RU2076384C1 - Plasma source of negative atomic ions - Google Patents

Abstract

FIELD: ion engineering. SUBSTANCE: plasma source has sealed electrical-discharge chamber with inlet and outlet holes. electrodes connected to power supplies, main ones being placed in chamber and accelerating electrode, behind its outlet hole. Chamber internal wall of length L is conducting in discharged region and is connected to power supply; pulsed power supplies are used for main and accelerating electrodes and for conducting surface; outlet hole is located in conducting wall and length L of conducting surface is found from condition L >2D, L<I, where D is width of electrical discharge chamber; I is length of discharge region. EFFECT: provision for obtaining electron-free pulsed beams of negative atomic ions directly at plasma source outlet. 2 dwg

Description

 The invention relates to the field of electrical engineering, in particular to electronic and gas-discharge devices, and can be used to create devices simulating the Earth's ionosphere conditions, in scientific studies of the characteristics of elementary processes in the collisions of negative ions with neutral and charged particles, as well as in the generation of atomic beams.

 Currently, the task of creating technological plasma sources of monoenergetic beams of negative low-energy ions (several eV) is very urgent. In the physical modeling of the effects of atomic oxygen fluxes on the outer airborne equipment of spacecraft operating in low Earth orbits, it is especially important to obtain ion beams without accompanying electrons, which complicate the modeling of natural conditions due to ionization and excitation.

 The authors are not aware of the plasma sources of atomic ion beams that do not contain an electronic component.

Plasma sources of negative ions are known. For example, a plasma source of negative oxygen ions (Danilina T.I. et al. PTE, 1968, No. 3, p. 158), based on the use of a Penning discharge with cold cathodes and ion extraction perpendicular to the magnetic field. It contains two cathodes of permanent magnet tips located in a sealed enclosure, a flat ferromagnetic circular anode with a hole in the center for ion extraction, and an accelerating electrode and an electrostatic lens. At a discharge current of 150 mA, an anode voltage of 500 V, and an accelerating potential of 2.3 kV, it provides a beam current of negative oxygen ions (O ^- ) of 50 μa with a concomitant electronic current of 30 mA.

The disadvantages of the source are the presence of an electronic component in the output beam, the current of which is 600 times higher than the beam current O ^- , as well as the comparative complexity of the design.

Closest to the proposed invention by the achieved effect is the plasma source of atomic hydrogen ions (H ^- ) chosen by us as a prototype [Antipov S.P. Elizarov L.I. Martynov M.I. Chesnokov V.M. PTE, 1984, N 4, p. 42-44] with axially symmetric geometry and radial magnetic field. The main elements of its design are: a sealed housing with an opening for introducing working gas, a heated hollow cathode located inside the housing, an auxiliary anode (used to ignite the discharge), a main anode in the form of a disk equipped with outlet openings near which an accelerating electrode is located, and a magnetic system, creating a radial magnetic field and consisting of internal and external magnetic poles, which are axially arranged hollow steel cylinders with a magnetic coil between them (ka Tod is in the inner pole). The discharge burns between the hollow cathode and the main anode. Ions are extracted in a direction perpendicular to the magnetic field through holes in the anode. The source with a discharge current of 50 A and an enclosed power of 3 kW allows to obtain an ion current of up to 0.7 A with a current of accompanying electrons of 1.2.1.4 A.

 The main disadvantage of this source is also the relatively large proportion of the electronic component in the beam of extracted particles. Another significant drawback is the design complexity, due to the need to use a magnetic coil, a heated cathode and an auxiliary anode.

 The technical effect of the plasma source of negative atomic ions of our design is to ensure that directly at the output of the plasma source, stable intense pulsed beams of negative ions of electronegative gases are obtained that practically do not contain accompanying electrons. The proposed source is structurally simple, compact, which allows it to be successfully used in laboratory facilities of limited size, simulating the conditions of the earth's ionosphere.

