US3257283A

US3257283A - Methods of heating ions in a plasma

Info

Publication number: US3257283A
Application number: US303606A
Authority: US
Inventors: Hamberger Sydney Maxwell
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1962-08-24
Filing date: 1963-08-21
Publication date: 1966-06-21
Anticipated expiration: 1983-06-21
Also published as: GB1014382A

Description

June 21, 1966 S. M. HAMBERGER METHODS OF HEATING IONS IN A PLASMA Filed Aug. 21, 1963 United States Patent O METHODS F HEATING IONS IN A PLASMA Sydney kMaxwell Hamberger, Abingdon, England, as-

signor to United Kingdom Atomic Energy Authority,

London, England Filed Aug. 21, 1963, Ser. No. 303,606 Claims priority, application Great Britain, Aug. 24, 1962,

32,531/ 62 2 Claims. (Cl. 176--3) This invention relates `to methods of heating ions in a plasma.

The electrons in a plasma contained by a magnetic field can readily be heated by feeding in electromagnetic waves of frequency equal to the electron cyclotron frequency. Heating the ions in a plasma by feeding in such energy at the ion cyclotron frequency is, however, more difficult, asin most pratical cases the ion cyclotron frequency is so much lower than the plasma frequency that electromagnetic Waves of frequency equal to the ion cyclotron frequency wave may not penetrate the plasma sufciently to effect useful heating of the ions.

According to the present invention the ions in a plasma contained by a magnetic field are heated by subjecting the plasma to two electromagnetic waves each of a frequency l high enough :to penetrate the plsama effectively and dif- A method in accordance with the present'invention for l heating ions in a plasma will now be described by way of example with reference to the accompanying drawing. The drawing shows diagrammatically an experimental apparatus in which the method has been carried out.

The apparatus comprises a cavity 1 of circular crosssection at the ends of which are coils 2 which when energised from a direct current source (not shown) produce a magnetic field within the cavity 1. This field is indicated by the dotted lines 3 and is substantially uniform over the central portion of the length of the cavity 1, but is constricted towards the ends. The cavity 1 contains hydrogen or helium maintained at a low pressure by a pumping arrangement 4.

The apparatus also includes two klystron oscillators 5 and 6 which are arranged to generate two sinusoidal electromagnetic waves and to supply them to the cavity 1 by way of a common path which includes a broad band, high power, klystron amplifier 7, a tuning device 8, and a waveguide 9. The tuning device -8 is used for matching purposes.

Projecting into the cavity 1 is a probe 10 which includes a small coil 11. During operation a voltage is induced in the coil 11 in dependence upon the rate of change of theaxial magnetic field in the cavity 1, and this voltage, after amplification by an amplifier 12, is displayed on an oscilloscope 13.

The method of operation is as follows. At the centre of the caviety 1 the magnetic field is approximately 3 kilogauss and the electron cyclotron frequency of the plasma contained by this field is therefore approximately 9400 megacyclesper second, at which frequency the cavity 1 is arranged to resonate. y

3,257,283 Patented June 21, 1966 ICC The electromagnetic waves produced by the oscillators 5 and 6 both have a frequency approximately equal to the electron cyclotron frequency, but the difference in the two frequencies is variable over a range' of about 0.5 to megacyclesper second. These two electromagnetic waves, which are sup-plied to the cavity 1 over the waveguide 9, are able to penetrate the plasma. The plasma acts as a non-linear impedance and generates within itself electromagnetic waves at the sum and difference frequencies. It is` to be expected that when the difference frequency is at least approximately equal to the ion cyclotron frequency the electromagnetic waves at the difference frequency will resonate with the ion motions and heating of the ions will result. f

The amplifier 12 responds to the difference frequency and therefore the amplitude of the voltage supplied by the amplifier 12 is a measure of the amplitude of electromagnetic waves of .this frequency in the plsamaproduced by the non-linear coupling of the two high frequency electromagnetic Waves.

If readings are taken of this amplitude for a range of difference frequencies and the readings plotted against4 the magnitude of the difference frequency, it is found that there is a dip in the curve in the region where the difference frequency is equal to the ion cyclotron. frequency in the mid-plane. This finding is consistent with the supposition that at that difference frequency the ions are being heated, with a consequent lossof energy by the electromagnetic waves having the difference frequency.

One suggested explanation of the mechanism, the explanation being simplified and ignoring the effect of collisions, is as follows. In the interior of a plasma the magnetic field is less than the applied magnetic field, due

. to the field resulting fromgyration of the charged particles .around the magnetic field lines. This effect is directly proportional to the summation of the energy of the particles transverse to the applied magnetic field. If the transverse energy of the particles fiuctuates, therefore, the internal magnetic field will fluctuate.

Thus if the electrons are additionally subjected to two sinu-soidally varying electric fields having frequencies just above and below the natural frequency of rotation (the electron cyclotron frequency), the electrons will execute oscillating spirals, the radii of which will vary at the difference'frequency. Thatis to say, the energy of the electrons transverse to the magnetic field varies at the difference frequency. An oscillatory magnetic eld is therefore produced within the plasma in the same direction and opposite to the applied field. This oscillatory magnetic field has associated with it an azimuthal electric field at the difference frequency which can couple to the lons.

Other known methods of generating the -two electromagnetic waves lmay be used, and the waves may then be supplied over the same or separate paths to the cavity 1.

The method is not restricted to heating of ions in a mirror field, but can be applied to any containment geometry in which there is an essentially uniform region of magnetic field.

I claim:

1. In a method of heating ions in a plasma having an ion cyclotron frequency and contained by a magnetic field, the steps of subjecting the plasma to two radio frequency electromagnetic waves each of a frequency high enough to penetrate effectively the plasma, said waves differing by a frequency which resonates with the ion cyclotron frequency.

2. In a method of heating ions in a plasma having an ion cyclotron frequency and an electron cyclotron frequency with said plasma being contained by a magnetic field, the steps of subjecting the plasma to two radio frequency electromagnetic waves each of a frequency approximately equal to the electron cyclotron frequency, said waves diiering by a frequency approximately equal to the ion cyclotron frequency.

References Cited by the Examiner UNITED STATES PATENTS Spencer 219-1055 Kamide 219--l0.55

Chang 313-161 Herold 313-161 3,090,737 5/1963 Swartz 313-161 3,104,305 9/1963 Crapuchettes 219-1055 3,105,803 10/1963 Weibel 313-161 L. H. BENDER, Assistant Examiner.

Claims

1. IN A METHOD OF HEATING IONS IN A PLASMA HAVING AN ION CYCLOTRON FREQUENCY AND CONTAINED BY A MAGNETIC FIELD, THE STEPS OF SUBJECTING THE PLASMA TO TWO RADIO FREQUENCY ELECTROMAGNETIC WAVES EACH OF A FREQUENCY HIGH ENOUGH TO PENETRATE EFFECTIVELY THE PLASMA,SAID WAVES DIFFERING