As seen in Fig. 1(a), the diabatic signal does indeed increase as the value of n increases. In addition, both ionization features are evident at slew rates differing by a factor of 2 from that used to obtain the data in Fig. 1.When both lasers are polarized parallel to the direction of the electric field, then theoretically no | m x | = 2 can be produced and, in this case, we observe a dramatic decrease in the signal obtained at the diabatic threshold. In view of this test, and since the probability for diabatic passage in a given field slew rate is larger for \m l \ = 2 states than for | raj = lor |raj = 0 states, we conclude that the diabatic high-field peak is due primarily, if not wholly, to |raj = 2.The signal at electric fields between the adiabatic threshold and the diabatic threshold is attributed to a combination of adiabatic and diabatic passage through level crossings. As n increases, the ratio of this intermediate signal to the diabatic signal drops rapidly.These measurements demonstrate, for the first time, the passage of highly excited atoms from low electric fields to ionizing electric fields along predominantly diabatic paths. Clearly, it is important to account for the possibility of diabatic thresholds whenever one observes atomic states of high n using field ionization.A compact, toroidal configuration of magnetized plasma is produced by a combination of Z-and 0~pinch discharges. A paramagnetic toroidal field is produced by currents circulating in the plasma on closed flux surfaces.We report here our initial results on the formation of a plasma confinement configuration with the generic name of spheromak. 1 ' 2 This compact toroidal configuration has both toroidal (B^) and poloidal (B r ,B 2 ) magnetic field components with the toroidal field maintained by circulating plasma currents rather than by an external coil through the toroidal hole, as in a tokamak. Configurations of this type were first studied theoretically in an astrophysical context. 3 * 4 Related laboratory experiments involving plasma guns, 5 * 6 electron beams, 7 and pinches 8 have been performed. Our experiment, called paramagnetic spheromak (PS-1), makes use of Z-and 6 -pinch techniques to produce a prolate spheroid configuration. The results show, for the first time, that it is possible to establish the desired closed poloidal flux surfaces with a stabilizing paramagnetic toroidal field (i.e., with the peak magnitude of B(p near the magnetic axis). Figure 1 illustrates the formation phase. We start with a cylindrical deuterium gas column of radius 11.4 cm and a pressure of 15 mTorr. The column contains an axial bias magnetic field {-B z ) produced by I 9 currents 9 in an external, single-turn mirror coil with a mirror ratio of 1.1. The bias field is produced by a 20-kV, 18-\xY capacitor bank capable of producing fields with magnitude up to 8 kG. Typically fields of 4 kG are used. The field rises in 3 to 5 jusee (depending on the external inductance) and is clamped. Following this a Z-directed current shell is produced by disc...

Spatially resolved measurements of the magnetic field of a spheromak have been analyzed and compared with expectations for the ratio of j\\/B from the pre s sure-gradientfree Taylor model and a model with pressure due to Morikawa. Better agreement is found with the model containing finite pressure.

Globally coherent modes which are observed during formation in the S-1 Spheromak plasma occur during flux conversion and plasma relaxation toward a minimum-energy state, suggesting that these modes provide a means for relaxation. A significant finding is the temporal progression of these modes through a sequence n = 5, 4, 3, 2, m = 1, as q rises through rational fractions m/n, where n and m are defined by the functional dependence e' "~+~o f the fluctuations on toroidal angle @ and poloidal angle 8 Co.mparison with theory of the observed modes and the sequence of occurrence suggests that these modes are due to resistive MHD instabilities.

Experimental measurements of the equilibrium in the S-1 spheromak [M. Yamada, J. Sinnis, H. P. Furth, M. Okabayashi, G. Sheffield, T. H. Stix, and A. M. M. Todd, in Proceedings of the US-Japan Symposium on Compact Toruses and Energetic Particle Injection (Princeton Plasma Physics Laboratory, Princeton, NJ, 1979), p. 171] by use of magnetic probes inside the plasma show that the final magnetic equilibrium is one that has relaxed close to the Taylor (minimum-energy) state, even though the plasma is far from that state during formation. The comparison is made by calculating the two-dimensional μ profile of the plasma from the probe data, where μ is defined as μ0 j∥/B. Measurements using a triple Langmuir probe proved evidence to support the conclusion that the pressure gradients in the relaxed state are confined to the edge region of the plasma.

Globally coherent modes have been observed during formation in the S-1 Spheromak plasma [Plasma Physics and Controlled Nuclear Fusion 1984 (IAEA, Vienna, Austria, 1985), Vol. 2, p. 535] by analysis of magnetic field fluctuations measured from outside the plasma. The modes are of low n number (2≤n≤5), where n is defined by the functional dependence einφ of the fluctuation on toroidal angle φ. These modes are shown to be related to flux conversion and plasma relaxation toward a minimum-energy state during the spheromak formation. The modes are active while the q profile is rapidly changing, with q on axis, q0, rising to 0.7. A significant finding is the temporal progression through the n=5, 4, 3, 2; m=1 mode sequence as q rises through rational fractions m/n. During formation, peak amplitudes of the n=2, 3, 4 modes relative to the unperturbed field have been observed as high as 20%, while more typical amplitudes are below 5%.

This paper summarizes the characteristics of the spheromak .lasmas obtained during the past five-year operation period of S-1 experiments. The S-1 Spheromak device, which began operation in 1983, generates a compact toroid in which the self-generated toroidal field in the plasma is comparable to the poloidal field. The S-1 experiment is unique in that spheromak plasmas are formed by inductive transfer of magnetic flux from a toroidal-shaped "flux core", and plasma stability is maintained by shaping of the externally applied equilibrium field and using loose-fitting passive conductors. Without stabilizer coils, the plasma was unstable to gross MHD modes, with the tilt or shift being dominant. Significant improvements in plasma stability and parameters occurred after the installation of passive Figure-8 stabilization coils or funr.el-shaped conductor plates. Stable spheromaks with up to 550 kA

Globally coherent modes have been observed during formation in the S-1 Spheromak plasma by analysis of magnetic field fluctuations measured from outside the plasma. The modes are of low n number (2 < n ^ 5), where n is defined by the functional dependence e * of the fluctuation on toroidal angle . These modes are shown to be related to flux conversion and plasma relaxation toward a minimum-energy 3tate during the spheromak formation. The nodes are active while the q ptofile is rapidly changing, with q on-axis, a, rising to 0.7. A significant finding is the temporal progression through the n = 5, 4, 3, 2; m = 1 mode sequence as q rises through rational fractions ra/n. During formation, peak amplitudes of the n = 2, 3, 4 modes relative to the unperturbed field have been observed as high as 20%, while raor<5 typical amplitudes are below 5%. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United Slates GovcZnt. N*£ the United States Government nor any W*?^^^. employees, nuke* any warranty. «pre» or implied, or assumes any legal lability " " £ M1 Mity foMhe accuracy, completeness, or usefulness of any information, apparatus, product or nc^dia^or represents that IB use would no. infringe privately owned nghla. Refer-S^mtoi^nV fleck cmrchl product, process, or service by Uade name, trademark, mlr^rer, o/otCise does m necessarily constitute or imply *. ^"^™£ mcndalior., or favoring by the United Slates Government or any agency thereof. The v.ews Tnd opMoas of authors expressed hevem do not rectsanly I1W or reflect those of the United States Government or any agency thereof.

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