 The technical effect is achieved by the fact that the plasma source of negative atomic ions is a sealed electric discharge chamber with inlet and outlet openings, electrodes connected to the power sources: main electrodes placed in the chamber and accelerating one located behind its outlet. What is new in it is that the inner wall of the chamber is of length L in the discharge zone (instead of the common concept of “discharge gap”, the term “discharge zone” is used, i.e., the region where gas ionization occurs, since the constructive implementation of the invention when a conducting wall is the inner surface of one of the main electrodes) is conductive and connected to a power source, the power sources are selected pulsed, the outlet is placed in the conductive wall, and the length L of the conductive erhnosti found from the condition of L ≥ 2S, L <l, wherein the width of the discharge chamber D, l the length of the discharge zone.

When studying the electrokinetic characteristics of a gas-discharge plasma, we theoretically substantiated and experimentally confirmed the possibility of controlling the diffusion of charged particles in a decaying plasma at scales significantly exceeding the Debye radius r _d by changing the potential of the conducting plasma boundary. This made it possible to obtain, in a decaying plasma of pulse-periodic discharges in electronegative gases, duration-controlled fluxes to the wall of negative atomic ions that are practically free of electrons. The theoretical justification and experimental data are presented in the Appendix.

 In FIG. 1 shows a diagram of the proposed plasma source, where a sealed chamber 1 with input 2 and output 3 holes, the main electrodes 4 (cathode), 5 (anode), accelerating electrode 6, switching power supplies 7, 8 and 9, a conductive surface (wall electrode) 10, the gas injection system 11, the vacuum chamber 12, the length of the conductive surface L, the length of the discharge zone l, the width of the electric discharge chamber D.

In FIG. Figure 2 shows the dependences of the electron concentration N _e and the diffusion flux of negative ions on the conducting wall Г ^- in a repetitively pulsed discharge as a function of time t, where T is the period of discharge pulses, T ₀ and T _1, respectively, the moments of formation of an ion-ion plasma in the absence and in the presence of (dashed) control voltage supplied to the conductive wall at time T _e .

The proposed device operates as follows. The sealed housing 1, connected from the side of the outlet 3 to the vacuum chamber 12, is previously pumped out. After that, with the help of the working gas input system 11, gas is pumped through the inlet 2; the pumping speed is chosen so that the working pressure in the source does not exceed 0.1.0.2 Torr (Gabovich MD, Physics and technology of plasma ion sources. M. Atomizdat, 1972, p. 177). Using a pulse source 7, a pulsed periodic discharge is excited in the working gas with a period of discharge pulses T. After time T _{0, the} electrons leave the volume, the plasma becomes ion-ion (see Appendix). When a positive voltage U is supplied to the accelerating electrode 6 from the power supply 9 for a time t <T T ₀ , a pulsed beam of negative ions with an energy eU is received at the output of the plasma source 3.

The transition to the ion-ion plasma mode at an earlier moment T ₁ <T ₀ can be achieved if small positive potentials U _{e are} applied to the wall electrode using a power source 8 starting from the moment of time T _e <T ₁ <T ₀ . In this case, T ₁ depends on both T _e and U _e . By varying the last two parameters, it is possible to adjust T ₁ , and thereby ensure the possibility of obtaining a pulsed beam of negative ions containing no electrons, with a pulse repetition period T and with their duration T-T ₁ > T-T ₀ . Thus, due to the fact that part of the wall in the discharge chamber is made conductive, an additional parietal electrode is present in the discharge zone, the control voltages on which allow the pulse duration of the beam of negative ions to be regulated.

