RU2012945C1 - Method of control over current of plasma emitter of large area - Google Patents

Abstract

FIELD: experimental physics, electron and ion sources with plasma emitters. SUBSTANCE: invention may also find in acceleration equipment. For control over current of plasma emitter of large area potential of emission electrode is set equal to potential of emitting plasma. With stable concentration of plasma of auxiliary discharge difference of potentials between plasma of auxiliary discharge and electrode separating this plasma from emitting plasma change within intervals from 4,3·10^-5·T_e·ln(Π/2·m_c/M_i) to 8,8×10^-8·n $\genfrac{}{}{0ex}{}{\text{2/3}}{\text{aux}}$ ·T $\genfrac{}{}{0ex}{}{\text{1}}{\text{e}}$ ^/3·r^4/3, where T_e and n_aux are temperature and concentration of emitrons of plasma of auxiliary discharge accordingly; m_e and M_i are correspondingly masses of electron and ion of plasma forming gas; r is radius of hole in electrode separating plasma of auxiliary discharge from emitting plasma. Setting of potential of emission electrode equal to potential of emitting plasma removes potential barrier for electrons close to this electrode acting on energy efficiency of emitter and homogeneity of emission from peripheral sections of emitter and control over emission current I_e of plasma emitter accompanied by reduction of power used to generate I_e is performed by adjustment of value of flux of electrons injected from auxiliary discharge into expander under change of difference of potentials between plasma of auxiliary discharge and electrode which separates this plasma from emitting plasma. with change of difference of potentials within specified limits length of ion layer close to wall of hole in separating electrode alters which makes it possible to control area of plasma of auxiliary discharge from which selection of electrons injected into expander takes place from its maximum value to zero. EFFECT: improved reliability of control. 1 dwg

Description

 The invention relates to methods for controlling the current of large-area plasma emitters and can be used in electronic and ion sources generating beams with a large cross section.

 A known method of controlling the current of a large-area plasma emitter [1], based on the creation of an auxiliary discharge generating an emitting plasma, including a change in the current of the auxiliary discharge, leading to a change in the parameters of the emitting plasma.

 The disadvantages of this control method are the change in the nature of the radial distribution of the concentration of the emitting plasma and, therefore, the uniformity of the distribution of current density over the surface of the emitter in the current control process, as well as the difficulty in obtaining low emission currents due to the instability of the auxiliary discharge plasma at its low concentration.

 Closest to the proposed one is a method for controlling the current of a large-area plasma emitter [2], based on the fact that the emitting plasma is created in the expander electrode filled with working gas as a result of injection of electrons into it from the auxiliary discharge plasma, and the emission current is controlled by changing parameters of the emitting plasma by changing the potential of the expander electrode.

(I_э- ток эмиссии, Р - мощность, затрачиваемая на его получение.The disadvantages of this method are the increase in the heterogeneity of the distribution of current density over the surface of the emitter with increasing emission current and low energy efficiency of the emitter H =

(I _e is the emission current, P is the power spent to obtain it.

 The first drawback is due to the fact that with an increase in the potential of the expander electrode, the shape of the space charge layer changes, in which the electrons injected into the expander electrode are accelerated. In this case, the divergence of the injected electron flux decreases, which leads to an increase in the emission current density in the center of the emitter and to a decrease at the periphery.

 The second drawback is that the control of the emission current is carried out at relatively high (up to a thousand volts) control potentials, which increases the power consumed from the control source and, accordingly, reduces the energy efficiency of the emitter.

 The aim of the invention is to preserve the uniformity of the distribution of current density over the surface of the emitter and increase the energy efficiency of the emitter when controlling current.

) до 8,8·10^-8·n $\genfrac{}{}{0ex}{}{\text{2}}{\text{в}}$ ^/3·T $\genfrac{}{}{0ex}{}{\text{1}}{\text{е}}$ ^/3·r^4/3, где Т_е и n_в - температура электронов и концентрация плазмы вспомогательного разряда соответственно; m_e и M_i - масса электрона и иона плазмообразующего газа; r - радиус эмиссионного отверстия в электроде, отделяющем плазму вспомогательного разряда от эмитирующей плазмы.The goal is achieved in that, to control the current of a large-area plasma emitter, the potential of the emission electrode is chosen equal to the potential of the emitting plasma, and the parameters of the emitting plasma are changed at a constant concentration of electrons of the auxiliary discharge plasma by changing the potential difference between the auxiliary discharge plasma and the electrode insulated from the expander electrode and separating the auxiliary discharge plasma from the emitting plasma, in the range from 4.3 · 10 ^-5 · T _e · ln (

) to 8.8 · 10 ^-8 · n $\genfrac{}{}{0ex}{}{\text{2}}{\text{at}}$ ^{/ 3} · T $\genfrac{}{}{0ex}{}{\text{1}}{\text{e}}$ ^{/ 3} · r ^4/3 , where T _e and n _in are the electron temperature and the plasma concentration of the auxiliary discharge, respectively; m _e and M _i are the mass of the electron and ion of the plasma-forming gas; r is the radius of the emission hole in the electrode separating the auxiliary discharge plasma from the emitting plasma.

