16.522, Space Propulsion Prof. manuel martinez-sanchez Lecture 17: NotES ON HALL THrUSTErs NOTES ON HALL THRUSTERS 1. Introduction Hall thrusters are electrostatic ion accelerators in which the grid system( which serves in classical ion engines to anchor the negative charges used to accelerate the ions) is replaced with a relatively strong magnetic field perpendicular to the flow This magnetic field impedes the counterflow of electrons in the accelerating field and, as will be shown does away with the space-charge limitation which restricts the flow and thrust of ion engines Hall thrusters have had a curious and long history. Discovered in the early 1960s the u.s. (, and possibly independently in the ussr 2), they were later abandoned in the West when it became apparent that there were strong instabilities which could not be completely eliminated, and when some additional work in german indicated higher effective plasma collisionality than had been expected Contributing to the demise was the simultaneous successful development of gridded ion engines of the Kaufmann type which appeared to satisfy most high Av mission equirements then envisioned, and the promise of efficient hydrogen arcjets for the low and intermediate Av ranges. It later developed that many important missions optim in the isp range of 1000-2000 sec. not well covered by either of these types of e engines, and this remained an unmet requirement for many years at least in the Development continued, however, in the USSR, particularly at the Kurchatov configurations were evolved there, and it was realized that the instabilities while c Institute in Moscow (2), under A I Morozov s leadership. Progressively more efficient present and annoying did not materially interfere with performance. The Soviet generic name for this type of engine is Thruster with a Closed Electron Drift and two competing implementations have been developed known respectively as the Stationary Plasma Thruster(SPT)and the Thruster with an Anode Layer(tAl). The general principles are the same in both, and will be discussed below By the early 1980s these engines had achieved operational status in the USSR, and have since flow in over 50 missions, which, however, were until recently limited to elatively small total impulses. Starting in 1991, with the complete removal of the earlier communications barrier development has re-started in the West in the form of collaborative efforts with Russian teams aimed at improved life, lighter electronics and flight qualification to the generally more stringent Western standards. The strong interest shown by the user community stems from the ability of Hall thrusters to operate with fairly good efficiency (50%)in the hitherto difficult specific impulse range around 1500 sec. and also from their relative simplicity compared to ion 22, Space Propulsion ture 17 ge 1 of7
16.522, Space Propulsion Prof. Manuel Martinez-Sanchez Lecture 17: NOTES ON HALL THRUSTERS NOTES ON HALL THRUSTERS 1. Introduction Hall thrusters are electrostatic ion accelerators in which the grid system (which serves in classical ion engines to anchor the negative charges used to accelerate the ions) is replaced with a relatively strong magnetic field perpendicular to the flow. This magnetic field impedes the counterflow of electrons in the accelerating field, and, as will be shown, does away with the space-charge limitation which restricts the flow and thrust of ion engines. Hall thrusters have had a curious and long history. Discovered in the early 1960’s in the U.S. (1), and possibly independently in the USSR (2), they were later abandoned in the West when it became apparent that there were strong instabilities which could not be completely eliminated, and when some additional work in Germany (3) indicated higher effective plasma collisionality than had been expected. Contributing to the demise was the simultaneous successful development of gridded ion engines of the Kaufmann type, which appeared to satisfy most high ∆V mission equirements then envisioned, and the promise of efficient hydrogen arcjets for the low and intermediate ∆V ranges. It later developed that many important missions optimize in the Isp range of 1000-2000 sec., not well covered by either of these types of engines, and this remained an unmet requirement for many years, at least in the West. Development continued, however, in the USSR, particularly at the Kurchatov Institute in Moscow (2), under A.I. Morozov’s leadership. Progressively more efficient configurations were evolved there, and it was realized that the instabilities, while present and annoying, did not materially interfere with performance. The Soviet generic name for this type of engine is Thruster with a Closed Electron Drift, and two competing implementations have been developed, known respectively as the Stationary Plasma Thruster (SPT) and the Thruster with an Anode Layer (TAL). The general principles are the same in both, and will be discussed below. By the early 1980’s these engines had achieved operational status in the USSR, and have since flow in over 50 missions, which, however, were until recently limited to relatively small total impulses. Starting in 1991, with the complete removal of the earlier communications barrier, development has re-started in the West, in the form of collaborative efforts with Russian teams aimed at improved life, lighter electronics and flight qualification to the generally more stringent Western standards. The strong interest shown by the user community stems from the ability of Hall thrusters to operate with fairly good efficiency (≈50%) in the hitherto difficult specific impulse range around 1500 sec., and also from their relative simplicity compared to ion engines. 16.522, Space Propulsion Lecture 17 Prof. Manuel Martinez-Sanchez Page 1 of 7
Propellent Manifold Magnetic Coils and Anode Channel B Field B J Dialectric Walls of Annular Cavity Cathode Conceptual Schematic of the Hall Thruster gure
Figure 1
