Microinstabilities in an inhomogeneous ion-beam plasma system

The authors present an experimental and theoretical study of the interaction of an energetic ion beam with a collisionless, magnetised target plasma generated by means of a microwave helix. The beam-plasma system is considered inhomogeneous with beam and plasma density gradients perpendicular to the homogeneous magnetic field which is directed parallel to the axis of the beam and the plasma column. Experimentally the authors find excitation of an instability with frequency near the ion cyclotron frequency. The wave propagates 1 azimuthally in the direction of the electron Larmor rotation and axially in the direction opposite to the motion of the beam ions. The measurements are compared with a linear fluid theory for an infinite system. The numerical results show that in the presence of a steep enough negative density gradient waves with positive perpendicular and negative axial phase velocities can be driven unstable by the beam, in agreement with the observations.

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