The claimed plasma source is implemented in the following design. A glass discharge tube with a length of 180 mm, an inner diameter of 35 mm, with aluminum electrodes and a wall nickel cylinder with a length of 70 mm was chosen as a sealed chamber. Oxygen was pumped through the tube and a pulse-periodic discharge was ignited with T 320 μs, voltage between the discharge electrodes 900 V, current in the pulse 50 mA. The stationary pressure in the tube was 0.04 Torr. The current of charged particles through an outlet of 1.2 mm diameter was measured using a flat molybdenum collector, the ratio of the electron and ion components in the beam was monitored by probe measurements near the outlet (O. Kozlov, Electric plasma probe. M. Atomizdat, 1969). The temporal resolution in the measurements was 10 μs. The results of measurements of the ion I _i and electronic I _e currents to the collector at U + 8 V at moments t ₁ 100 μm, t ₂ 180 μs after the discharge pulse are presented in the table. As can be seen from the table, the current of the ion component of the beam exceeds the electron current by more than an order of magnitude.

 A plasma ion source was used in laboratory simulation of the flight conditions of spacecraft in the Earth's ionosphere. During operation for 120 hours, the main parameters of the source (pulse current at a given discharge voltage, operating pressure at a given gas pumping rate) remained virtually unchanged; operating modes when the electronic component in the output beam is practically absent (see table) were reproduced.

 Thus, the plasma source of negative atomic ions provides the possibility of obtaining stable intense ion beams, which allows it to be used to create devices simulating the Earth’s ionosphere in scientific studies of the characteristics of elementary processes in the collisions of negative ions with neutral and charged particles, as well as in the generation of atomic bundles. In addition, such devices can be used as sources of negative ions of electronegative gases with a sufficiently large (more than 0.5 eV) electron affinity energy (H, Cl, I, etc.) in electrostatic accelerators and in industrial technological plants for various purposes.

 Application.

It is known that when a charged body is placed in a plasma near its surface, plasma polarization occurs, resulting in shielding of the corresponding electric field. The characteristic spatial scale of such shielding is equal to the Debye radius
r _d [cm] 500 (T _e [eV] / N [cm ^-3 ]) ^1/2
(where T _e and N are the temperature and concentration of charged particles). For typical gas discharge conditions r _d <11 mm. Therefore, the application of small (several volts) electric potentials to the conductor at the plasma boundary, which do not cause an independent (or non-independent) electric discharge, does not directly lead to a change in the distribution of electric fields in the plasma, and therefore, to a change in the nature of diffusion of charged particles. However, after a time of order
t [c] = 1 / ω _p = 1.79 • 10 ^-5 / N [cm ^-3 ] ^1/2
(time scale of separation of charges in a plasma) after the inclusion of such a potential, the value of the near-wall potential jump at the plasma boundary will change. Depending on the value of the established potential eF at this boundary, the nature of the recombination of charged particles on it will change: particles having the normal component of the kinetic energy E _n > eF will recombine at the boundary, with an energy E _n <eF reflected from the potential barrier. If the characteristic size of the conductor at the plasma boundary is L> Λ (diffusion length), then, in accordance with the changed boundary conditions, the profile of the ambipolar potential in the plasma also changes, which controls the rate of diffusion of charged particles to its boundaries.

The nature of the diffusion motion of charged particles was studied by us experimentally in a decaying plasma of a repetitively pulsed discharge (T 500. 1000 μs, pulse duration 3.30 μs) in helium, nitrogen and oxygen of low pressure (0.03.1 Torr) excited in a glass tube with a diameter of 35 mm s molybdenum electrodes. A hollow metal cylinder 70 mm long, adjacent to the walls of the tube, was placed inside the tube, made of nickel foil. Probe measurements of the distribution of the electric potential, as well as electron concentrations in the decaying plasma inside the nickel cylinder, were performed under conditions when small positive and negative potentials were applied to it. It is established that the application of such potentials entails a change in the rate of diffusion escape of charged particles from the volume. So, in a decaying helium plasma (helium pressure 0.7 Torr, current pulse 0.7 A, pulse duration 15 μs, T 440 μs), through T _e 20 μs after the end of the discharge, a positive voltage pulse with a duration of 200 μs and amplitude was applied to the wall cylinder 5 V. As a result, using probe measurements, a faster decrease in the electron concentration on the axis of the discharge tube was recorded (from 1.1 • 10 ¹¹ to 6.0 • 10 ¹⁰ cm ^-3 instead of 9.0 • 10 ¹⁰ cm ^-3 ) the specified 200 μs. Thus, the ability to control the rate of diffusion processes during the evolution of a gas-discharge decaying low-pressure plasma by small changes in the conductor potential at the plasma boundary is confirmed experimentally.