 A positive effect is achieved due to the fact that the current control by the proposed method is carried out not by changing the shape of the space charge layer, in which the electrons injected into the expander electrode are accelerated, and the voltage drop across the layer, but by adjusting the magnitude of the electron flow injected from the auxiliary discharge into the electrode expander.

In the emitters of this type, the emission current is provided by two groups of electrons: high-energy electrons of the stream injected into the expander electrode from the auxiliary discharge, and low-energy electrons, mainly formed at the peripheral sections of the emitter as a result of secondary electron emission from the walls of the expander electrode. Setting the potential of the emission electrode equal to the potential of the emitting plasma removes the potential barrier for electrons near this electrode, which can significantly increase the contribution to the emission current of low-energy electrons in the peripheral parts of the emitter and thereby increase the energy efficiency of the emitter. In addition, this excludes one of the factors affecting the uniformity of emission during current control. Uniform distribution of current density over the surface of the emitter is provided under the condition _in n / n _e> 1, where n _e and n, respectively, _{in the} emitting plasma concentration and auxiliary discharge plasma, and the plasma potential emitting U _e exceeding the potential of the auxiliary discharge plasma _in U. In this case, a diverging stream of electrons is injected into the electrode expander, generating a uniform emitting plasma in it.

 In the proposed method, the emission current is controlled by changing the magnitude of the electron flux injected into the electrode expander. To fulfill the above conditions, ensuring the uniformity of emission, the concentration of the auxiliary discharge plasma is kept constant, and the electron flux is regulated by a change in the potential difference between the auxiliary discharge plasma and the electrode electrically isolated from the expander electrode and separating this plasma from the emitting plasma. When the potential difference changes, the length of the ion layer at the wall of the hole in the electrode, which separates the auxiliary discharge plasma from the emitting plasma, changes, which allows controlling the area of the auxiliary discharge plasma from which electrons injected into the expander electrode are selected. The proposed method allows to control the emission current without compromising the uniformity of the emission.

) . При меньшей разности потенциалов резко возрастает электронный ток на электрод, что приводит к значительному изменению распределения концентрации эмитирующей плазмы, а в ряде случаев к погасанию вспомогательного разряда. Увеличение разности потенциалов вызывает рост протяженности пристеночного ионного слоя в отверстии отделяющего электрода и уменьшение площади плазменной поверхности, определяющей величину инжектируемого в расширитель потока электронов. При разности потенциалов, равной 8,8 ˙10^-8 ˙n_в ^2/3 Те ^1/3 ˙r ^4/3, протяженность пристеночного ионного слоя в канале отделяющего электрода становится такой, что плазма из вспомогательного разряда не проникает в отверстие отделяющего электрода. В этих условиях выход электронов из вспомогательного разряда в электрод-расширитель возможен только через потенциальный барьер, что резко снижает инжектируемый поток электронов и соответственно ток эмиссии, поэтому дальнейшее увеличение разности потенциалов приводит к существенному уменьшению энергетической эффективности эмиттера. Кроме того, при образовании потенциального барьера электроны инжектируются из вспомогательного разряда в виде узкого пучка, что обуславливает появление существенной неоднородности эмиссии (максимум плотности тока в центре эмиттера).The purpose of the invention is achieved by providing a certain range of variation of the potential difference between the auxiliary discharge plasma and the separating electrode. The minimum potential difference corresponding to the largest current of electrons injected from the auxiliary discharge is equal to the potential difference between the plasma and the "floating" electrode in the plasma. This potential difference depends on the kind of plasma-forming gas and the electron temperature and is equal to 4.31 · 10 ^-5 · T _е · ln (

) With a smaller potential difference, the electron current to the electrode sharply increases, which leads to a significant change in the concentration distribution of the emitting plasma, and in some cases to the extinction of the auxiliary discharge. An increase in the potential difference causes an increase in the length of the near-wall ion layer in the hole of the separating electrode and a decrease in the plasma surface area, which determines the magnitude of the electron flux injected into the expander. With a potential difference of 8.8 ˙ 10 ^-8 ˙n _in ^2/3 Te ^1/3 ˙r ^4/3 , the length of the near-wall ion layer in the channel of the separating electrode becomes such that the plasma from the auxiliary discharge does not penetrate the hole of the separating electrode . Under these conditions, the escape of electrons from the auxiliary discharge into the expander electrode is possible only through the potential barrier, which sharply reduces the injected electron flux and, accordingly, the emission current, therefore, a further increase in the potential difference leads to a significant decrease in the energy efficiency of the emitter. In addition, when a potential barrier is formed, electrons are injected from the auxiliary discharge in the form of a narrow beam, which leads to the appearance of a significant inhomogeneity of emission (maximum current density at the center of the emitter).

