Modulation of magnetosonic waves by background plasma density in a dipole magnetic field: 2‐D PIC simulation

A weak E layer in a non-uniform magnetic field will tend to precess as a rigid body in response to the radial focusing of external magnetic fields and fields due to wall currents. We study the interaction of this precessional mode with a background plasma, and we explicitly include dissipation mechanisms in the plasma, walls and external resistors. When the plasma background is treated in the MHD approximation, we find that the mode changes character from a precessional mode at low density to a compressional Alfvén wave at high density. For a very weak E layer, instability is found, even without dissipation, when a sufficiently high background plasma density is present. However, for moderate E-layer strengths, the modes are found to be stable, even with dissipation.

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Effect of axial magnetic field tapering on whistler-pumped FEL amplifier in collective Raman regime operation

International Journal of Engineering & Technology ◽

10.14419/ijet.v7i4.15298 ◽

2018 ◽

Vol 7 (4) ◽

pp. 2044

Author(s):

Ram Gopal ◽

Pradip Kumar Jain ◽

Pradip Kumar Jain

Keyword(s):

Magnetic Field ◽

Growth Rate ◽

Magnetic Fields ◽

Plasma Density ◽

Cyclotron Frequency ◽

Axial Magnetic Field ◽

Electron Cyclotron ◽

Interaction Region ◽

Background Plasma ◽

Transfer Energy

The dispersion relation of the FEL Amplifiers is sensitive to the linear tapered strong axial magnetic fields, electron cyclotron frequency and plasma frequency of electrons. For the synchronism of the pumped frequency, it should be closed to electron cyclotron frequency which is resonantly enhanced the wiggler wave number that produces the amplifier radiation for higher frequency from sub millimeter wave to optical ranges. The guiding of radiation signal into the waveguide and charge neutralization phenomenon, the beam density should be greater than the background plasma density with tapered strong axial magnetic field. It is quite considerable that radiation signal slowed down at much higher background plasma density comparable to the density of beams and enhanced the instability growth rate also. In Raman Regime operation, the growth rate decreases as increases with operation frequency of the amplifier, however, the growth rate is larger in this regime. It is noted that as increases with background plasma density, the beat wave frequency of the Ponderomotive waves is increases thus the mechanism of background plasma density can serve for tenability of the higher frequencies. The tapering of the strong guided magnetic field is a crucial role for enhancing the efficiency of the net transfer energy as well as reduction of interaction region along the axis. It is observed that, an efficiency of the transfer energy enhanced by while the reduction along the interaction region of about with the variation of tapering in a strong axial guided magnetic fields.

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Dispersion relation and growth rate in a free-electron laser with a background plasma

Journal of Plasma Physics ◽

10.1017/s0022377815000033 ◽

2015 ◽

Vol 81 (3) ◽

Cited By ~ 3

Author(s):

T. Mohsenpour ◽

B. Maraghechi

Keyword(s):

Magnetic Field ◽

Growth Rate ◽

Dispersion Relation ◽

Plasma Density ◽

Free Electron ◽

Free Electron Laser ◽

Axial Magnetic Field ◽

Background Plasma ◽

Positive Energy ◽

Induced Velocity

The method of perturbation has been applied to derive a general dispersion relation for a free-electron laser (FEL) with background plasma and helical wiggler in the presence of an axial magnetic field. This dispersion relation is solved numerically to find unstable interactions among all of the wave modes. Numerical calculations show that new coupling between the left wave and positive-energy space-charge of electron beam are found when wiggler induced velocity is large. This coupling does not change with increasing the plasma density. The growth rate of FEL is changed with increasing the plasma density and the normalized axial magnetic field.

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