A plasma source of negative atomic ions

 

(57) Abstract:

Usage: ion technique. The device allows you to get directly at the exit of the plasma source does not contain electrons pulsed beams of negative atomic ions in electronegative gases. The essence of the invention: a source of negative atomic ions comprises a sealed discharge chamber with an inlet and outlet connected to the power supply electrodes: basic, placed in the chamber, and accelerating beyond its outlet. The inner wall of the chamber length L in the discharge zone is made conductive and connected to a power source, the power supply main and the accelerating electrode and the conductive surface of the selected pulse output hole is placed in the conductive wall, and the length L of the conductive surface is found from the condition L 2D L < 1, where D is the width of the electric discharge chamber, I is the length of the zone of discharge. 2 Il.

The invention relates to the field of electrical engineering, in particular to electronic discharge devices, and can be used to create devices that simulates the Earth's ionosphere, in the research of the characteristics of elementary tap the x beams.

At the present time is a very urgent task of creating technology plasma sources monoenergetic beams of negative ions of low energy (a few eV). Physical modelling of the effects of flows of atomic oxygen on the outer onboard equipment spacecraft operating in low earth orbits, it is especially important to obtain ion beams without accompanying electrons, complicating the modeling field conditions due to ionization and excitation.

The authors are not known plasma sources beams of atomic ions that do not contain electronic components.

Known plasma sources of negative ions. For example, a plasma source of negative ions of oxygen (Danilin T. I. and other operating rules, 1968, No. 3, S. 158), based on the use of the penning discharge with cold cathodes and ejecting the ions perpendicular to the magnetic field. It contains housed in a sealed enclosure two cathode pole piece of the permanent magnets, flat ferromagnetic circular anode with a hole in the center for the extraction of ions, and accelerating electrode and an electrostatic lens. When the discharge current of 150 mA, the anode voltage of 500 V and yuushin electronic current of 30 mA.

Disadvantages source are the electronic components of the output beam current which is 600 times the beam current OF-and also the comparative complexity of the design.

Closest to the proposed invention achieved the effect is chosen as a prototype plasma source for atomic hydrogen ions (H-) [Antipov, S. P. Elizarov L. I. Martynov M. I. Chesnokov C. M. PTE, 1984, N 4, S. 42-44] with axially symmetric geometry and radial magnetic field. The main elements of its design are: a sealed enclosure with an opening for entry of the working gas inside the enclosure to be heated hollow cathode, an auxiliary electrode (used for ignition of the discharge), the main anode in the form of a disk, provided with the outlets, which is located near the accelerating electrode, a magnetic system that creates a radial magnetic field and consisting of inner and outer magnetic poles, representing axially spaced hollow steel cylinders with magnetic coil between them (the cathode is located in the inner pole). The discharge burns between the hollow cathode and the main anode. Ions are extracted in the direction perpendicular magnetic p of the ion current to 0.7 And at a current associated electrons 1,2.1,4 A.

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

The technical effect of the plasma source of negative atomic ions of our proposed design is to provide directly at the exit of the plasma source of stable intense pulsed beams of negative ions in electronegative gases, containing no accompanying electrons. Proposed source is structurally simple and compact, allowing you to use it in laboratory facilities are limited in size, simulating the conditions of the earth's ionosphere.

The technical effect is achieved in that the plasma source of negative atomic ions is a sealed discharge chamber with an inlet and outlet connected to the power supply electrodes: basic, placed in the chamber, and accelerating placed at its outlet. New it is, " used the term "zone of discharge" i.e. the area where the ionization of the gas as possible constructive implementation of the present invention, when the conductive wall is the inner surface of one of the main electrodes) are made conductive and connected to a power source, the power source is selected pulse, the output hole is placed in the conductive wall, and the length L of the conductive surface is found from the condition L 2S, L < l, where D is the width of the electric discharge chamber, l is the length of the zone of discharge.

Studies of electrokinetic characteristics of gas discharge plasma by us theoretically substantiated and experimentally confirmed the ability to control the diffusion of charged particles in decaying plasma at scales much greater than the Debye radius rdby changing the potential of the conducting boundaries of the plasma. It is possible to obtain in the decaying plasma pulse-periodic discharge in electronegative gases regulated by the duration of the threads on the wall of negative atomic ions, containing no electrons. Theoretical substantiation and experimental data is given in the Appendix.

