Results obtained on the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 13 1 ( 199 1) ] after installation of the design poloidal field winding are presented. Values of Be, = 2~, nd) T&/B s (a) -12% are achieved in low-current (I = 220 kA) operation; here, n, and T, are central electron density and temperature, and Be (a) is the poloidal magnetic field at the plasma edge. An observed decrease in pod) with increasing plasma current may be due to inadequate fueling, enhanced wall interaction, and the growth of a radial field error at the vertical cut in the shell at high current. Energy confinement time varies little with plasma current, lying in the range of OS-l.0 msec. Strong discrete dynamo activity is present, characterized by the coupling of m = 1, n = 5-7 modes leading to an m = 0, n = 0 crash (m and n are poloidal and toroidal mode numbers). The m = 0 crash generates toroidal flux and produces a small (2.5%) increase in plasma current.

In the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 13 1 (1991)] magnetic oscillations become stationary (locked) in the lab frame as a result of a process involving interactions between the modes, sawteeth, and field errors. Several helical modes become phase locked to each other to form a rotating localized disturbance, the disturbance locks to an impulsive field error generated at a sawtooth crash, the error fields grow monotonically after locking (perhaps due to an unstable interaction between the modes and field error), and over tens of milliseconds of growth confinement degrades and the discharge event.ually terminates. Field error control has been partially su&essful in eliminating locking.

Electrostatic fluctuations have been measured in a large reversed-field pinch, and are large ihe/n ~20%-40%, 7'^/r~10%-25%). Frequency and wave-number spectra are broad (AA2~70-150, Am-3-6), and differ from measured magnetic fluctuation spectra. The transport inferred from correlation measurements indicates that electrostatic fluctuations can account for significant particle losses, but contribute < 15% to energy loss.PACS numbers: 52.55.-s In toroidal magnetic confinement devices, cross-field transport exceeds diffusion predicted by collisional processes. Some theoretical models invoke electrostatic turbulence to explain this anomalous transport. Charge separation results in a fluctuating electric field, E = -VO; the ExB drift then drives transport. Electrostatic fluctuations may be responsible for particle transport in tokamak [1-4] and stellarator [5] edge plasmas, and perhaps also energy transport. Early measurements on ZETA [6] indicated that the role of electrostatic losses could be significant. More recent investigations have begun in several reversed-field-pinch (RFP) experiments [7][8][9]. The RFP and tokamak have similar edge plasma equilibrium density and temperature. However, the RFP contains greater magnetic shear, unfavorable magnetic curvature, and fewer magnetically trapped particles.In this Letter, we report measurements of edge electrostatic fluctuations (in density, potential, and electron temperature) in the MST RFP. We find that the amplitudes are large and the frequency and wave-number spectra are broad, similar to fluctuations in tokamaks. The deduced fluctuation-induced particle transport is comparable to the total particle losses. However, the fluctuation-induced energy transport is relatively small. MST [10,11] is a large RFP (^=0.52 m, 7^ = 1.5 m), with typical plasma parameters /^ < 600 kA, AZ^=(0.5-2.0) X 10'^ cm~^ r^o<500 eV, and pulse length < 80 msec. The present studies were conducted in low-current plasmas [/,, < 250 kA, /z,-=(0.6-0.8) x 10^^ cm~\ T^o < 180 eV], shown in Fig. 1. For these conditions, a single-turn loop voltage K/ = 15.5 V, pinch parameter O = Bp(a)/{Bt) = \.S5 (where (Bf) is the volume-averaged toroidal field), and reversal parameter F=Bt(a)/{Bt} = -0.15 were obtained.Probe measurements were made at r/a>:0.92 (where r/a = l at the wall), 40° above the outer midplane. Graphite toroidal rail limiters extend 1 cm from the wall at the inner and outer midplane. Triple probes were constructed using 0.5-mm-diam tungsten tips, spaced 1.6 mm apart. Two triple clusters, separated by 11.4 mm, were fixed on a single probe support. Measurements were made using a triple probe technique [12]. Ion saturation current Js was collected by a floating double probe biased to -300 V [> (5-10)A:rJ. The floating potential V/ was measured across a 100 kn impedance to the ground. The local plasma density rie, electron temperature Te, and (1) plasma potential Opi were then inferred bywhere ks is Boltzmann's constant, V^ is the potential of the positive-biased tip, and a, p, and f are consta...

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This paper presents a scheme for three-step laser-based stripping of an H ÿ beam for charge exchange injection into a high-intensity proton ring. First, H ÿ atoms are converted to H 0 by Lorentz stripping in a strong magnetic field, then neutral hydrogen atoms are excited from the ground state to upper levels by a laser, and the remaining electron, now more weakly bound, is stripped in a strong magnetic field. The energy spread of the beam particles gives rise to a Doppler broadened absorption linewidth, which makes for an inefficient population of the upper state by a narrow-band laser. We propose to overcome this limitation with a ''frequency sweeping'' arrangement, which populates the upper state with almost 100% efficiency. We present estimates of peak laser power and describe a method to reduce the power by tailoring the dispersion function at the laser-particle beam interaction point. We present a scheme for reducing the average power requirements by using an optical ring resonator. Finally, we discuss an experimental setup to demonstrate this approach in a proof-of-principle experiment.

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Measurements of edge turbulence and the associated transport are ongoing in the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 13 1 (199 1)] using magnetic and electrostatic probes. Magnetic fluctuations are dominated by m= 1 and n-R/a, tearing modes. Particle losses induced by magnetic field fluctuations have been found to be ambipolar ((4, g,.)/&=O). Electrostatic fluctuations are broadband and turbulent, with mode widths Am-3-7 and An-70-150. Particle, parallel current, and energy transport arising from coherent motion with the fluctuating ExB drift have been measured. Particle transport via this channel is comparable to the total particle loss from MST. Energy transport (from (FE+)/B,) due to electrostatic fluctuations is relatively small, and parallel current transport (from (J,, E,+)/B,) may be small as well.

Thin carbon foils are used as strippers for charge exchange injection into high intensity proton rings. However, the stripping foils become radioactive and produce uncontrolled beam loss, which is one of the main factors limiting beam power in high intensity proton rings. Recently, we presented a scheme for laser stripping an H ÿ beam for the Spallation Neutron Source (SNS) ring. First, H ÿ atoms are converted to H 0 by a magnetic field, then H 0 atoms are excited from the ground state to the upper levels by a laser, and the excited states are converted to protons by a magnetic field. In this paper we report on the proof-ofprinciple demonstration of this scheme to give high efficiency (around 90%) conversion of H ÿ beam into protons at SNS in Oak Ridge. The experimental setup is described, and comparison of the experimental data with simulations is presented.