a magnetoplasmadynamic arcjet

The plasmadynamic recombination laser concept is verified experimentally in a high-power quasisteady MPD arcjet operating at 4 kA and 12 g/sec of argon. Measurements of the spatial variation of electron temperature electron density, and population densities in the arc exhaust flow confirm that inverted populations of the 4p to 4s ArII transitions are established by collisional-radiative recombination of the ArIII ion. Using an optical cavity aligned transversely to the flow, recombination lasing of four such transitions, 5145, 4880, 4764, and 4727 Å, is observed spectrophotographically and photoelectrically over the entire 1-msec discharge.

Effect of Applied Magnetic Fields on Physical Processes in an MPD Arcjet

which produced an essentially axial slowly diverging magnetic field in the electrode region. Argon was used as the propellant and the background pressure was maintained at about 5 x 10 ~2 torr. In the present experiments, discharge current (/) was varied from 300 to 1500 A and propellant mass flow rate (w), from 15 to 75 mg/sec, at applied magnetic field (B), measured at cathode tip, up to about 4000 gauss. The electrode assembly was mounted on a parallelogrampendulum thrust stand, and the electrical power for the arc was brought onto the stand through mercury pots. Deflection of the stand was sensed by a linear differential transformer. The total reaction force (thrust), Tty was measured, mounting the magnetic coil on electrode assembly. The reaction force which acts on the electrode assembly, rea, was measured, attaching the magnetic coil to the coil holder connected rigidly with a vacuum tank. In this case, the interaction force between current passing through the electrode assembly and applied magnetic field was excluded, evaluating it by reversing the direction of magnetic field. To check this technique, the interaction force was also measured at some operating points, using an electrode assembly in which the cathode is shorted to the anode. Using Tt and Tea, the reaction force which acts on the magnetic coil, Tmc, can be obtained from Tmc =Tt- TeSL. The current distribution on the cathode was examined, using the divided cathode, divided into central and outer parts, and operating the arcjet in quasisteady mode (operation time 1-3 msec). The current distribution on the anode surface was evaluated by furnishing electrically-insulated small electrodes (Fig. 2) on it. These electrodes were endurable in steady operation of the arcjet, and they were also used for the detection of rotating spokes.

Establishment of Quasi-Steady Operation in a Pulsed MPD Arcjet

XPERIMENTAL studies were made on the properties of the plasma stream emanating from a pulsed MPD arcjet, expecially with respect to the quasi-steady operation.14 From diagnostic measurements, a large fraction of the discharge current was found to extend in the plasma downstream of the electrodes. It has been questioned whether this extending current is desirable in the operation of an MPD arcjet. The purpose of the present work is to answer this question with respect to the establishment of the quasi-steady operation. The quasi-steadiness of the operation should be inferred from the properties of the plasma stream as well as the current and voltage of the arc discharge. Contents The pulsed MPD arcjet used in the present experiment is similar to that described in Ref. 1. The maximum discharge current is 12.5 ka and the maximum power is about 2 Mw. The discharge current and voltage are steady and last for more than 800 jusec. An annular coil is mounted concentric to the arcjet system with its center plane at the front surface of the anode. The maximum strength of the magnetic field is 20 kgauss at the center of the coil. The working gases are hydrogen, helium and argon. The mass-flow rate m is variable—for instance, from 50 mg/sec to 300 mg/sec for hydrogen. The MPD arcjet is installed on an end flange of a vacuum tank which is 1.5 m in diameter and 2.8 m in length. The metal surface inside the tank is completely covered with Mylar sheet in order to prevent the discharge current from flowing in the tank wall. The tank pressure before each run is about 10 ~5 torr. The working gas injection phenomena was studied to determine the trigger time of the arc discharge so that the amount of the neutral gas injected before triggering the arc discharge could be minimized, and also to find how a quasi-steady injection of the working gas is established in the discharge region. To this end, the pressure was measured in the jet of neutral gas by a newly developed fast ionization gauge.5 From the gauge signal, the gas pressure was found to rise in about 600 jusec after opening the gas ports and reach a constant value. A steady state lasts for 3 ~ 5 msec. Figure 1 demonstrates the time variation of the pressure distribution along the center line in the jet of neutral argon. The origin of time in this figure is taken arbitrarily and the normal trigger time is 1 msec in this case. As the time increases, the experimental curve tends to follow a Z~2 law. The coordinate Z is taken along the center line and measured from the anode surface. The pressure dependence on Z~2 law is predicted from the spherically expanding steady flow at frozen

Cathode Region of a Quasi-Steady MPD Arcjet

Arc luminosity in cathode region of quasi-steady MPD arc jet, using high speed photography and electric and magnetic field probes

Experiments using a 25-kw hollow cathode lithium vapor MPD arcjet

The performance is reported of a 25-kw, lithium fueled, applied field MPD arcjet which incorporates a unique feed system with an open-ended heat-pipe vaporizer and a hollow cathode. The arc typically operates at currents of 250-500 amp, voltages of 40-60 v, magnetic field strengths between 500 and 3000 gauss, and produces a highly ionized lithium beam which transports 70% of the input electrical power to the beam stop. The ambient tank pressures range as low as 2 X 10 ~ torr. A comparison of hollow cathode and conventional MPD arc performance is made and it is concluded that the hollow cathode arc is superior to the conventional design. Diagnostic experiments performed in the exhaust plume include the determination of axial ion and atom velocities by measurements of Doppler shifts with a scanning Fabry-Perot interferometer and of ion energies by use of electrostatic energy analyzers; the determination of ion temperatures by comparison of experimental and calculated spectral line profiles; the determination of electron temperatures and densities with electrostatic probes; and the determination of current densities by Rogowski loops. These measurements indicate that at a station 25 cm downstream of the arc head the directed ion velocity exceeds 2 X 10 cm/sec. At a station 90 cm downstream the plasma typically has an electron temperature of about 2.3 ev, a density of approximately 1.5 X 10 cm~, and under certain operating conditions more than 40% of the arc current extends 90-cm downstream of the arc head. The efficiency of a process that converts input electrical power into kinetic energy is discussed in terms of a model which sets the plasma into rotation with subsequent expansion in a magnetic field.

Spectroscopically measured velocity profiles of an MPD arcjet

Abstract : The plasma jet was investigated with respect to the Doppler shift of spectral lines resulting from macroscopic velocities in axial and rotational directions. (Author)

Measurements of the current density distribution in the exhaust of an MPD arcjet.

The plasma current patterns of an MPD arcjet were determined experimentally by detecting the current induced magnetic field changes with Hall-effect sensors. Insulated water-cooled probes were traversed radially and axially to define the magnetic field gradients that reflect the current density components jr, J0, and j2. The nature of the discharge was explored for variations in mass flow, power from 25-75 kw, and applied magnetic field up to 1500 gauss, using hydrogen, ammonia, nitrogen, and argon propellant gases. The surveys reveal a discharge plume characterized by a centralized cathode current, with restricted radial current flow, conforming in essence to a magnetic nozzle channeling shaped by the applied field. The coaxial anode current sheath extends over a considerable volume. An axial extension of the current occurs with increase in the applied magnetic field. However, less than 15% of the total current was detected in the plume of ammonia and argon, with 20-40% for nitrogen and hydrogen. Electromagnetic interactions in the portion of the beam that was probed account for only a small fraction of the MPD arcjet thrust. Current leakage within the nozzle may manifest azimuthal energy addition and special swirl probes were employed to search for this. Measurements taken with magnetic probes to evalua e azimuthal Hall-current flow did not display measurable levels.

Thrust from an MPD arcjet.

Thrust from an MPD arcjet.