private:writeups:hallthrusters
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private:writeups:hallthrusters [2020/08/06 14:53] admin [1. Breathing mode - Axial (m=0) ionization instability] |
private:writeups:hallthrusters [2020/08/06 15:59] (current) admin |
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| ====== Instabilities ====== | ====== Instabilities ====== | ||
| - | Several common low-frequency instabilities in HETs | + | Several common low-frequency instabilities in HETs. |
| + | |||
| + | Four main instabilities (plus a few lesser observed ones) | ||
| + | - Breathing mode (m=0 ionization instability) | ||
| + | - Spoke mode (m>1 ionization instability) | ||
| + | - Axial transit time oscillations (due to high electron resistivty) | ||
| + | - Azimuthal $E \times B$ electron drift instabilities (Related to strong $\nabla_z B_r$ gradients) | ||
| + | |||
| + | Impact HETs by | ||
| * these oscillations can indeed lead to violent conditions that could potentially damage the thruster discharge supply | * these oscillations can indeed lead to violent conditions that could potentially damage the thruster discharge supply | ||
| Line 233: | Line 241: | ||
| * Oscillating azimuthal electric field creates a $ v_z=$ $\frac{E_\theta \times B_r }{B_r^2}$ $=\frac{E_\theta}{B_r}$ axial drift velocity that is part of the ``anomalous transport" | * Oscillating azimuthal electric field creates a $ v_z=$ $\frac{E_\theta \times B_r }{B_r^2}$ $=\frac{E_\theta}{B_r}$ axial drift velocity that is part of the ``anomalous transport" | ||
| * Claims the detailed physics describing this instability are poorly explored. | * Claims the detailed physics describing this instability are poorly explored. | ||
| + | |||
| + | [Boeuf2017] | ||
| + | * density fluctuations moving azimuthally in the direction of the electron drift with a phase velocity of 0.2E/B. | ||
| + | * Dominant in the anode region | ||
| + | * plasma potential in that region appears to fluctuate in a helical fashion and that this could also play a role in driving cross-field electron transport | ||
| + | * A non-zero fluctuating azimuthal electric field $E_\theta$ results in an $E_\theta \times B_r$ axial drift. | ||
| + | * Nature of this instability is not completely clear. | ||
| + | * The CIV (Critical Ionization Velocity) concept implies that the rotation of the spoke is associated with a rotating double layer (charge separation) with an azimuthal potential drop on the order of the ionization energy of the gas, | ||
| + | * the presence of spokes does not seem to affect the performance of the thruster because it primarily effects the region near the anode and NOT the acceleration region. | ||
| [Me] | [Me] | ||
| * Maybe another way to think of this is an azimuthally rotating ionization region with an azimuthal structure $exp \left( i(m\theta - \omega t) \right) $ where the mode number, $m$, is an integer greater than 0. | * Maybe another way to think of this is an azimuthally rotating ionization region with an azimuthal structure $exp \left( i(m\theta - \omega t) \right) $ where the mode number, $m$, is an integer greater than 0. | ||
| - | ==== 3. Azimuthal drift instability related to axial gradients in $\nabla_z B_r$ ==== | + | ==== 3. Azimuthal |
| Line 251: | Line 268: | ||
| * For the purpose of stability, $\nabla_z B_r$ is designed to be positive along the discharge chamber, with the minimum magnetic field at the anode and maximum near the exit plane. | * For the purpose of stability, $\nabla_z B_r$ is designed to be positive along the discharge chamber, with the minimum magnetic field at the anode and maximum near the exit plane. | ||
| + | [Boeuf2017] | ||
| + | * large amplitude, small wavelength azimuthal oscillations of the azimuthal electric field | ||
| + | * result from the coupling between electron Bernstein waves and ion acoustic waves | ||
| - | ==== 4. Poorly understood ... ==== | ||
| - | [Choueiri2001] | + | ==== 4. Axial " |
| - | * Typically 20 to 100 kHz | + | |
| - | * Non-azimuthal | + | |
| - | * Poorly reported in literature, partially because of the overlap with the previous azimuthal instabilities. | + | |
| - | * Thought to be related to 1) neutral collisions or 2) ionization. | + | |
| - | + | ||
| - | + | ||
| - | ==== 5. Axial " | + | |
| [Choueiri2001] | [Choueiri2001] | ||
| Line 271: | Line 283: | ||
| * Well predicted by linear theory of gradient driven magnetosonic waves with a primarily axial propagation | * Well predicted by linear theory of gradient driven magnetosonic waves with a primarily axial propagation | ||