In Fig. 1 shows a diagram of the proposed plasma source, hectrol 6, switching power supplies 7, 8 and 9, a conductive surface (wall surface electrode) 10, the system of the gas inlet 11, the vacuum chamber 12, the length of the conducting surface L, the length of the discharge zone l, the width of the electric discharge chamber D.

In Fig. 2 depicts the dependence of the electron concentration Neand the diffusion flux of negative ions in the conductive wall G-in pulse-periodic discharge in dependence on time t, where T is the repetition period of the discharge pulses, T0and T1accordingly, the moments of the formation of ion-ion plasma in the absence and in the presence (dashed line) of the control voltage applied to the conductive wall at time Te.

The proposed device operates as follows. The hermetic housing 1, is connected from the outlet 3 to the vacuum chamber 12, a pre-pumped. After using the system I working gas 11 provide the gas flow through the inlet 2; leveling speed is chosen such that the working pressure in the source does not exceed 0,1.of 0.2 Torr (Gabovich, M. D. Physics and technology of plasma ion sources. M Atomizdat, 1972, S. 177). Using the pulse source 7 in the working gas excite yodat of volume, the plasma becomes ion-ion (see Appendix). When applying for accelerating electrode 6 from the power source 9 of the positive voltage U at the time t < T T0at the exit of the plasma source 3 receives the pulsed beam of negative ions with energy U.

The transition to the regime of ion-ion plasma in an earlier time T1< T0can be achieved if you apply using a power source 8 at parietal electrode small positive potentials Uesince the time Te< T1< T0. In this case, T1depends on Teand Ue. By varying the last two parameters can be adjusted T1and thereby to provide a pulsed beam of negative ions that do not contain electrons, with the repetition period of pulses T and duration T-T1> T-T0. Thus, due to the fact that part of the wall of the discharge chamber is made conductive in the discharge zone, there is an additional parietal electrode, the control voltage which allow you to adjust the pulse width of the beam of negative ions.

Declared plasma source is implemented in the following design 35 mm, with aluminum electrodes and parietal Nickel cylinder length 70 mm tube was pumped oxygen and lit periodic-pulsed discharge with T 320 μs, the voltage between the discharge electrodes 900, the current pulse 50 mA. Stationary pressure in the tube when this was 0.04 Torr. The current of charged particles through the outlet diameter of 1.2 mm was measured using a flat molybdenum collector, the ratio of the electronic and ionic components in the beam was monitored with a probe measurements near the outlet (Kozlov, O. C. electric probe in the plasma. M Atomizdat, 1969). The temporal resolution in the measurements was 10 μs. The results of measurements of ion Iiand e Iecurrents at the collector when U + 8 in moments of t1100 μm, t2180 μs after the discharge presented in the table. As can be seen from the table, the current of the ion beam components by more than an order exceeds the electron current.

A plasma ion source was used for laboratory simulation of space flight spacecraft in the ionosphere of the Earth. During operation within 120 hours basic source parameters (pulse current at a specified voltage discharge, the working pressure is ucke electronic component is virtually absent (see table), played.

Thus, the plasma source of negative atomic ions provides the possibility of obtaining stable intense ion beams, so you can use it when creating a device that simulates the Earth's ionosphere, in the research of the characteristics of elementary processes in collisions of negative ions with neutral and charged particles, and in the generation of atomic beams. In addition, such devices may be used as sources of negative ions in electronegative gases with a sufficiently large (>0.5 eV) energy of the electron affinity (H, CL, I, and others) in electrostatic accelerators in industrial process plants for various purposes.

The application.

It is known that when placed in the plasma charged body near its surface polarization of the plasma, leading to shielding of the corresponding electric field. The characteristic spatial scale of such shielding is equal to the Debye radius

rd[cm] 500 (Te[eV]/N[cm-3])1/2< / BR>
(where Teand N are the temperature and concentration of charged particles). For typical conditions Gal t) electric potentials, non-occurrence of the independent (or dependent) electric discharge directly does not change the distribution of electric fields in plasma, and therefore, changing the nature of diffusion of charged particles. However, after a time of order

t[c] = 1/p= 1,7910-5/N[cm-3]1/2< / BR>
(the time scale charge separation in the plasma after enabling this potential will change the magnitude of the parietal spike potential on the boundary of the plasma. Depending on the magnitude of the steady-state potential Eph at this boundary will change the nature of the recombination of charged particles: particles having the normal component of the kinetic energy En> Eph will recombine at the border, with energy En< Eph reflected by a potential barrier. If the characteristic size of the conductor at the boundary of the plasma L> (diffusion length), in accordance with the changed boundary conditions changes the profile of ambipolar potential in the plasma, controlling the speed of diffusion of charged particles to its borders.

