## Abstract

Two new, previously unknown types of dissipative streaming instabilities (DSI) are substantiated. They follow from new approach, which allows solving in general form the classical problem of an initial perturbation development for streaming instabilities (SI). SI is caused by relative motion of the streams of plasma components. With an increase in level of dissipation SI transforms into a DSI. The transformation occurs because dissipation serves as a channel for energy removal for the growth of the negative energy wave of the stream. Until recently, only one type of DSI was known. Its maximal growth rate depends on the beam density nb and the collision frequency ν in the plasma as ∼nb/ν. All types of conventional beam-plasma instabilities (Cherenkov, cyclotron, etc.) transform into it. The solution of the problem of the initial perturbation development in systems with weak beam-plasma coupling leads to a new type of DSI. With an increase in the level of dissipation, the instability in these systems transforms to the new DSI. Its maximal growth rate is ∼nb/ν. The second new DSI develops in beam-plasma waveguide with over-limiting current of e-beam. Its growth rate ∼nb/ν. In addition, the solutions of abovementioned problem provide much information about SI and DSI, significant part of which is unavailable by other methods.

### Keywords

- beam-plasma instability
- dissipative instability
- development of initial perturbation
- growth rate
- absolute/convective instability

## 1. Introduction

Streaming instabilities (SI) occupy a prominent place among other plasma instabilities. They are caused by a motion of some plasma components relative to others. An example is the well-known beam–plasma instability [1]. With this instability, the directed motion of a group of fast electrons passing through the background plasma excites potential oscillations with a large growth rate near the plasma frequency. Particular attention to this instability is mainly due to the idea of creating sources of powerful electromagnetic radiation on its basis. At present, these sources have many advantages over the known vacuum sources [2, 3]. One more example (we mention these two only) is the Buneman instability [4], in which plasma electrons move relative to ions.

In the overwhelming majority of investigations beam–plasma interaction is considered without any noticeable dissipation. It, actually, was assumed that the dissipation is small and cannot have any noticeable effect on the physical processes. In this case, the development of instability leads to an increase in the amplitude of electromagnetic oscillations in the plasma, as well as their energy at the expense of beam kinetic energy. In the absence of dissipation, the level of excited oscillations may be quite high, and their energy can even be comparable to the initial energy of the beam [5].

However, generally speaking, dissipation in the system (collisions between plasma particles, heating of metal surfaces due to their complex impedance, etc.) can play an essential role in plasma–beam interaction. It can become not only a decisive factor in limiting the spatial and temporal growth, determining the field amplitude and the mode structure and limits the growth rates. In addition to these properties, which are common to all systems, it is necessary to pay special attention to the unique role of dissipation in systems with a stream of charge particles: dissipation of high level does not suppress the SI completely. Strong dissipation transforms each SI to instability of other type – to dissipative streaming instability (DSI) [1]. This type of instabilities is due to the presence of the negative energy wave (NEW) in a stream of charge particles [6, 7]. In fact, dissipation serves as a channel for energy removal for excitation of this wave. This leads to instabilities of a new physical nature, to DSI. Dissipation is the cause of this instability.

The physical nature of SI is not as simple as it might seem at first glance. It takes a lot of effort^{1} to understand it clearly. This is all the more so, if we are dealing with the transformation of SI into a DSI. The transformation (in general, the transformation of one type of instability into another) makes the behavior of SI in a system with dissipation especially interesting. In addition, there are other reasons that significantly increase interest in the study of problems associated with dissipation and the DSI caused by it. Some of them are as follows.

Modern high-frequency microwave electronics, both plasma and vacuum, have two basic trends of development: an increase in the frequency and power of the output radiation [2]. With increasing frequency, the thickness of the skin layer on the resonators’ walls decreases. This, in turn, leads to an increase in active energy losses. Actual dissipation in the system increases.

