Cyclotron resonance heating

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Electron cyclotron resonance heating ( English electron cyclotron resonance heating , ECRH) and ion cyclotron resonance heating ( ion cyclotron resonance heating , ICRH) are methods for heating in the magnetic field confined plasmas , such as for the development of fusion reactors to be used as an energy source.

physics

Schematic representation (orbital radii not scaled) of the gyration movement of electrons and a hydrogen ion (proton) in the magnetic field.

Both methods are based on charged plasma particles, electrons or ions , being forced onto helical paths in the magnetic field by the Lorentz force . These are composed of a movement along the imaginary magnetic field line and - perpendicular to it - a circular movement around the field line (gyration). For the latter, the so-called gyration frequency results because of (Lorentz force = centrifugal force) . Here is the magnetic field strength, the mass and the charge of the gyrating particle. If you radiate an electrical wave with exactly this frequency, it can be absorbed by the gyrating particle. Since the gyration frequency is fixed by the magnetic field, it does not change due to the absorption of the microwave, but the increased energy of the particles leads to an increase in the rotational speed (gyration speed) and thus in a larger gyration radius due to the increased centrifugal force.

For electron cyclotron resonance heating, a wave of the frequency is required according to the above equation , whereby the magnetic field is to be used in Tesla . Since nuclear fusion experiments are carried out with magnetic fields of a few Tesla, frequencies around 100–200 GHz, corresponding to wavelengths of 3–1.5 mm, ie in the microwave range, result . Since the mass of the hydrogen ion (proton) as the lightest ion is already 1840 times higher than that of the electron, the gyration frequency of the hydrogen ions is lower by this factor for the same magnetic field. With the same kinetic energy, i. H. the same temperature, the gyration radius is larger because of the increased centrifugal force. For the ion cyclotron resonance heating, the frequencies are in the range of 50-100  MHz , the wavelengths are a few meters. The radii of the gyration movement depend on the speed because of the centrifugal force. Under the conditions of a fusion reactor with thermal energies of approx. 10 keV (corresponds to a temperature of approx. 100 million ° C), the thermal speed of the hydrogen ions is 1,700 km / s; that of the lighter electrons 73,000 km / s.

Electron and ion cyclotron resonance heating are resonance phenomena , i. H. the absorption takes place only at a fixed frequency given by the above equation. A wave that penetrates the plasma is therefore only locally absorbed by plasma particles where the magnetic field meets the above resonance condition. This can be seen in contrast to the microwave oven in the kitchen, where the absorption of the microwaves takes place at the dipole moments of the water molecules in the food, which are thrown back and forth by the electric fields of the electromagnetic wave and give off heat through collisions with neighboring atoms. This process is largely independent of the frequency of the wave.

Because of the different frequencies of electron and ion cyclotron resonance and thus the different wavelengths of the electromagnetic waves involved, the technical implementation and application for heating the plasmas are so different that both methods will be described separately below.

Electron cyclotron resonance heating (ECRH)

Since the magnetic field in the plasma is location-dependent, the location at which the microwave is absorbed and the electrons are heated can be selected very precisely via the resonance condition. Under standard conditions, the magnetic field strength increases in a fusion experiment towards the inside of the torus , where the magnetic field coils are closer. The microwave is radiated radially from the outside into the toroidal plasma and absorbed where the resonance condition is met. A change in the poloidal (vertical) angle of incidence allows heating on the plasma axis or further outside. B. the temperature distribution (radial temperature profile) can be modified. A toroidal (horizontal) tilting of the microwave beam, on the other hand, creates a toroidal asymmetry in the electron movement and thus a toroidal current. This current drive ( electron cyclotron current drive , ECCD) is one of the ways in which an attempt is made to permanently maintain the toroidal current required in a tokamak . Such local currents in the plasma that can be generated radially in a targeted manner can also influence the magnetic field configuration or heal dynamic instabilities (so-called tearing modes ).

The ECRH is initially limited to low and medium particle densities, since with increasing density the plasma for the microwaves - like a metal for visible light - becomes reflective and the heating steel can no longer penetrate to the resonance zone. This can be circumvented by choosing other polarizations of the microwave or by emitting waves with a multiple of the gyration frequency (with N = 1, 2, 3, ...) that are not reflected under the conditions in question. ECRH can also be used for diagnostic purposes by locally generating a small temperature perturbation - e.g. B. with a heating pulse with moderate power - and then observed how the temperature disturbance spreads in the plasma.

technical realization

Gyrotrons are being developed as microwave sources at frequencies around 100–200 GHz . The currently most powerful tubes achieve outputs of 1 MW over a period of 30 minutes. The microwave beams generated by the gyrotrons with a diameter of a few centimeters can be brought to the torus either freely via mirrors or within waveguides , where they are guided into the plasma by possibly movable mirrors. Since the microwave beams with wavelengths around 2 mm are comparatively insensitive to surface roughness, robust metal mirrors can be used. Because of the high power, the components have to be cooled despite the low losses (high reflectivity of the metal mirror / waveguide). ECRH has a number of technical advantages that make this heating method particularly suitable for reactor operation: Only comparatively small windows to the torus are required for the microwave rays and, with mirrors, labyrinth-like openings, e.g. B. through the radiation protection walls, so that the gyrotrons can be operated and maintained away from the torus hall. The disadvantage that primarily only the electrons are heated, but a high temperature of the hydrogen ions is required for nuclear fusion, does not play a major role, since at the densities required in reactors, electrons and ions quickly balance their energy through collisions.

A 50 MW ECR heater is intended for the tokamak ITER , primarily for the necessary power drive and for stabilizing instabilities. The Wendelstein 7-X stellarator is heated with a 10 MW ECRH (first expansion stage 7 MW) in long pulse mode. In addition to the heating effect, the current driven by ECRH can be used to change the magnetic field configuration.

Individual evidence

  1. ^ Charles Kittel, Walter D. Knight, Malvin A. Ruderman, A Carl Helmholz, Burton J Moyer: Berkeley Physics Course Volume 1: Mechanics . Ed .: Springer. 2001.