| * Due to the coupling of the "beam mode" ($k_z u_i$) with $\nabla_z B_r$ driven oscillations. | * Due to the coupling of the "beam mode" ($k_z u_i$) with $\nabla_z B_r$ driven oscillations. | ||
| + | |||
| + | [Boeuf2017] | ||
| + | * 100 to 500 kHz | ||
| + | * associated with the transit time of ions in the acceleration region | ||
| + | * The dispersion relation for these waves can be obtained by writing and linearizing a system of 1D fluid equations based on electron and ion continuity equations, collisionless momentum equation for cold ions, and a drift-diffusion momentum equation for electrons, with the constraint of constant current. | ||
| + | * amplitude of the oscillations was strongly dependent on the electron resistivity and increased with decreasing electron mobility, in total agreement with the resistive mode | ||
| + | * associated with important kinetic effects coupled with oscillations of the acceleration zone | ||
| + | * During the oscillations of the acceleration zone, some ions may “see” a total potential drop larger than the applied voltage while a group of low energy ions is formed. | ||
| + | * due to a too large electron resistivity | ||
| + | |||
| + | [Me] | ||
| + | * The dependence on large electron resistivity to me is counter intuitive. | ||
| + | |||
| + | ==== 5. Poorly understood and unnamed instability ... ==== | ||
| + | |||
| + | [Choueiri2001] | ||
| + | * Typically 20 to 100 kHz | ||
| + | * Non-azimuthal | ||
| + | * Poorly reported in literature, partially because of the overlap with the previous azimuthal instabilities. | ||
| + | * Thought to be related to 1) neutral collisions or 2) ionization. | ||
| + | |||
| Line 391: | Line 424: | ||
| ====== Magnetic field design ======= | ====== Magnetic field design ======= | ||
| - | * [Hofer2014] | + | [Hofer2014] |
| + | * While a complete understanding of electron dynamics is currently unavailable, | ||
| of Hall thruster design. | of Hall thruster design. | ||
| + | [Bouef2017] | ||
| + | * The radial electric field (location and magnitude) is directly related to | ||
| + | - Electron transport | ||
| + | - electric field distribution | ||
| + | - ionization | ||
| + | - ion beam divergence through the potential distribution (I don't get this) | ||
| + | - Wall erosion by ions | ||
| ====== Open issues related to HETs ====== | ====== Open issues related to HETs ====== | ||
| (Need citations...) | (Need citations...) | ||
| Line 406: | Line 446: | ||
| The goal of this section is to list common (or even uncommon) diagnostics associated with measuring " | The goal of this section is to list common (or even uncommon) diagnostics associated with measuring " | ||
| - | * Laser Induced Fluorescence (LIF) - Measures ion and atom (presumably neutral?) mean velocity. | + | * Laser Induced Fluorescence (LIF) - Measures ion and atom (presumably neutral?) mean velocity. |
| * Langmuir probes - temperature, | * Langmuir probes - temperature, | ||
| * Segmented electrodes - Used to study instabilities (need specifics) [Boeuf2017]. | * Segmented electrodes - Used to study instabilities (need specifics) [Boeuf2017]. | ||
| * "Fast imaging probes" | * "Fast imaging probes" | ||
| * Discharge current measurements - Measurement of total current, I_elec + I_ion. | * Discharge current measurements - Measurement of total current, I_elec + I_ion. | ||
| + | * Highspeed videos of exhaust. | ||
| + | |||
| + | |||
| + | ====== Ionization region ====== | ||
| + | |||
| + | ==== Length of ionization region ==== | ||
| + | |||
| + | [Boeuf2017] | ||
| + | * TODO | ||
| + | |||
| + | |||
| ====== Questions ====== | ====== Questions ====== | ||
| * Why is there no ion saturation limit to Hall thrusters? | * Why is there no ion saturation limit to Hall thrusters? | ||
| Line 422: | Line 473: | ||
| * [Boeuf2017] As the thruster size and power change, the current density does not change and stays on the order of 1000 A/m^2 in most Hall thrusters. | * [Boeuf2017] As the thruster size and power change, the current density does not change and stays on the order of 1000 A/m^2 in most Hall thrusters. | ||
| * Would secondary electrons generated by electron striking channel walls be an alternative source of electrons? | * Would secondary electrons generated by electron striking channel walls be an alternative source of electrons? | ||
| + | * There' | ||
private/writeups/hallthrusters.1596725599.txt.gz · Last modified: 2020/08/06 14:53 by admin
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