Patent Number: 047553456
Section: summary

Radio frequency (rf) heating in the 10 to 100 megahertz (MHz) range is a practical and efficient means of heating magnetically confined plasmas to high temperatures for fusion energy devices. The high-frequency waves are generated in an oscillator outside a vacuum vessel containing the magnetically confined plasma. If the waves have particular frequencies (or wavelengths), part of their energy can be transferred to the nuclei or electrons in the plasma. These particles then collide with other particles and thereby increase the plasma temperature. In ion cyclotron resonance heating (ICRH), the frequency of the energy source is adjusted to be roughly equal to the frequency at which the ions in the plasma spiral about the magnetic field lines. The ions acquire energy from the rf waves and share it with other particles in the plasma by collisions. ICRH is generally preferred over electron cyclotron resonance heating because the frequency for a given magnetic field strength is lower due to the larger mass of the ions. As the heating demands of medium and large fusion devices, such as the Tore Supra tokamak in France, for example increases, greater power handling demands over longer periods of operation are placed on the antenna systems used in rf heating of the plasmas in these devices. Due to limited access ports to the plasma, smaller antenna structures with higher power and higher frequency capabilities are required to maximize the power conveyed through each antenna. Such demands are forcing each component of the ICRH system antenna, Faraday shield, and rf vacuum feedthrough to be improved. An improved vacuum feedthrough for ICRH heating is the subject of a copending U.S. Pat. Application Ser. No. 836,776, now U.S. Pat. No. 4,694,264, filed Mar. 5, 1986, by Thomas L. Owens et al, for Improved Coaxial Vacuum Feedthrough and having a common assignee with the present invention. The subject matter of this reference is incorporated herein by reference thereto. Various inductively coupled, antenna designs, such as the short loop, long loop, resonant cavity and U-slot antennas have been proposed, or used, for fusion plasma heating. However, the first three of these antenna designs have disadvantages, stemming from the requirement of external impedance matching networks which limit their usefulness. The U-slot has internal matching but is limited in load range that can be matched at high frequencies. For example, the simple half-loop antenna has been used on many plasma confinement experiments. This antenna design, although it offers good magnetic coupling to a plasma, has additional disadvantages due to its high voltage requirements at the input because of its high inductance, its high voltage standing wave ratio (VSWR) in the vacuum coax/feedthrough region between the antenna and the required matching network, usually stub tuners, and its need for multiple-port access for installation and removal in a typical use. The resonant cavity configuration includes an inductive post radiating element which is coupled to ground at one end through a capacitor and fed by a coaxial line attached a fractional distance from the grounded end. By proper choice of the feedpoint and capacitance, this antenna can be matched to the transmission line for a single value of load resistance. However, since the load resistance due to the plasma may vary significantly, the antenna requires a matching network external to the vacuum system, as is the case with other antenna designs. Thus, it will be apparent to those skilled in the art that there is a need for an improved inductive type antenna for high power and high frequency rf plasma heating applications which is of compact design, does not require external tuning networks, and provides reduced voltage and current requirements at the vaccuum feedthrough. SUMMARY OF THE INVENTION In view of the above need, it is an object of this invention to provide an improved compact, inductive-loop type antenna for use in rf frequency inductive heating of a magnetically confined plasma which does not require an external impedance matching network for varying plasma loads. Another object of this invention is to provide an antenna as in the above object which requires lower voltages and currents in the coaxial feed line to the antenna for higher power and frequency applications. Other objects and many of the attendant advantages of the present invention will be apparent from the following detailed description of the invention taken in conjunction with the drawings. In summary, the invention is a resonant double-loop antenna for inductively coupling rf energy into a magnetically confined plasma within a vacuum housing. Basically, a resonant circuit is formed by connecting an inductive radiating element between capacitors C1 and C2 which have their opposite electrodes connected in common. The inductive element is in the form of a current strap positioned adjacent the plasma to be heated. The length of the inductor is much less than a half wavelength of the operating frequency range, typically 50 to 110 megahertz. A real input impedance is obtained by tapping into the resonant circuit at a near midpoint of the current strap between capacitors C1 and C2. The impedance can be matched by adjusting capacitors C1 and C2 for a given tap arrangement, or by keeping C1 and C2 fixed and adjusting the tap position. Because the complete circuit loop consisting of C1, C2, and the antenna inductor or current strap is resonant, current flows in the same direction along the entire length of the antenna and is essentially equal in each branch of the circuit divided by the input tapping point. This reduces the voltage and current requirements at the input of the antenna. An electrostatic shield (Faraday shield) may be placed between the inductive radiating element and the plasma being heated to prevent capacitive coupling between the plasma and the antenna.