2. Hall Thruster Physics The Hall thruster schematic is shown in Fig. 1. It consist of a coaxial annular cay where plasma is created by passing current between the annular anode on the vity upstream end of an otherwise dielectric cavity and the externally located cathode The propellant enters this plasma cavity via an annular manifold at the anode. a radial magnetic field is applied, either by ring-shaped permanent magnets, or through coils and soft iron yokes. The magnetic field greatly slows down the axial mean velocity of the electrons, which due to the low collisionality prevailing, are forced to execute mostly E xB drift around the annulus, while being radially confined by sheaths on the insulating walls The ions meanwhile, are only weakly affected by the magnetic field, and, if the density is low enough that collisions are rare, are simply accelerated by the electrostatic field to an exit velocity (1) where V is the potential at the place where the ion is created( with respect to the outside potential). Because of the quasi-neutrality faciliated by the presence of the electrons, no space-charge limitation arises in this type of thruster(as opposed to gridded ion thrusters), and the acceleration distance can be several cm, compared to the typical 0. 5-1 mm gap used in ion engines. This flexibility is one of the main advantages of the Hall thruster. It also removes the strong thrust density limitation dictated by the Child-Langmuir law in ion engines To the extent that collisions do occur, but, more importantly because of electron electrons also travel axially across the b field under the influence of the applied axial E field. They are then collected by the upstream anode, and pumped by the power supply to an external cathode. The emitted electrons mainly join the accelerated ions to form a neutralized plasma beam, but, inevitably a fraction also diverts upstream into the accelerator section This fraction is to be minimized(this is the role of the magnetic field) because their acceleration to the anode potential is one of the devices loss mechanisms. On the other hand not all of this energy is lost to the anode because a good part of it is used to produce ionization of the injected neutral The name Hall Thruster"arises form the mechanism by which thrust forces are exerted on the solid parts of the engine. As indicated ions are simply accelerated by the electrostatic e field but since the ions are in a quasineutral plasma throughout an equal and opposite electrostatic force is exerted on the free electrons in tha plasma. In the presence of the radial magnetic field b, however these electrons are not free to accelerate towards the anode; instead, they drift"azimuthally (perpendicular to both e and B)at such a velocity as to generate an equal and opposite magnetic force on themselves. If we denote by x the forward axial direction (Fig. 2) the electrons end up balanced 16. 522, Space Propulsion Lecture 17 Prof. Manuel martinez Page 3 of 7
2. Hall Thruster Physics The Hall thruster schematic is shown in Fig. 1. It consist of a coaxial annular cavity where plasma is created by passing current between the annular anode on the upstream end of an otherwise dielectric cavity and the externally located cathode. The propellant enters this plasma cavity via an annular manifold at the anode. A radial magnetic field is applied, either by ring-shaped permanent magnets, or through coils and soft iron yokes. The magnetic field greatly slows down the axial mean velocity of the electrons, which, due to the low collisionality prevailing, are forced to execute mostly E × B drift around the annulus, while being radially confined by sheaths on the insulating walls. The ions meanwhile, are only weakly affected by the magnetic field, and, if the density is low enough that collisions are rare, are simply accelerated by the electrostatic field to an exit velocity ci = 2eV mi (1) where V is the potential at the place where the ion is created (with respect to the outside potential). Because of the quasi-neutrality faciliated by the presence of the electrons, no space-charge limitation arises in this type of thruster (as opposed to gridded ion thrusters), and the acceleration distance can be several cm, compared to the typical 0.5 - 1 mm gap used in ion engines. This flexibility is one of the main advantages of the Hall thruster. It also removes the strong thrust density limitation dictated by the Child-Langmuir law in ion engines. To the extent that collisions do occur, but, more importantly, because of electron scattering by a combination of plasma electrostatic fluctuations and wall collisions, electrons also travel axially across the B field under the influence of the applied axial E field. They are then collected by the upstream anode, and pumped by the power supply to an external cathode. The emitted electrons mainly join the accelerated ions to form a neutralized plasma beam, but, inevitably, a fraction also diverts upstream into the accelerator section. This fraction is to be minimized (this is the role of the magnetic field) because their acceleration to the anode potential is one of the device’s loss mechanisms. On the other hand, not all of this energy is lost to the anode, because a good part of it is used to produce ionization of the injected neutral gas. The name “Hall Thruster” arises form the mechanism by which thrust forces are G E field, but since the ions are in a quasineutral plasma throughout, exerted on the solid parts of the engine. As indicated, ions are simply accelerated by the electrostatic an equal and opposite electrostatic force is exerted on the free electrons in that plasma. In the presence of the radial magnetic field G B , however, these electrons are G E and not free to accelerate towards the anode; instead, they “drift” azimuthally (perpendicular to both G B ) at such a velocity as to generate an equal and opposite magnetic force on themselves. If we denote by x the forward axial direction (Fig. 2) the electrons end up balanced as 16.522, Space Propulsion Lecture 17 Prof. Manuel Martinez-Sanchez Page 3 of 7