A theoretical analysis of the balance of electron concentrations, as well as positive and negative ions in a decaying plasma of electronegative gases, has been performed. The ionic composition of such a plasma is determined mainly by molecular ions A $\genfrac{}{}{0ex}{}{\text{+}}{\text{2}}$ formed in the discharge due to ionization by electrons, as well as atomic ions A ^- , appearing in the discharge as a result of dissociative attachment processes. The analysis showed that the diffusion decay of the plasma of electronegative gases occurs in two stages. At the first stage, ambipolar diffusion of electrons and positive ions occurs; negative ions are “locked” in volume due to the presence of an ambipolar electric field and a near-wall potential jump. At the end of the stage, as the electron concentration decreases, the radial electric field ceases to hold the electrons, and they almost instantly leave the volume towards the walls. In the second stage, the plasma consists of positive and negative ions with an almost complete absence of electrons; plasma decay is determined by the joint diffusion of ions of different signs. In a decaying plasma, the electron temperature usually does not exceed 0.1 eV; therefore, the processes of ionization and dissociative adhesion, which have a large energy threshold, are insignificant. If the detachment processes, which depend on the concentration of atoms and metastable molecules, do not play a noticeable role in the balance of A ^- concentrations, then, as follows from our theoretical relations, the transition to the second stage occurs after time
T ₀ = Λ ² / 2D _p ln (1 + 1 / a ₀ )
after the end of the discharge pulse, where Λ is the diffusion length, D _{p is} the diffusion coefficient of positive ions; and _{0 is the} ratio of the concentration of negative ions to the concentration of electrons by the time the discharge pulse ends.

Experiments have been performed to study a periodic pulsed discharge in oxygen plasma using various gas-discharge devices. From measurements of probe current-voltage characteristics in a discharge in a glass tube with a diameter of 35 mm at a pressure of 0.07 Torr, T 560 μs, pulse duration 32 μs, and a current in a pulse of 10 mA, it was found that after T ₀ 200 μs after the end of the discharge pulse, an abrupt the transition from electron-ion to ion-ion plasma, due to the diffusion escape of electrons from the volume while maintaining most of the positive and negative ions. A similar effect was observed through T ₀ 20.50 μs after a discharge pulse in a periodic pulse with T 150.500 μs, pulse duration 5 20 μs, current in a pulse of 500 mA at pressures of 0.5-3 Torr and a discharge zone diameter of 3 mm. In these experiments, the current densities of negative ions from the ion-ion plasma to the wall probe at a potential of + 5V were 10 µa / mm ² or more.

In accordance with the foregoing, the sharing effect of formation of ion-ion plasma techniques and control diffusion of charged particles evolution electron density N _e and the diffusion flux of negative ions to the wall F ^- will correspond to FIG. 2. By applying potentials U _e to the wall electrode of the discharge device at a certain moment of time T _e (dashed line in Fig. 2), it is possible to provide a stream of negative ions with a duration of T T ₁ > T T ₀ (where T ₁ depends on T _e and U _e ). Note that in this way it is possible to obtain a pulsed flow of negative ions to the wall even under conditions when it is not "realized" in a natural way, i.e. T <T ₀ .

Claims (1)

 Plasma source of negative atomic ions, including a sealed electric discharge chamber with inlet and outlet openings, electrodes connected to power sources: main electrodes placed in the chamber, and accelerating electrodes located behind its outlet, characterized in that the inner wall of the chamber of length L in the discharge zone is made conductive and connected to a power source, the power sources are selected pulsed, the outlet is placed in the conductive wall, and the length L of the conductive surface is found from the condition L ≥ 2D, L <l, where e D is the width of the electric discharge chamber, l is the length of the discharge zone.

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