The nature of the diffusive motion of charged particles was studied experimentally in the decaying plasma pulse-periodicacademic in a glass tube with a diameter of 35 mm with molybdenum electrodes. Inside the tube was placed made of Nickel foil hollow metal cylinder with a length of 70 mm, adjacent to the walls of the tube. Were made probe to measure the distribution of electric potential and electron concentrations in the decaying plasma inside the Nickel cylinder in conditions when he was served a small positive and negative potentials. It is established that the application of these potentials entails changing the rate of diffusion care of charged particles from the volume. So, in the decaying plasma of helium (helium pressure of 0.7 Torr, a pulse of current of 0.7 a, pulse duration 15 μs, T 440 ISS) through Te20 μs after the end of the discharge wall of the cylinder pulse was applied positive voltage with duration of 200 μs and an amplitude of the 5th Century In the probe measurements has been reported in more rapid decrease of the electron concentration on the axis of the discharge tube (1,110 land only11up to 6,01010cm-3instead 9,01010cm-3within these 200 µs. Thus, the ability to adjust the speed of diffusion processes in the evolution of the gas discharge decaying plasma of low pressure by small changes of the potential of the conductor Eazy electrons, as well as positive and negative ions in the decaying plasma of electronegative gases. The ionic composition of such a plasma is determined mainly molecular ions (A+2formed in the discharge due to ionization of electrons and atomic ions (A-appearing in the discharge in the processes of dissociative sticking. The analysis showed that the diffusive decay of the plasma of electronegative gases occurs in two stages. In the first stage is the ambipolar diffusion of electrons and positive ions, negative ions are locked in volume due to the presence of ambipolar electric field and wall jump potential. At the end of phase with decreasing electron density radial electric field ceases to hold the electrons, and they instantly go from volume to the walls. In the second stage, the plasma consists of positive and negative ions in the almost complete absence of electrons; the collapse of the plasma is determined by the joint diffusion of ions of different sign. In the decaying plasma electron temperature usually does not exceed 0.1 eV, so the processes of ionization and dissociative sticking with large energy is l, not play a prominent role in the balance of concentrations of A-then, as follows from the obtained theoretical relationships, the transition to the second phase occurs through time

T0=2/2Dpln(1+1/a0)

after the discharge, where the diffusion length, Dpthe coefficient of diffusion of positive ions; and0the ratio of the concentration of negative ions to the concentration of electrons by the end of the discharge.

Performed the experiments on the study of pulse-periodic discharge in oxygen plasma using different discharge devices. The measurement probe volt-ampere characteristics of the 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, the current pulse of 10 mA found that through T0200 μs after the discharge occurs an abrupt transition from electron-ion to ion-ion plasma, due to diffusion care of electrons from the volume while maintaining the most part positive and negative ions. A similar effect was observed at T020.50 μs after the discharge in a pulse-periodic discharge T 150.500 ISS, detailspayment density currents of negative ions from an ion-ion plasma in murine probe at a potential of + 5 V was 10 μa/mm2and more.

In accordance with the foregoing, when combined effect of formation of the ion-ion plasma and methods of control diffusion of charged particles the evolution of the electron density Neand the diffusion flux of negative ions on the wall G-will fit shown in Fig. 2. Feeding on parietal electrode discharge device adjustable on the magnitude and duration of the potentials Ueat some time Te(the dotted line in Fig. 2), it is possible to provide on the boundary of the plasma stream of negative ions by the duration t of T1> T T0(where T1depends on Teand Ue). Note that this way you can get a pulse stream of negative ions to the wall even when the "natural" way, it will not be realized, i.e., T < T0.

A plasma source of negative atomic ions, including hermetic discharge chamber with an inlet and outlet connected to the power supply electrodes: basic, placed in the chamber, and accelerating placed at its outlet, characterized in that the inner wall of the chamber length L in the discharge zone is made conductive and Soea wall, and the length L of the conductive surface is found from the condition L 2D L < l, where D is the width of the electric discharge chamber, l is the length of the zone of discharge.

 

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