In this review we consider new typw of nonlinear interactions of particles and waves in plasmas which allows us to convert the wave energy with large frequency difference without inverted particle population and special matching conditions between the wave frequencies and the wave numbers. The main emphasis is given to recent concepts clarifying the physics of the plasma-maser phenomena. Investigations on the basis of these concepts include studies of closed and open nonstationary systems, influence of the wave stochasticity on the plasma-maser interactions, transition effects, and the role of certain symmetries (as well as their violation) on the efficiency of the plasma maser.

1. Introduction.........................................................................................................................

2. Dielectric function of the nonresonant waves.............................................................................

2.1 Direct third-order contribution........................................................................................
2.2 Polarisational contribution...............................................................................................
2.3 Weak nonstationarity......................................................................................................

3. Closed and open plasma systems.............................................................................................

3.1 Conservation of nonresonant quanta in closed systems...............................................
3.2 Open systems..................................................................................................................
3.3 Regular sources...............................................................................................................
3.4 Wave amplification in nonstationary plasmas...............................................................

4. Transition processes in plasma-maser interactions....................................................................

4.1 Stochastic properties of waves and plasma microinhomogeneities..............................
4.2 Regular resonant waves..................................................................................................
4.3 Random resonant waves.................................................................................................
4.4 Transition interaction.......................................................................................................

5. Evolution of resonant waves and particle distributions..................................................................

5.1 Nonlinear evolution of resonant waves...........................................................................
5.2 One-dimensional problem...............................................................................................
5.3 Wave stochasticity and maximum rates for resonant waves........................................
5.4 Inverse plasma-maser effect...........................................................................................

6. Interactions in magnetized plasmas..........................................................................................

6.1 The nonlinear dielectric permittivity................................................................................
6.2 Plasma-maser interaction...............................................................................................
6.3 Transition to unmagnetized plasmas...............................................................................

Acknowledgements...................................................................................................................
References.............................................................................................................................

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