Plasma wave
A plasma wave is a wave that propagates in a gas plasma . In a plasma there can be a multitude of different waves depending on the temperature , the applied magnetic field and other properties. Most of them are electromagnetic , but there are waves without a magnetic component.
Preliminary remark
In physics, plasma is a gaseous, electrically conductive and externally electrically neutral mixture of charged and uncharged particles. By supplying energy there is (often only partial) ionization of the original gas, e.g. B. a separation of electrons from atoms or from molecules . The investigation of plasma waves and the radiation emitted by plasmas (plasma spectroscopy) is used to research plasmas and to monitor the current state of the plasma (plasma diagnostics).
species
Plasma without a magnetic field
In a plasma that is not subject to a magnetic field, there are two types of waves, the transverse electromagnetic (light) waves and longitudinal waves :
- Electromagnetic waves can only propagate above the plasma frequency , below which they are reflected by the plasma. This occurs, for example, with short waves in the earth's atmosphere when they hit the ionosphere . At frequencies well above the plasma frequency, the plasma loses its influence on the propagation of the wave.
- The longitudinal (electrostatic) waves include the classic sound waves , but the additional electrical interaction leads to new effects. At the plasma frequency , a new type of wave occurs, the plasma oscillation , in which the light electrons swing against the inert ions.
Magnetized plasmas
In the presence of a magnetic field, the vibration behavior changes fundamentally in some cases. A direction is defined by the direction of the magnetic field and the refractive index depends heavily on the direction of propagation relative to the magnetic field and on the direction of polarization. The most important effects can be described by looking at the propagation of waves parallel or perpendicular to the magnetic field.
Propagation parallel to the magnetic field
Since the magnetic field does not exert any force on charged particles that move along the magnetic field lines, the longitudinal waves behave as if there were no magnetic fields. In the case of transverse waves, the refractive index depends on the direction of rotation of the circular polarization . Waves can propagate even at low frequencies, but at cyclotron frequencies there are resonant energy interactions with the electrons or ions, which move in circular orbits around the magnetic field lines and partially reflect again. At very high frequencies the wave behaves again as in a vacuum.
Propagation perpendicular to the magnetic field
Here there are no longer any pure longitudinal waves and with transverse waves it depends on the direction of the electric field of the wave compared to the direction of the magnetic field. If the electric field is aligned parallel to the external magnetic field, the wave is not influenced by the magnetic field and the propagation corresponds to the behavior in the magnetic field-free case (O-mode). Longitudinal waves and transverse waves, whose electric field is perpendicular to the magnetic field, couple to the so-called X-mode. Waves with low frequencies can propagate again, but resonances only occur at two frequencies (lower and upper hybrid resonance), followed by a frequency range in which the waves cannot propagate and are reflected at the boundary layer.
Alfvén waves at low frequencies
Slow plasma phenomena can be described with magnetohydrodynamics , this also applies to the low-frequency ranges of the waves above. There are 3 types of this description:
- 1. Sound wave
- In a plasma, as in gases, a sound wave can arise if there is an increase in pressure at one point.
- 2. Shear Alfvén wave
- When a magnetic field is applied, waves can propagate parallel to the magnetic field lines. The ions "pull" the field lines with them. This results in a disturbed field in the plasma, which in turn creates a restoring force and a wave is created.
- 3. Compression Alfvén wave
- This longitudinal wave propagates perpendicular to the magnetic field lines and behaves similarly to the sound wave. The classic pressure is increased by a "magnetic field pressure" and thus increases the speed of the wave.
The Alfvén waves are named after Hannes Alfvén . The shear Alfvén waves move at the Alfvén velocity of the same name .
Artificial production
In order to generate a plasma wave in this dynamic equilibrium, the plasma is also specifically stimulated.
Using short pulsed lasers
The use of a high energy and very short pulsed laser is one possibility. Here, the laser is set to a point focused . Due to the extreme increase in energy at a single point, the free electrons are additionally excited and move away from the atomic nuclei in this area. After the pulse, the electrons strive back to the residual atoms. However, the electrons are not completely captured by the atoms, some shoot over the target and then return again. This oscillating dipole , formed by the negatively charged electrons and the positively charged residual atoms, creates an electromagnetic plasma wave for a very short time.
In this way, researchers have succeeded in accelerating electrons up to 200 MeV (megaelectron volts).
Using accelerated positrons
Another possibility is the generation of a plasma wave by means of already accelerated positrons , the antiparticles of the electrons.
Here positrons are shot through a plasma. Along the flight path, the positron disturbs the equilibrium and creates an effect similar to that of the laser pulse. However, the plasma wave spreads here over the entire trajectory.
In this way, another positron that followed the first could be accelerated by another 80 MeV.
Possible uses
These techniques are currently still in the experimental stage. However, practical use in the following areas is foreseeable in the near future:
- in the field of experimental elementary particle physics , since these Kielfeld accelerators are significantly smaller and cheaper than today's particle accelerators with kilometer-long acceleration tubes.
- in medicine: tumors can be treated with proton beams. With the new equipment, patients would no longer have to be brought to accelerator centers, but could be treated directly in hospital.
- in drive technology for the use of plasma engines , e.g. B. for space expeditions to more distant areas of space.
literature
- Neil F. Cramer: The physics of Alfvén waves. Wiley-VCH, Berlin 2001, ISBN 3-527-40293-4
- Abraham C.-L. Chian: Alfvén waves in cosmic and laboratory plasmas. Royal Swedish Academy of Sciences, Stockholm 1995, ISBN 91-87308-33-9
- Rodney Cross: An introduction to Alfven waves. Hilger, Bristol 1988, ISBN 0-85274-245-2