. .—-. ., . CIC-14REPORTCOLLECTION LA=6044-PR ProgressReport REPRODUCTION UC-20 Cmy Issued: Awust 1975 LASL Controlled Thermonuclear Research Program January —December 1974 Compiled by F. L. Ribe I 1 / h J ! IosiQ4a Iamos k scientific laboratory of the University of California LOS ALAMOS. NEW MEXICO 87545 4 “b An AffirmativeActian/EqualOpportunityEmplayer UNITED STATES ENERGY RESEARCH AND DEvELOPMENT ADMINISTRATION CONTRACT W-7405 -ENG. 36 The four most recent reports in this series, unclassified, are LA-4585-MS, LA-4888-PR, LA-5250-PR, and LA-5656-PR. In the interest of prompt distribution, this report was not edited by the Technical Information staff. PrintedintheUnitedStatesofAmerica.Availablefrom NationalTechnicalInformationService U.S.DepartmentofCommerce S265 PortRoyalRoad Springfield,VA 22151 Price:PrintedCopy$7.60Microfiche$2.25 . .= .. _. \ -— ~===%.—- SUMMARY AND INTRODUCTION ,.— .- .— F. L. Ribe and G. A. Sawyer Since the last Annual Progress Report (LA-5656- In the case of the Scylla I-B and I-C experiments. PR, .July 1974) Scyllac has been rearranged into its detailed studies of the plasma implosion were com- final, complete toroidal configuration with 4-meter pleted and the device has been rebuilt for lower- radius. The equilibrium and m = 1 instability temperature, higher density plasma operation ap- properties of the 4-meter-radius sector were found to propriate to heating and scattering experiments us- be unchanged, i.e., plasma confinement was ter- ing a 1OO-.J, 4 X 1010 W/cm2 C02 laser furnished by minated by the m = 1 instability with a growth rate Group L-1. y = 0.7 MHz at a compression field of 40 kG and an The implcwion-heating experiment (Chapter VII) / = 1,0 wavelength of 33 cm. The full-torus con- has been in operation for some months after I@rat ion allowed the toroidal mode structure of the successfully (wercoming numerous problems of high- m = 1 instability to be measured. It follows the wjltage technology. The data are yieldin~ a consis- theory with a cutoff minimum wavelength of about 4 tent picture of’ the irnplosi(m process. meters. No m = 2 or higher m instabilities were In the ZT-1 diffuse toroidal pinch experiment, ion observed. These results were reported at the 1974 temperatures of 140 eV at c(mfinement times of 10 — IAEA Fusion Conference in Tokyo. psec have heen obtained. Detailed studies of Since the Tok.vo Conference the main attention of magnet ic-i’ield structure show that stahilitv is the Scyllac group has been to closing the gap limited by diffusion of the poloidal and toroidal” between the plasma dynamics of the instabilities fields. A new configuration (ZT-S) with 50”( larger and the current risetime (gain-bandwidth) minor radius and more closelv matched poloidal and characteristic of the feedback circuit (see Chapter III toroidal lield risetimes is being set up to lengthen the below). Bv lengthening the / = 1,0 wavelength to 63 cfmfinement by raising the fielci-diffusi(m times. A cm and decreasing the compression field to 18 kG parameter study of a Z-pinch fusion reactor has been the growth rate has decreased to 0.3 MHz in accord made. wit h theory. The plasma confinement time has been In the realm of plasma diagnostics HF-laser. side- correspondingly increased from 10 to 25 psec. Im- on holography has been successfully applied to provements in the feedback circuit have decreased Scyllac, yielding results of primary importance. In its risetime from 1.5 to 1.1 Ksec and raised its current addition the basic plasma-beta and coupled-cavity capability from 700 to 1100 A. At this writing interferometer measurements have been com- manipulation of the plasma column by programmed puterized with large savings in data-reduction time. currents in / = O feedback coils is proceeding to test Tw(~-dimensional displays of Thompson scattering the theoretical finding that the plasma response will data are being developed. be excessive. i = 2 feedback coils are being elec- The Experimental Plasma Physics group con- trically tested and will probably be used, since they t inued to extend its studies of the electrical resistivi- have the virtue of instantaneous plasma response ty by utilizing the detailed information about the and produce three to four times greater force on the threshold for parametric instabilities or anomalous plasma. With the new plasma characteristics, im- absorption that was obtained by them in previous proved circuit, response and I = 2 coils, we are years. A particularly important demonstration of cautiously optimistic that the next year will see the these precision measurements is this group’s recent purpose of Scyllac accomplished—the production of * observation that intense ac electric fields can plasmas with staged heating and low compression produce a fundamental change in the electron-ion rat ios (b/a z 2.5) appropriate for wall stabilization collision rate. of’the m = 1 mode. Much significant new technology The Theoretical Physics Program has continued