The second trend – an increase in the power of output radiation – leads to the need to increase the beam current. The role of space charge phenomena increases also, as well as the role of the NEW. In these circumstances it becomes important to take into account all factors that also lead to the buildup of the same wave i.e. to dissipation. In a sense, dissipation becomes associated with the space charge phenomena. In addition with an increase in the beam current, the return current increases also. With account the decrease in the skin layer and the finite conductivity of metallic surfaces, this leads to an increase in the level of dissipation in the system. All this indicates that dissipation, along with the space charge of the beam plays an important role in microwave electronics. A detailed understanding of the role of all these phenomena is vital for many problems aimed at achieving high-intensity beams and their applications.

Until recently, only one DSI was known in beam–plasma interaction theory [1]. Its maximal growth rate depends on collision frequency

However, recent studies have shown that there are other DSI also [9, 10, 11]. The interaction of the stream with the background plasma critically depends on some basic parameters of the system and/or on its geometry. Their changes lead to new physics of the beam-plasma interaction and to previously unknown types of DSI. The parameters are: the level of correlation between the beam and the plasma fields and the value of the beam current.

Available methods of instability investigation do not allow getting complete information on the process of transformation of given instability into another type. Is known the most complete information on instability can be obtained by solving the problem of the evolution of fields in space and time during the development of an initial perturbation. This problem is classical in theory of instabilities [12]. Its results can clear up how the fields of given instability transform to the fields of another one along with many other accompanying details. The character of the space–time evolution of an initial perturbation is an important issue in many branches of physics. However, the results of this problem are hardly achievable. Ultimately its mathematical solution reduces to calculation of the integral with complete dispersion relation (DR) in the denominator of the integrand. For the result the DR should be specified and solved before integration. This sharply reduces generality of results. And even in the special cases, it is not always possible to carry out the integration. In [13] an approach is presented that allowed overcome difficulties and obtain analytical expression for the fields’ space–time structure for all types of conventional beam-plasma instabilities. Results show that with increase in level of dissipation all types of beam-plasma instabilities transform to the only known type of DSI.

This review shows that the number of DSI is not limited by the above-mentioned DSI. Two new types of DSI are substantiated. They follow from solution of the same classical problem of initial perturbation development. One of the DSI manifests itself in the results of solving the problem in systems with weak beam-plasma coupling. Weak interaction realizes if the beam and the plasma are spatially separated by a considerable distance. Under weak coupling the beam actually is left to its own and its proper oscillation come into play. Moreover, among them is the NEW. Its interaction with plasma causes instability, the growth rate of which reaches maximum at resonance of the plasma wave with the NEW. This resonance of wave–wave type was called “Collective Cherenkov effect” [14]. An increase in the level of dissipation leads to a new DSI with the growth rate

The second new DSI appears in results of solving of the same problem in uniform cross-section beam-plasma waveguide with over-limiting e-beam. With an increase in the beam current the fields of its space charge affects more and more on the beam-plasma interaction. This manifests itself in two ways. Along with the increasing of the role of space charge oscillations, static fields of the beam space charge set an upper limit on the beam current that can pass through a given vacuum electro-dynamical system. The limit can be overcome by plasma filling. Plasma neutralizes the space charge of the beam. Plasma-filled waveguides can transmit e-beams with a current that is several times higher than the limiting current in vacuum waveguide. The fields of overlimiting e-beam space charge changes the character of its instability. The instability of over-limiting beams is not associated with any radiation mechanism [9, 14]. Its growth rate reaches maximum at the point of exact Cherenkov resonance and depends on the beam density as

In present review special attention is paid to systems, the geometry of which is similar to geometry of plasma microwave sources. These devices are a cylindrical waveguide with thin annular plasma and spatially separated thin annular e-beam. In this geometry the new types of DSI manifest themselves also [10].

In order to dispel all possible doubts about the correctness of the results, both new DSI are also substantiated by conventional analysis of the corresponding DR. To obtain a geometry-independent result for weak beam-plasma coupling we use perturbation theory based on smallness of the coupling parameter.

