Patent Number: 052251462
Section: description

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 illustrates an electron injection scheme for controlling transport in a tokamak plasma. An electron injector 10 is shown external to a tokamak plasma 12. The plasma has a toroidal magnetic field B.sub.T. Preferably, the injector 10 is placed where the natural grad-B drift direction is radially inward. This would either be above or below the plasma 12 depending on the direction of B.sub.T. For purposes of illustration, FIG. 1 shows the injector 10 below the plasma. Electrons are emitted from a heated cathode 20. An electro cyclotron heating (ECH) waveguide cavity 16 has an acceleration region 17 adjacent to the cathode for accelerating the electrons by radio frequency waves at the electron cyclotron frequency. Referring to FIG. 1a, ripple or bending magnets 18 are locally placed around the torus and provide a ripple field region in the plasma. The cathode 20 is connected to power source 19. The electrons are injected into the plasma and then allowed to drift vertically, by means of the grad-B drift, into the region of field ripple. The field created by the bending magnets is a relatively small perturbation to the magnetic field to cause the electrons to be trapped and drift radially inward into the center of the plasma. This charges the interior of the plasma negative and thereby creates a radial electric field at the edge of the plasma. The arrangement provides a means of biasing large tokamak plasmas without the insertion of material objects, and a way of attaining H-mode regime with substantially lower applied power. A detailed view of the cathode 20 is illustrated in FIG. 2. The cathode includes a carbon heater 22 surrounded by tantalum shields 24, and inside tantalum shields 24a. The heater and tantalum shields are supported by current feed and support tubes 26. Preferably, the tubes 26 are coaxial copper tubes for supplying a current feed as well as providing for water flow. The carbon heater 22 is also attached at one end to copper mounting blocks 24a. Adjacent to the carbon heater 22 at an end opposite the mounting blocks is an electron emitting faceplate 28 of LaB.sub.6 coated on molybdenum or tungsten. In accordance with the above description, a method for creating a radial field at the edge of a plasma of a tokamak includes providing a ripple field region in the plasma by the localized placement of a plurality of bending or ripple magnets 18 around the torus of the tokamak. Electrons having a predominantly perpendicular energy with respect to the toroidal magnetic field direction B.sub.T, are externally or non-invasively injected into the ripple field region and are trapped. The plasma center is negatively charged by allowing the electrons to grad-B drift vertically toward the plasma interior until they are detrapped, thereby creating a radial electric field at the edge of the plasma. The electron injector 10 described is capable of injecting approximately 20A of electron current with electrons having low KeV energy and a perpendicular to parallel energy ratio (v.sub..perp. /v.sub..vertline.) of greater than 1. For trapping the electrons it is also necessary that: EQU v.sub..perp. /v.sub.51 .gtoreq.(2.delta.).sup.1/2 (1) where .delta. is the ripple fraction. As an example, if v.sub..perp. /v.sub..vertline. is .gtoreq. 10, then the electron becomes detrapped when .delta..ltoreq.0.005. A cathode having electro cyclotron heating as described would obtain electrons with v.sub..perp. /v.sub..vertline. &gt;&gt;1. The radial penetration distance of injected electrons, L, can be controlled by adjusting the injected electron energy: EQU L=V.sub.D /.nu..sub.eff =40E.sup.2.5 (2.delta.).sup.0.5 (RBn.sub.e).sup.-1 (2) where L is expressed in centimeters (cm); E in KeV; R in meters (m); B in Teslas (T); n.sub.e is (10.sup.12 cm.sup.-3), and: EQU .nu..sub.eff sec.sup.-1 =2.5.times.10.sup.3 n.sub.e E.sup.-1.5 (2.delta.).sup.-0.5 (3) and further: EQU V.sub.D =10.sup.5 E(RB).sup.-1 (4) where V.sub.D is in cm/sec. For a typical tokamak fusion test reactor (TFTR) having parameters of R.apprxeq.2.7, B.apprxeq.5, n.sub.e .apprxeq.3, .delta..apprxeq.0.02, and E=5, the expected penetration distance, L, is 11 cm. The required injection current is modest (on the order of 10A), even for large tokamak plasmas. The minimum required injected current is determined by the leakage radial current. Taylor et. al gives an expression of the leakage radial current density from the force balance equation as: EQU J.sub.r B.sub..phi. /c=n.sub.i m.sub.i cE.sub.r /(B.sub..phi. .tau..sub.p) (5) where .tau..sub.pp is the momentum damping time which is thought to be of the order of ion-ion collision time (T. H. Stix, Phys. Fluids 16, 1260 (1973)). Integrating equation 5 over the surface, the total radial current obtained is: EQU I.sub.r =4.pi..sup.2 aRn.sub.i m.sub.i c.sup.2 E.sub.r B.sub..phi..sup.-2 .tau..sub.p.sup.-1 (6) Assuming .tau..sub.p to be the ion-ion collision frequency, one obtains: EQU I.sub.r (A)=10aR.mu.n.sub.e.sup.2 E.sub.r B.sub..phi..sup.-2 T.sub.i.sup.-1.5 (7) where a is in meters, R is in meters, n.sub.e is 10.sup.12 cm.sup.-3, E.sub.r is 100 V/cm, B is in Teslas, and R.sub.i is 10 eV. As an example, in terms of the above units, on the Continuous current Tokamak (CCT) at UCLA, a=0.3, R=1.5, .mu.=1, n.sub.3 =1, E.sub.r =2, B.sub..phi. =0.3, and T.sub.i =3, the radial current is about 20 amperes, which is very close the observed required injected current to maintain the high confinement mode in the CCT. For the TFTR, a=0.8, R=2.7, .mu.=2.7, .mu..ltoreq.2, n.sub.3 =3, E.sub.r =5, B.sub..phi. =5, and R.sub.i =30, the radial current is 0.47A, which is relatively small. For CIT (Compact Ignition Takamak) parameters, a=0.6.times. 1.4, R=2.1, .mu.=2.5, n.sub.3 =20, E.sub.r =5, B.sub. .phi. =11, and T.sub.i = 30, and the I.sub.r =4.4A, which is still quite small. Thus, the CIT plasma can be induced to go into the high mode with less than 100 KW of injected power. There has thus been shown na electron injection scheme for controlling transport in a tokamak plasma. Electrons with predominantly perpendicular energy are injected into a ripple field region created by a group of localized poloidal field bending magnets. The trapped electrons then grad-B drift vertically toward the plasma interior until they are detrapped, charging the plasma negative. Calculations indicate that the highly perpendicular velocity electrons can remain stable against kinetic instabilities int he regime of interest for takamak experiments. The penetration distance can be controlled by controlling the "ripple mirror ratio", the energy of the injected electrons, and their v.sub..perp. /v.sub..vertline. ratio. In this scheme, the poloidal torque due to the injected radial current is taken by the magnets and not by the plasma. Injection is accomplished by the flat cathode containing an ECH cavity to pump electrons to high v.sub..perp.. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiment was chosen and described to best explain the principles of the invention and its practical application and thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.