of high-voltage, high-charge capability, low- strong programs in magnetohydrodynamic (MHD) inductance spark gaps have been developed for this theory including diffuse plasma and field profiles, experiment. Vlasov analysis of stability properties of plasmas, A capability of conventional, long linear theta and computer modeling of plasmas. pinches is being restwed with the nearing comple- tion of’ the 5-meter Scylla IV-P experiment. Engineering support of the CTR program includes of superconducting wire, high-current interrupters, . mechanical design development of specialized elec- and fabrication of a 300-k.J storage coil. trical components, such as spark gaps, construction Fusion Technology and Reactor Design have con- of new experiments, and engineering support of the tinued in the areas of reactor system studies, ● major confinement devices. neutronics data assessment, and insulator and struc- Magnet ic Energy Transfer and Storage (METS) t ural alloy research and development of conceptual studies are progressing in the areas of development design of a Scyllac Fusion Test Reactor (SFTR). 2 II. SCYLLAC FULL TORUS EXPERIMENTS E.L. Cantrell, W. R. Ellis, B.L. Freeman, K.B. Freese, H. W. Harris, F. C. Jahoda, R. Kristal, M.D. Machalek, J. R. McConnell, W. E. Quinn, A.S. Rawcliffe, R. E. Siemon A. Introduction MHD theory to within -5%. The unstable m = 1 plasma motion follows the predictions of both the The main objective of the Scyllac experiment is to old-ordering6’7 and Freidberg’s generalized theory8 study methods of producing stable high-beta plasma to within 15ri both in absolute value and the equilibria and confinement in a high-beta, relative scaling of the growth rate from the 5- and 8- stellarator, toroidal geometry. The long range goal is m sector experiments. Furthermore, conditions in the development of a toroidal fusion reactor based on both the sectors and the torus satisfied Freidberg’s the theta-pinch principle. Preliminary Scyllac stabilization criterion for m=2 modes. 9 These experimental were performed in 5-m (R=2.4 m) and studies provide no indication of higher order poloidal 8-m (R=4.O m) toroidal sectors to study the plasma mode structure than m =1, supporting experimental- equilibrium and stability and their scaling. The 5-m ly the validity of the finite gyroradius stabilization sector experiments, which demonstrated the ex- criteria. istence of the high-beta, Q= 1,0 toroidal equilibrium, Experimental evidence gives a strong indication were completed in 1972. Conversion to the Scyllac that the unstable plasma motion lies in the plane of full torus with a major radius of 4.0 m began in early the torus. Theory predicts a toroidal mode structure 1973 and was completed, including device checkout, for the m= 1 instability with a cut-off at a minimum in April 1974. wavelength which is dependent on the major radius From April to November 1974, experimental con- and the bending of the magnetic field lines. An finement studiesz on the Scyllac full torus were analysis of the mode structure in Scyllac gives performed without the aid of the planned feedback further credence to the theoretically predicted scal- system. The results on the full torus characterize the ing of the absolute magnitude of the m =1 instability y confined plasma by the following parameters: growth rates. Measurements of the toroidal mode $=0.8; a=l.O cm; n=2.7X1016 cm-3; Te=500 eV; Ti numbers have been made up to 5, and the =0.8 keV; and Iz = 1.3 kA. A toroidal helical plasma measurements reveal the theoretically expected fall- equilibrium is achieved as predicted by them-y.3i4’5 off of the unstable displacement amplitude past The stably confined plasma is terminated after 6 to n =3. Theory also predicts that the maximum un- 8 PS by an m = 1 unstable motion of the column, stable mode number, n, which can exist in the whose pro erties are those of the theoretically Scyllac torus is 6. ? predicted6 S8m = 1 instability y and coincide with the Toroidal currents exhibiting peak values as high observations on the 8-m sector experiment. Streak as 1.3 kA have been observed in the Scyllac dis- photographs give an m = 1 instability growth rate of charges. Currents of this magnitude exceed the 0.7+0.3 MHz and indicate a toroidal mode structure Kruskal-Shafranov limit for Scyllac, but apparently in agreement with theory. Comparison of the do not influence the plasma stability on the measured plasma parameters and magnetic fields available time scales. This conclusion is based on with MI-ID sharp-boundary theory confirm the the experimental observations that the unstable plasma equilibrium and instability growth rate and plasma motion is predominantly planar, whereas their scaling. kink modes would produce plasma motion out of the The plasma exhibits an ini~ial 6 to 8 ps stable plane, and the observed growth rates are a factor of 5 phase during which equilibrium conditions are greater than the predicted values of the Kruskal- achieved.