## 2. The only known DSI and transition to it

For the beginning we shortly present rezoning, from which follow: all types of beam-plasma instabilities (Cherenkov, cyclotron, beam instability in spatially periodical structure) transform to the only known DSI with the maximal growth rate

In general, the dispersion relation (DR), describing a plasma system penetrating by an electron beam can be written as

where

The beam electrons interact with the proper oscillations of the system and the interaction leads to instability. Developing instability manifests itself most effectively at frequencies and wavelengths close to the proper frequencies of the system in the absence of the beam, and, at the same time, close to the beam natural frequencies. In fact, along with (1) following condition is met

All (conventional) beam-plasma instabilities, including DSI, follow from (1)–(3). With an increase in level of dissipation all types of no-dissipative instabilities (Cherenkov, cyclotron etc) transform into the well-known DSI. If one searches the solutions of DR (1) in the form

All types of no-dissipative instabilities follow the first and the right-hand side term. In this case the dissipative (second) term in (4) is small. The DSI follows from the second term (when it is greater than the first term) and the right-hand side term. The relation between the respective growth rates

where the frequency of collisions in plasma

## 3. Weak beam-plasma coupling. New type of DSI

### 3.1 Solution of the problem of initial perturbation development under weak beam-plasma coupling

The best way to study an instability in detail and its possible transformation to that of other type is the solving of the problem of initial perturbation development. The information obtained by other ways is insufficient and does not give any details. Here we present general (geometry independent) solution of the problem for weakly coupled beam-plasma systems.

Consider a system consisting of a mono-energetic rectilinear electron beam and cold plasma. To begin with, suppose the following: the plasma and the beam are weakly coupled (e.g. in a consequence of a sufficiently large distance between them). Let an initial perturbation arises at a point

In given case of weak beam-plasma coupling the instability is the result of the interaction of the beam negative energy wave (NEW) and the slowed down wave in the plasma. The interaction is of Collective Cherenkov type. We proceed from the theory of wave interaction in plasma [16]. In terms of this theory the problem of the initial perturbation evolution under instability development in non-equilibrium plasma can be considered based on the set of partial differential equations for the amplitudes of the interacting waves: beam charge density wave

where

The solution of the set (6) gives the dependence of the field’s amplitude on longitudinal coordinate and time under instability development. Applying the Laplace transformation with respect to time

The field’s amplitude

where

Thus, the problem has been reduced to the problem of integration in (8). It is somewhat simpler in comparison to the integral, which represents classical solution. Instead of full DR its analog stands. The analog is determined by interaction of the waves, participating in the instability development. This replacement simplifies integration. However, it remains difficult and many authors use roundabout methods carry out an expression for possible estimation of the fields behavior [17, 18]. Presented here method easily leads to the desired result i.e. to expression for space–time distribution of the fields. We merely transform the variables

The second integration (over * t*. According to this method, the contour of integration should be deformed to pass through the saddle point in the direction of the steepest descent. The saddle point is found from the condition

and is equal to

As a result we arrive to the following expression for the field’s space time structure under development of the instability in spatially separated beam-plasma system

### 3.2 Analysis of the instability development

The expression (12) looks very complicate. At first glance it is impossible to extract any information on the instability behavior from it. However, it turned out, the expression may be easily analyzing. Moreover, the results are obtained from scratch, i.e. they are not based on prior research. Substantial part of the information is unavailable by other way. In particular, the analysis clearly shows that with increase in level of dissipation the no-dissipative instability turns to a new type of DSI and provides detailed information on both instabilities.

The properties of the instability is determined mainly by the exponential factor

which provides many information: the temporal and the spatial growth rates, the spread of the unstable perturbations’ velocities, the nature of the instability (absolute or convective), the effect of dissipation on instability, etc.

First consider some general properties of the instability, which follow from (13).

It is easily seen that in the absence of dissipation unstable perturbations have velocities in the range from

Now we turn to determination of the meaning of the denotation

Its root is

In the wave theory the velocity (15) is called convective velocity. It characterizes the spatial convection of the fastest growing perturbations. (15) shows that the peak of the wave packet disposes in its middle. The packet is symmetric with respect to its peak. Substitution of

The meaning of the parameter

At a fixed point

Then the field decreases, and at the time

Thus, the solution of the problem of initial perturbation development along with other detailed information, gave results of conventional initial and boundary problems. This coincidence confirms correctness of developed approach (initial assumptions, mathematics, etc.). An additional advantage of the approach is in its geometry-independence. At first glance, the presented approach seems more complicated than traditional approaches, but this complexity is only apparent.