 Using the proposed method in comparison with the prototype method increases the energy efficiency of the emitter, since with a decrease in the emission current the power spent on its production is reduced. This is because when controlling the proposed method, in contrast to the regulation by the prototype method, when the emission current decreases, the current of electrons injected into the expander decreases. In addition, control is carried out at lower voltages.

 An example of practical implementation. The proposed method changes the current of the large area electron emitter based on the injection of electrons into the expander electrode from a low-voltage reflective discharge with a cold hollow cathode.

 The drawing shows a diagram of the emitter.

The emitter contains an auxiliary discharge chamber, including a hollow cathode 1, anode 2 and a separating electrode 3 with an aperture of radius r = 1.7 mm for electrons to exit into the expander electrode 4 with the emission electrode 5 at the end. The discharge voltage U ₁ from the source 6 is supplied between the anode 2 and the cathode 1. The potential of the separation electrode U ₂ is set using the source 7, and the potential of the expander U ₃ is set by the source 8. The accelerating voltage U _accele from the power source 9 is supplied between the emission electrode and the collector 10 The ratio of the concentrations of the emitting plasma and the auxiliary discharge plasma, as well as the concentration of the latter, is measured by two Langmuir cylindrical probes 11 and 12. When voltage is supplied from 6.8 power sources, a discharge is ignited in the discharge chamber, trona from a plasma discharge electrode penetrate the expander 4 via the hole in separating electrode 3, forming therein emitting plasma.

To obtain a uniform distribution of current density over the surface of the emitter by appropriately adjusting the voltage of sources 6 and 8, the potential of the emitting plasma is set 1.3–3.3 times the potential of the auxiliary discharge plasma. In the emitters of this type, the potential of the emitting plasma is equal to the potential of the expander, and the plasma potential of the auxiliary discharge is 0.8 ˙ U ₁ . Further, the recording devices 13 and 14, the currents on the probes measured concentration ratio emitting plasma n _e and n _{in the} auxiliary discharge plasma, by appropriately adjusting the setting of the auxiliary discharge current source 6 _to n / n _e> 1.

To implement the proposed method, the potential of the emission electrode is set equal to the potential of the emitting plasma, and the potential difference between the auxiliary discharge plasma and the separating electrode is changed. The first condition for the emitter of the type in question is fulfilled when the emission electrode is connected to the expander electrode. To determine the plasma concentration of the auxiliary discharge and the electron temperature, the probe characteristic of the Langmuir probe 11 is used, the measurement of which is performed at the maximum possible current of the auxiliary discharge I _m (for the case under consideration, I _m = 1 A). Since the plasma-forming gas is He Т _е ≈10 ⁵ K, and the plasma concentration of the auxiliary discharge n is _at ≈ 1˙ 10 ¹⁸ m ^-3 . Substituting n _in , T _e , r, we find the potential difference between the auxiliary discharge plasma and the separating electrode, respectively, for the lower 36.4 V and the upper limit of 826 V. The tests performed showed that changing the potential difference within the indicated limits allows you to smoothly and inertialessly control the emission current I _e from 200 to 2 mA.

 When the potential difference is lower than the lower limit, the auxiliary discharge is extinguished, and when the upper limit is exceeded, the emission current decreases to almost zero. The latter is due to the locking of the holes in the separating electrode by the parietal ionic layer. When changing the emission current within the specified limits, the heterogeneity of the distribution of current density over the surface of the emitter increased no more than 1.3 times, while when using the prototype method no less than 4 times. Tests have shown that the energy efficiency of the emitter N in comparison with the prototype method increases by 1.35-1.50 times.

Claims (1)

METHOD OF CONTROL THE CURRENT PLASMA EMITTER LARGE AREA, including the creation of an emitting plasma by injecting electrons into an expander electrode filled with a working gas from an auxiliary discharge plasma and changing the parameters of the emitting plasma bounded by the emission electrode, characterized in that the potential of the emitting electrode is chosen to be equal to the emitting electrode and a change in the parameters of the emitting plasma is carried out at a constant concentration of electrons of the plasma of the auxiliary discharge by changing potential difference between the auxiliary discharge plasma and the electrode electrically isolated from the expander electrode and separating the auxiliary discharge plasma from the emitting plasma, in the range
4.3 · 10 ^-5 · T _e · ln (

) . . 8.8 · 10 ^-8 · n $\genfrac{}{}{0ex}{}{\text{2}}{\text{at}}$ ^{/ 3} · T $\genfrac{}{}{0ex}{}{\text{1}}{\text{e}}$ ^{/ 3} · r ^4/3 ,
where T _e and n _in - respectively, the temperature and concentration of electrons in the plasma of the auxiliary discharge;
m _e and M _i are the mass of the electron and ion of the plasma-forming gas, respectively;
r is the radius of the emission hole in the electrode separating the auxiliary discharge plasma from the emitting plasma.

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