Alfvén wave

A plasma wave is a wave which propagates in a gas plasma. In a plasma, it may vary according to temperature, to give an applied magnetic field, and other properties, a variety of different shafts, most are electromagnetically, but there are no waves magnetic portion.

• 3.1 means short pulsed laser
• 3.2 By means of accelerated positrons

Preliminary

The plasma is referred to in physics, a gaseous, electrically conductive and electrically neutral to the outside mixture of charged and uncharged particles. By supplying energy takes place (often only partially ) ionization of the original gas, eg separation of electrons from atoms or of molecules. Analysis of plasma waves and the votes of plasmas radiation ( plasma spectroscopy ) is used for the research of plasmas and to monitor the current state of plasma ( plasma diagnostics ).

Species

Plasma without a magnetic field

In a plasma to which no magnetic field acts, there are two types of waves, the transverse electromagnetic ( light) waves and longitudinal waves:

• Electromagnetic waves can propagate only above the plasma frequency, including they are reflected from the plasma. This occurs, for example in case of short waves in the Earth's atmosphere when they meet on the ionosphere. At frequencies significantly higher than the plasma frequency, the plasma loses its effect on the propagation of the wave.
• To the longitudinal ( electrostatic ) waves are the classic sound waves, however, the additional electrical interaction leads to new effects. In the plasma frequency occurs, a new wave type, the plasma oscillation, in which the light electrons oscillate against the sluggish ions.

Magnetized plasmas

In the presence of a magnetic field, the vibration behavior of partially changed fundamentally. By the direction of the magnetic field, a direction is excellent, and the refractive index is strongly dependent on the direction of propagation relative to the magnetic field and on the polarization direction. The main effects can be described, when considering the propagation of waves in parallel or perpendicular to the magnetic field.

Propagation parallel to the magnetic field

Since the magnetic field only exerts forces on particles that can move perpendicular to the plasma, the longitudinal waves as in the magnetic field free case behavior. Wherein the transverse refractive index by the rotation of the circular polarization depends. Waves can propagate even at low frequencies, but at the cyclotron frequencies leads to resonant energy interactions with the electrons or ions, which move in circular orbits around the magnetic field lines and also partially a reflection. At very high frequencies, the wave behaves again as in the vacuum.

Propagation perpendicular to the magnetic

There is no longer pure longitudinal waves and transverse waves in it depends on the direction of the electric field of the wave compared with the direction of the magnetic field at. The electric field is oriented parallel to the external magnetic field, the shaft is not affected by the magnetic field and the propagation is the same behavior in the magnetic field -free case (O- mode). Longitudinal waves and transverse waves whose electric field is perpendicular to the magnetic field coupled to so-called X-Mode. Can propagate waves with low frequencies, again, at two frequencies, it first comes into resonance ( upper and lower hybrid resonance), which is followed by a frequency domain in which the wave can not propagate and are reflected at the boundary layer.

Alfvén waves at low frequencies

Slow plasma phenomena can be described by the MHD, which also applies to the low-frequency portions of the above waves. We distinguish three types in this description:

First sound wave

In a plasma can be created as in gases, a sound wave, when it comes to a position in a pressure increase.

2 shear Alfvén wave

When applying a magnetic field, waves to propagate parallel to the magnetic field lines. The ions "drag" it to the field lines. This results in a distorted field in the plasma, which in turn gives rise to a restoring force, and a shaft is formed.

3 compression Alfvén wave

This longitudinal wave propagates perpendicular to the Magnetfeldinien and behaves similarly to the sound wave. The classic pressure is amplified by a " magnetic pressure " and thus increases the speed of the shaft.

The Alfvén waves are named after Hannes Alfvén. The shear Alfvén waves move continuously with the same Alfvén speed.

Generation

To form a plasma wave in this equilibrium, the plasma is further energized selectively.

By means of short pulsed laser

The use of a high-energy and short-pulse laser is a possibility. Here, the laser is focused to a point. Due to the extreme increase in energy at a single point, the free electrons are additionally excited and take off from the atomic nuclei in this area. After the pulse, the electrons seek back to the rest atoms. The electrons are not fully captured by the atoms shoot partially over the top and turn back again. This oscillating dipole formed by the negatively charged electrons and the positively charged residue atoms, creates an electromagnetic plasma wave for a very short time.

In this way, researchers have been able to accelerate electrons up to 200 MeV ( mega-electron volts) (Science 298, 1596-1600 (2002) ).

By means of accelerated positrons

Another possibility is the creation of a plasma wave by means of an accelerated positrons, the antiparticle of the electron.

These positrons are shot through a plasma. Along the flight path disturbs the equilibrium and the positron produces an effect similar to the laser pulse. However, the plasma wave propagates from here over the entire trajectory.

Purpose

• In the field of experimental particle physics, as they Kiel field accelerators are significantly smaller and cheaper than today's particle accelerators with miles of acceleration tubes.
• In medicine, tumors can be treated by proton beams. The new apparatus, the patient should not be placed on the accelerator centers, but could be directly treated in the hospital.
• In drive technology for the use of plasma engines, eg for space expeditions to remote areas of the universe.