### 3.3 The influence of dissipation. New type of DSI

Dissipation significantly influences on the presented picture of the instability development and changes it. First of all, it suppresses slow perturbations. The wave packet shortens. The threshold velocity

Only high-velocity perturbations (in the range

As expected, the growth rates fall down. Dissipation distorts the symmetry of the induced wave packet. In presence of dissipation the dynamics of the fields can be obtained from the same Eq. (14) accounting for dissipation. It has the form

The solution of (18) gives the point of the field maximum

This expression shows that with an increase in the level of dissipation, the peak shifts more and more to the front of the wave packet. This takes place along with the decreasing of the wave packet’s length. Substitution of

The function

As a criterion for the type of DSI this relation between the growth rates of DSI dependence of the growth rate on dissipation. Below this result is confirmed by conventional electro-dynamical analysis of the DR for weakly coupled beam-plasma system.

### 3.4 Substantiation of the new DSI by conventional analysis of the DR

From electro-dynamical point of view, a spatially separated beam-plasma system is nothing, but a multilayer structure. The traditional analytical consideration of such systems leads to a very cumbersome DR, which, in addition, is highly dependent on the geometry and greatly complicates with an increase in the number of layers. However, the importance of the problem and the need for its analytical investigation has led to development of specific methods. Here an approach is presented that allows avoiding abovementioned difficulties. Also, the approach has an important advantage: the procedure for obtaining the DR does not depend on specific shape/geometry. In other words, obtained results can be adapted to systems of any geometry. The approach considers the problem of weak beam-plasma interaction by perturbation theory. The small parameter, which underlies the theory, is the parameter of weak beam-plasma coupling. We briefly present here the basics of this approach accounting for dissipation [11].

Consider a system consisting of a mono-energetic rectilinear electron beam and cold plasma. To begin with, suppose the following: the plasma and the beam are weakly coupled (e.g. a consequence of a sufficiently large distance between them). We also assume their homogeneity in the cross section. The geometry of the system is not specified. It also is assumed that the beam current is less than the limiting current in the vacuum waveguide. Dissipation in the system is taken into account by the introduction the collisions in plasma. For simplicity, consideration is limited to the case of a strong external longitudinal (to the beam propagation direction) magnetic field, which prevents the transverse motion of the beam and plasma particles.

The small parameter underlying the perturbation theory is the parameter of weak coupling between the beam and the plasma (that is, the smallness of the integrals describing the overlap of beam and plasma fields). In the zero order approximation, the perturbation theory assumes independence of the beam and plasma. In the first-order approximation, the theory leads to the DR [11, 14].

The expressions

where

where

In absence of dissipation the instability is due to NEW interaction with the plasma. Its growth rate is

We emphasize unusual dependence on the beam density as

where

We arrive to the same new type of DSI presented in (22). The expression (28) shows a gradual transition of the growth rate of no-dissipative instability caused by NEW interaction with plasma into the growth rate of new type of DSI. It develops under weak coupling and differs from the conventional DSI (with an growth rate ∼

## 4. Uniform cross section beam-plasma waveguide. One more new type of DSI

### 4.1 Evolution of the initial perturbation in plasma waveguide with over-limiting electron e-beam

One more new DSI arises under consideration of the problem of the initial perturbation development for the instability of over-limiting beam (OEB) in uniform cross-section plasma waveguide.

