Results 1 - 10 of 78
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[en] Minimizing the energy deposited in the electron current in neutralized flows, such as in the Hall thruster, is examined. Modifying the electron current along the channel by inserting emitting electrodes, can enhance the efficiency. By employing variational methods, an optimal electron current distribution is found. The efficiency enhancement due to this effect, however, is shown to be small
[en] Propagation of extraordinary mode waves in nearly parallel stratified plasmas (magnetic beach geometry) is investigated. Since the plasma is unable to shield out the right circular component of the electric field, these waves are very heavily damped. The resonance region is treated by a boundary layer expansion which reduces the problem to an integrodifferential system in one dimension. It is proved analytically that for moderate to high density, waves incident from the high field side are totally absorbed with no reflected wave. At very low density some transmission is possible, where the transmission coefficients are being given correctly by cold plasma theory. Numerical solution of the integrodifferential system shows that the power deposition profile can differ significantly from that predicted from a local WKB theory
[en] Control of the electric field profile in the Hall Thruster through the positioning of an additional electrode along the channel is shown theoretically to enhance the efficiency. The reduction of the potential drop near the anode by use of the additional electrode increases the plasma density there, through the increase of the electron and ion transit times, causing the ionization in the vicinity of the anode to increase. The resulting separation of the ionization and acceleration regions increases the propellant and energy utilizations. An abrupt sonic transition is forced to occur at the axial location of the additional electrode, accompanied by the generation of a large (theoretically infinite) electric field. This ability to generate a large electric field at a specific location along the channel, in addition to the ability to specify the electric potential there, allows one further control of the electric field profile in the thruster. In particular, when the electron temperature is high, a large abrupt voltage drop is induced at the vicinity of the additional electrode, a voltage drop that can comprise a significant part of the applied voltage
[en] Measurements of heat flux into a probe in plasma, combined with measurement of electron temperature, are used to extract information about the plasma composition. In particular, such measurements in oxygen and nitrogen plasmas at a pressure of several millitorrs indicate that these plasmas are composed mostly of molecular ions. The measurement is based on comparing the rates of heating and cooling of a probe during its exposure to and isolation from the plasma flow. The measured heat flux into the negatively biased probe is in good agreement with the calculated heat flux carried by the impinging plasma ions
[en] Full Text:In low temperature weakly-ionized plasmas the electron temperature is usually determined by particle balance while the plasma density is determined by energy balance. Ashen the power deposited in the plasma is large enough so that neutrals are depleted, the electron temperature and the plasma density are coupled. We calculate analytically the plasma steady-state for a substantial neutral depletion. A case is presented in which, surprisingly, the plasma density QTRitdecreases when the power deposited in the plasma QTRitincreases. Implications to our Helicon plasma source are discussed
[en] It is shown that the net momentum delivered by the large electric field inside a one-dimensional double layer is zero. This is demonstrated through an analysis of the momentum balance in the double layer at the boundary between the ionosphere and the aurora cavity. For the recently observed double layer in a current-free plasma expanding along a divergent magnetic field, an analysis of the evolution of the radially averaged variables shows that the increase of plasma thrust results from the magnetic-field pressure balancing the plasma pressure in the direction of acceleration, rather than from electrostatic pressure
[en] The snowplow motion and the convective skin effect are shown to be the two opposite limits in a unified model of plasma pushing by magnetic pressure. During the snowplow motion the plasma is compressed to a high density in a thin layer and the ion velocity equals the shock velocity. If, on the other hand, the spatial scale of the density gradient is smaller than the ion skin depth, the magnetic field penetrates with a velocity much higher than the ion velocity and the plasma compression is small
[en] Ions in partially ionized argon, nitrogen, and helium gas discharges are accelerated across a magnetic field by an applied electric field, colliding with neutrals during the acceleration. The momentum delivered by the electric force to the ions, which is equal to the momentum carried by the mixed ion-neutral flow, is found by measuring the force exerted on a balance force meter by that flow exiting the discharge. The power deposited in the ions is calculated by measuring the ion flux and the accelerating voltage. The ratio of force over power is found for the three gases, while the gas flow rates and magnetic field intensities are varied over a wide range of values, resulting in a wide range of gas pressures and applied voltages. The measurements for the three different gases confirm our previous suggestion [G. Makrinich and A. Fruchtman, Appl. Phys. Lett. 95, 181504 (2009)] that the momentum delivered to the ions for a given power is enhanced by ion-neutral collisions during the acceleration and that this enhancement is proportional to the square root of the number of ion-neutral collisions.
[en] Neutral depletion can significantly affect the steady state of low temperature plasmas. Recent theoretical analyses predicted previously unexpected effects of neutral depletion in both collisional and collisionless regimes. In this paper we address the effect of the energy deposited in the neutral gas by a collisional plasma. The fraction of power deposited in the neutrals is shown to be independent of the amount of power. The first case we address is of a thermalized neutral gas. It is shown that a low heat conductivity of the neutral gas is followed by a high neutral temperature that results in a high neutral depletion even if the plasma pressure is small. In the second case neutrals are accelerated through charge exchange with ions leading to what we call neutral pumping, which is equivalent to ion pumping in a collisionless plasma. Neutral depletion is found in the second case for both a closed system (no net mass flow) and an open system (a finite mass flow). A thruster that employs a collisional plasma and pumped neutrals is compared with the thruster analyzed before that employs collisionless plasma.
[en] The dependencies in a gas discharge of the plasma density and of the neutral-gas depletion on the magnetic-field intensity and on the plasma particle flux are studied. It is shown that if plasma particle flux density outward of the discharge is fixed, varying the magnetic field intensity does not affect neutral-gas depletion. When there are plasma end losses along magnetic-field lines while neutral-gas is depleted, an increase in the magnetic field intensity results in a nonmonotonic plasma density profile across field lines. The plasma density then has a local minimum at the center of the discharge.