In the heart of the cathode spot

This post is dedicated to an amazing physical phenomenon, for the discovery of which a group of Soviet scientists could be awarded the Nobel Prize in physics. The award did not happen for various reasons, but there was a breakthrough in the development of technologies for the formation of powerful energy flows, electron and ion beams, and technologies for the modification of various materials. I will talk about these technologies in future articles, and today I will introduce readers to the phenomenon of explosive electron emission - the discovery of Soviet scientists, registered on June 25, 1976 and included in the list of fundamental discoveries in physics along with the discovery of electron, x-rays, laser radiation and superconductivity.

Historical excursion

Any discovery is preceded by a chain of various events, observations and experiments. So in this case, the basis of the development of many technologies that surround us are the simplest (for us now), but unique things. These are the experiments of Faraday, Galvani, Volta, Petrov, Stark.


After almost a hundred years, in 1896, bright points were noticed on the surface of the mercury cathode, which Stark later examined in detail . He proved that these points are inherent in all forms of arc discharge regardless of the cathode material, and in 1904 he first introduced the concept of a “cathode spot”. However, the causes and mechanism of the occurrence of cathode spots remained unclear.

I do not accidentally write this article. In 1998, after graduating from Tomsk State University , I came to work at the Institute of High Current Electronics as a very young man. To the vacuum electronics lab. After the “young fighter” course on assembling and disassembling fore-vacuum equipment and passing various tolerances, I decided to get and study the cathode spots on the installation for generating high-current (in general, according to the norm of the Russian language, it is more logical to say “high-current”, but the term “high-current” is established, therefore it will be be used further.App. author) beams. The skills of amateur photography came in handy, since the laboratory did not yet have high-speed video cameras and matrices, shooting was carried out with the Zenit camera on a special Mikrat film. Wonderful times! Through the smell of hydroquinone, cathode spots appeared.

The experimental design is quite simple. In the vacuum chamber between the cathodes 1 and 3 and the plasma anode 2, a reflective discharge is ignited . Through the transparent window 4 , cathode spots can be observed. It was through this window that the phenomenon was photographed. In order to fix the spots, the cathode of the reflective discharge 3 was made of a metal mesh. Approximately such photographs were taken in this experiment. Here are three frames from several hundred received then.

The discharge exists only a few tens of milliseconds, but this is enough to register the cathode spots in the "open shutter" mode on a sensitive Mikrat film. Bright points are cathode spots. At the same time, they appear equally probable both on cathode 1 and on grid 3 . Obviously, the blurry spots belong to cathode 1 (out of focus), and the clear spots belong to the grid. Of course, this is an integral shot. It is impossible to understand the dynamics of cathode spots, the moment and time of the formation of the first spot, and much more. But this experiment was very useful for me, since I saw with my own eyes the phenomenon itself as a whole.

Explosive electron emission development mechanism

More than sixty years have passed from the time of the Stark experiments until the understanding of the detailed mechanisms of explosive electron emission. But first things first.

Step 1

Imagine a vacuum chamber. Inside the chamber is the cathode and anode. Voltage applied. At the cathode - a negative potential, at the anode, respectively - positive.

An emission center (EC) arises on the surface of the cathode. This stage is characterized by scanty currents - from nanoamps to units of milliamps. This stage is called prebreakdown.

Step 2

Initiation of breakdown. There is a microexplosion of the emission center. Now it is an explosive emission center (VEC). A “ball” of cathode plasma is created, from the surface of which electrons are emitted towards the anode. This is the start of the explosive electron emission process.

In this case, the cathode plasma expansion velocity is much higher than the corresponding thermal velocity of the particles; therefore, it is customary to speak of the explosive nature of the phenomenon, in exact similarity to the explosion, for example, dynamite, when the shock wave arrives before the fragments that hit the explosion. Therefore, the phenomenon is called "explosive" electron emission.

Step 3

Stage of development of vacuum breakdown. A growing number of electrons are emitted from the cathode plasma. The heating of the anode begins, but there is still no anode plasma.

Step 4

The beginning of the appearance of the anode plasma and its spread into the discharge gap.

Step 5

Complete shorting of the discharge gap. Plasma fills the entire volume of the interelectrode space, the phenomenon of explosive electron emission ceases. The discharge goes into an arc and here's a miracle - a cathode spot (KP) appeared on the cathode, which I talked about above. Now the role of the cathode spot in the arc discharge is to maintain the charge carrier density due to electron thermal emission. This process leads to even greater additional heating of the cathode spot.
The development time of explosive electron emission is not long - from units to hundreds of nanoseconds. Cathode spots can burn much longer - as long as there is an arc.

In this case, explosive electron emission is the only type of emission that makes it possible to obtain a current density of 10 ⁹ A / cm ² and electron fluxes with a power of 10 ¹³ W.

Thus, the cause of the appearance of the cathode spot is explosive electron emission, and what is the very notorious emission center with which it begins? It turns out everything is simple. This is the heterogeneity and surface roughness of the cathode and, sometimes, elemental dirt!

There are inhomogeneities on the cathode surface. These are the so-called micropoint or microemitters. The figure shows how the phenomenon of explosive electron emission proceeds directly in the emission center. Thus, poor-quality cathode processing, the presence of heterogeneity and surface roughness increases the likelihood of emission centers and, as a result, cathode spots in the future. And it looks like the surface of the cathode after explosive emission. The presence of craters and traces of the expansion of the cathode substance is observed.

If you smooth the surface or melt it with a powerful electron beam, the number of emission centers drops sharply, which greatly complicates the process of cathode spot formation. This is very important in technological applications where high vacuum insulation strength is required, for example, in arresters, where the main task is to keep a high voltage between the cathode and anode until the stored energy is transferred (usually from a high voltage capacitor) to the payload. And vice versa. If a high emissivity of the cathodes is required, then they are made with a “developed” surface. For example, from shaving blades, braids of copper cable or foamed conductive materials. The need for inventions is cunning, as they say ...

On this we will consider the theoretical part sufficient. Despite the seeming simplicity of everything that I said in this article, in fact, this is the years of fascinating scientific research of various groups of scientists from around the world and, first of all, Soviet and Russian scientists under the leadership of Gennady Mesyats , his followers and students - the creators of a whole trend in the study of physics of powerful energy flows. The main thing in the discovery of explosive electron emission is applied applications associated with the creation of equipment for the generation of powerful electron beams, X-ray sources, and technologies for the modification of metals and alloys. In total, more than a hundred unique technologies.

Soon, for example, I will describe the technology and equipment for the generation of high-current electron beams for the modification of metals and alloys. Such equipment is supplied to Japan, the USA, China and Europe. There are no sanctions in science. Well, or much less than in politics)))

The photo shows an example of processing a part of a high voltage arrester. After machining from the original part (left), a part with unprecedented electric strength is obtained. I’ll write more about this technology and interesting areas of its application (for example, in dentistry) in the coming days! As they say, do not miss!))) The
article is published. Link
I also want to thank my colleagues for their help in preparing this material. I thank Alexander Batrakov, Alexei Markov and Evgeny Nefedtsev for useful advice. All these people are scientists of the ISE SB RAS in Tomsk. You can watch about this unique Institute and about technologies based on, in particular, explosive electron emission in the popular science video below (13 minutes)


Have a nice day!