Consider a cylindrical waveguide, fully filled with cold plasma. A mono-energetic relativistic electron beam penetrates it. The external longitudinal magnetic field is assumed to be strong enough to freeze transversal motion of the beam and the plasma electrons. We also assume that the beam and plasma radii coincide with the waveguide’s radius and consider only the symmetrical * E*-modes with nonzero components

*,*E

_{r}

*, and*E

_{z}

*. The development of resonant instability in this system is described by the DR and resonant condition those are [1].*B

_{φ}

* ω*and

*are the frequency and the longitudinal (along beam propagation direction that is*k

*is the waveguide’s radius,*R

*= 1,2,3 …,*s

*is the speed of light.*c

The character of the beam-plasma interaction changes depending on the beam current value. If the beam current is less than the limiting current in vacuum waveguide the instability is due to induced radiation of the system eigenwaves by the beam electrons. But, if the beam is over-limiting, its instability has the same nature as the instability in medium with negative dielectric constant [9, 14, 15]. We introduce a parameter

However, the resonant frequency, which is determined by the expressions (30), remains unchanged [15].

In order to show the variety of possible approaches to the solution of the problem of the initial perturbation development, in given case we solve it by other way. We turn to the set of origin equations, which describes e-beam instability in magnetized plasma waveguide

where * t*is time,

*are the cylindrical coordinates,*r

*and*

_{b,p}

*′*n

*are the perturbations of velocity and density for the beam and the plasma respectively,*

_{b,p}

where

The fields’ growth in the linear stage reveals itself most effectively on frequencies, closely approximating to roots of the DR and, simultaneously, to

where the carrier frequency

Thus, the problem of the initial pulse behavior reduces to determination of the slowly varying amplitude (SVA)

where

where

The solution of (37) is, actually, known. If one returns to the set (6) and transforms it (under

Comparison of (38) with (22) indicates one more new type of DSI. It develops in uniform cross section beam-plasma waveguide under over-limiting beam current and high level of dissipation. Its growth rate depends on the beam density and collision frequency as

### 4.2 Substantiation of the second new DSI by conventional method

Now we substantiate the second new DSI by solving the DR (30). We look for its roots in the form

where

The solutions of (39) depend on the beam current value that is on the value of parameter α. If

If the beam current increases and become comparable or higher than the limiting vacuum current i.e.

We emphasize new dependences on

## 5. The role of the new DSI in no-uniform-cross-section beam-plasma waveguide

### 5.1 Statement of the problem. Dispersion relation

In this section we pay special attention to systems, the geometry of which is similar to the geometry of plasma microwave sources and possible development of the new types of DSI in such systems. The simplest theoretical model of plasma microwave generators assumes relativistic e-beam propagating along axis of a plasma filled waveguide of radius * R*. The beam and plasma are assumed to be completely charge and current neutralized. In the waveguide cross-section the plasma and beam are annular, with mean radii

For theoretical study of the problem we use an approach [10], which gives result for arbitrary level of beam-plasma coupling. This condition is obligatory for obtaining comprehensive results. The DR, which follows from the approach, has a form, which clearly shows interaction of the beam and plasma waves. The approach proceeds from equation for polarization potential

Here * z*is the longitudinal coordinate,

*is the time,*t

*is the speed of light. The longitudinal electric field expresses as*c

where

The DR, which follows from the statement, is still very cumbersome (of integral type). To reduce the DR to a simple algebraic form one should make following expedient for theoretical model assumption: the plasma and the beam are not just thin but infinitesimal thin. In this case the DR becomes

where * k*is the wave vector along axis,

*is the frequency,*ω

(* G*is the coupling parameter. It depends on the overlap of the plasma and the beam fields and shows efficiency of their interaction

An important property of * G*is:

*=1 for*G

*< 1 in other cases. In long wavelength limit (for definiteness*G

*).*l

### 5.2 Growth rates

The DR (44) determines proper oscillations of transversally no uniform beam-plasma waveguide. The changes of the physical character of beam-plasma interaction must reveal themselves on its solutions.

where

where

This expression coincides to (27). Dissipation coming into interplay transforms this instability to DSI of new type with growth rate (coincides to (29))

As it should be, this is the instability discovered under consideration of the classical problem of the initial perturbation development in weakly coupled beam-plasma systems.