E×B E Figure 2 B The ions have no such azimuthal drift (or have a very small one because their Gyro radius is larger than the device's length), and so a net azimuthal current density arises, called a hall current E -en Given a current JHall in a plasma, the magnetic (Lorentz) force density on it is f=jHall xb )and an equal and opposite force is exerted by the plasma currents on the magnetic structure). We then have -en and using Eg.(2), = +en E i. e. the same as the forward electrostatic force on the ions as it should be. The important point is that the structure is not electrostatically acted on(electric fields and electric pressures are too weak here), but magnetically through the Hall current hence the name the device is a hall thruster but an electrostatic accelerator a duality which has led to some confusion 3. Thrust Capability Unlike the case of a gridded ion engine, there is not a clear-cut upper limit to the obtainable thrust density in a Hall thruster. In principle increasing the mass flow rate through a given engine would increase thrust by the same factor, provided the same degree of ionization can be maintained the voltage is presumed to be kept the same, since this controls specific impulse fairly directly. The ion beam current and the electron current leaking into the accelerating channel would increase as the mass Prof. Manuel martinez-Sanchez Page 4 of 7
( ) −e/ [ e ] G B G υ G (2) E + × = 0 or G υ e = − Drift G B G E × B2 (3) ⎛ ⎜υ = − Ex ⎟ ⎞ e ϑ (4) ⎝ B ⎠ r The ions have no such azimuthal drift (or have a very small one, because their Gyro radius is larger than the device’s length), and so a net azimuthal current density arises, called a Hall current: G G j = −ene Drift υ G B G E × = en (5) e B2 Given a current Hall in a plasma, the magnetic (Lorentz) force density on it is = Hall × G j G B ) and an equal and opposite force is exerted by the plasma currents on G j G f the magnetic structure). We then have e e × Drift G υ G f = −en G B and using Eq. (2), G E G f = +ene (6) i.e., the same as the forward electrostatic force on the ions, as it should be. The important point is that the structure is not electrostatically acted on (electric fields and electric pressures are too weak here), but magnetically, through the Hall current - hence the name. The device is a Hall thruster, but an electrostatic accelerator, a duality which has led to some confusion. 3. Thrust Capability Unlike the case of a gridded ion engine, there is not a clear-cut upper limit to the obtainable thrust density in a Hall thruster. In principle, increasing the mass flow rate through a given engine would increase thrust by the same factor, provided the same degree of ionization can be maintained. The voltage is presumed to be kept the same, since this controls specific impulse fairly directly. The ion beam current and the electron current leaking into the accelerating channel would increase as the mass 16.522, Space Propulsion Prof. Manuel Martinez-Sanchez Lecture 17 Page 4 of 7
flow rate, so that the energetic ability of these electrons to ionize the increased neutral flow would be maintained Since no space change develops these increases in flow, current and power could go on indefinitely The above reasoning has ignored the various effects of collisions (a)As far as the electrons, as the flow rate, and hence the gas density increases, so does the scattering collision frequency ve=ny C Qen (ny=heavy particle density, mean electron speed, QeH=mean scattering cross-section). When this frequency becomes comparable to the gyro or cyclotron frequency o el the electrons can no longer maintain pure azimuthal drift because each collision will allow the electric field to nudge them in the direction of the anode by about one Larmor radius v/o correspondingly to a velocity u=e /B, which is the azimuthal drift velocity. Thus, electrons are now able to cross the magnetic barrier at a velocity E Dx B which increases with be, hence with nH and ultimately with nY. The electron density n, in the acceleration region is governed by the flow of ions from the upstream ionization region as shown in the schematic, and increases generally in the same proportion as mr or I. This means that the electron"leakage "current E (8) will increase faster than IB, due to the ny factor in Ue. As will be discussed later, one of the factors determining the overall efficiency n of the device and from the schematic, ignoring wall losses, 18=1,-leak,so I 1+L,/I 22, Space Propulsion re 17 5 of 7
x flow rate, so that the energetic ability of these electrons to ionize the increased neutral flow would be maintained. Since no space change develops these increases in flow, current and power could go on indefinitely. The above reasoning has ignored the various effects of collisions: (a) As far as the electrons, as the flow rate, and hence the gas density increases, so does the scattering collision frequency υe = nHceQEH (nH =heavy particle density, frequency becomes comparable to the gyro or cyclotron frequency ω ce = 8 π kTe me mean electron speed, QeH =mean scattering cross-section). When this eB c = the me electrons can no longer maintain pure azimuthal drift, because each collision will allow the electric field to nudge them in the direction of the anode by about one Larmor radius V / ω correspondingly to a velocity υ = Ex / B, which is the azimuthal c drift velocity. Thus, electrons are now able to cross the magnetic barrier at a velocity Ex e V υ ≅ (7) B ω c which increases with υ , hence with nH and ultimately with mÝ. The electron density e n in the acceleration region is governed by the flow of ions from the upstream e ionization region, as shown in the schematic, and increases generally in the same proportion as mÝ or IB . This means that the electron “leakage” current Ex e υ Ileak = ene B ω (8) c will increase faster than IB , due to the nH factor in υ . As will be discussed later, e one of the factors determining the overall efficiency η of the device η = IB a Ia and from the schematic, ignoring wall losses, IB = I − Ileak a , so 1 η =1 − Ileak a = I 1 + Ileak / I a B 16.522, Space Propulsion Lecture 17 Prof. Manuel Martinez-Sanchez Page 5 of 7