Of particular interest are limit of high, over-limiting currents of e-beam

For * G*<< 1). But under collective Cherenkov effect

The instability (52) under * G*∼ 1 has mixed mechanism: it is caused simultaneously (i) by a-periodical modulation of the beam density in media with negative dielectric constant and (ii) by excitation of the NEW. But the lower expression is the growth rate of instability caused only by excitation of the NEW of overlimiting e-beam. The presence of dissipation intensifies the growing of the slow beam wave. Instability turns to be of dissipative type with growth rate that again is inverse proportional to dissipation.

However, the dependence on the beam density is completely different. This is the same DSI, which develops in uniform cross-section beam-plasma waveguide under over-limiting currents. Instabilities of the same type may be substantiated for finite thicknesses of the beam and plasma layers in waveguide. In this case one must use perturbation theory based on smallness of coupling coefficient.

As follows from this section, in the geometry of microwave plasma sources, the development of both new DSI is possible. Basic parameters of the both new DSI, and the conditions of their development should be taken into account upon design of the high power, high frequency plasma microwave devices.

## 6. Conclusion

Thus, based on very general initial assumptions, we have found out that the number of DSI in the beam-plasma interaction theory is not limited by the only previously known type. Two new, previously unknown types of DSI are presented. The new DSI reveal themselves in the analysis of solution of the problem of initial perturbation development. This problem is classical in the theory of instabilities. The first new DSI is the dissipative instability under weak beam-plasma coupling. In absence of dissipation the instability in these systems is caused by the interaction of the beam NEW with the plasma. With an increase in the level of dissipation this instability gradually transforms to the new type of DSI. Its maximal growth rate depends on the beam Langmuir frequency

The second new type of DSI is dissipative instability of over-limiting e-beam in uniform cross section waveguide. With increase in the beam current, its space charge and inner degrees of freedom reveal themselves more efficiently. If the beam current becomes higher than the limiting current in vacuum waveguide then the instability mechanism changes. In uniform cross section beam-plasma waveguide the instability becomes due to a-periodical modulation of the beam density in medium with negative dielectric constant. In this case the increase in the level of dissipation leads to one more new type of DSI with the maximal growth rate

The same types of DSI develop in systems having geometry, similar to microwave sources: cylindrical waveguide with thin annular beam and thin annular plasma. If the coupling between the beam and the plasma hollow cylinders is weak and the beam current is under-limiting the first type of DSI develops, but under over-limiting currents – the second. However, if the coupling of the beam and the plasma cylinders is strong, conventional type of DSI develops with well-known growth rate

Both new DSI are confirmed by conventional analysis of the respective DR.

Some words about the approach used. It has many advantages. First of all, it is based on very general initial assumptions and gives results regardless on geometry and specific parameters. The same approach is used for solving the same problem for conventional beam-plasma instabilities of all types (Cherenkov type, cyclotron type etc) [13], for the Buneman instability [22] etc. Obtained expressions for the spatial–temporal distribution of growing fields clearly show that with increase in the level of dissipation in background plasma, all these SI transform into DSI of conventional type. In addition, the analysis of obtained expressions gives much more detailed information on SI than other methods give. Part of the information on SI is not available in any other ways. The coincidence of other information to the results of conventional analysis confirms the validity of the approach (initial assumptions, mathematics etc).

Also, the presented approach shows that the DR describing the SI of given type can serve not only for solving of the initial/boundary problems and obtaining the dispersion curves. This point of view is very simplified. The approach shows that much more additional information is available from the DR. It, in fact, provides results on the initial perturbation development.

Summarizing, one can state that the presented approach can serve as an independent and very effective method for studying of any SI. There is no need to solve the problem again. One should only substitute the parameters of given instability in general expression for the field’s space–time distribution. The usage of this approach instead of traditional initial/boundary problems gives complete picture of the instability development. At first glance, it might seem that this method of analyzing instabilities is more complicated. However, this complexity is only apparent. In addition, this complexity, if any, is overlapped by the completeness of the information received.

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## Notes

- The instability of low density e-beam in plasma is a vivid example demonstrating this sense. It is discovered in 1948, experimentally proven in early sixties; however its physical meaning became finally clear in the middle of seventies (see [8]).