J. Lei

Sweep system design for a heavy ion beam probe on Madison Symmetric Torus

The sweep system for the heavy ion beam probe on the Madison Symmetric Torus (MST) is described. The two components of the system are the primary sweep optics and secondary collimation plates. Key issues in the sweep system design are the small entrance and exit ports available on MST, the significant toroidal beam motion induced by the strong poloidal magnetic field, and the excessive current loading due to plasma and ultraviolet (UV). The design accommodates these issues using a crossover sweep plate design in two dimensions for the primary beam as well as two dimensional sweeping on the secondary beam. The primary beam sweep design results in a sweep range of ±20° in one direction and ±5° in the perpendicular direction. The secondary beam sweep design results in entrance angles to the energy analyzer of <3° in radial and ∼0° in toroidal directions. The procedure for calculating sweep performance including fringe fields, a system for active trajectory control, and initial experiments on plasma and UV lo...

A heavy ion beam probe for the Madison Symmetric Torus

A heavy ion beam probe (HIBP) is being developed for application on the Madison Symmetric Torus (MST). It will be used to make measurements of the plasma space potential Φ(r), fluctuations in potential Φ(r), the electron density ne(r), fluctuating electron density ne(r), and radial electric field Er(r) from the core to the edge region of the plasma in MST. While information on these quantities can and has been obtained with probes inserted in the surface region, none of the above measurements have been made in the core of a hot reversed field pinch. Measurements of Φ(r), Er(r), and ne(r) have been well established in previous HIBP systems on tokamaks such as Impurities Studies Experiment, Texas Experimental Tokamak (TEXT), and TEXT Upgrade, stellarators such as Advanced Toroidal Facility and Compact Helical System and Bumpy Tori such as Elmo Bumpy Torus and Nagoya Bumpy Torus. Less well developed in terms of HIBP measurements are equilibrium and fluctuating magnetic fields. Because the confining field on...

Calibration and initial operation of the HIBP on the MST

For the first time, a heavy ion beam probe (HIBP) has been installed on a reversed field pinch, i.e., Madison symmetric torus (MST), to measure the plasma potential profile, potential, and electron density fluctuations, etc. The application of a HIBP on MST has presented new challenges for this diagnostic. The primary sources of difficulty are small access ports, high plasma, and, ultraviolet (UV) flux and a confining magnetic field produced largely by plasma currents. The requirement to keep ports small so as to avoid magnetic field perturbations led to the development of the cross-over sweep system. The effectiveness and calibration of this sweep system will be reported. In addition, this diagnostic is now operating with greater plasma/UV loading effects than most previous Rensselaer HIBPs. The plasma flux is reduced by using a magnetic suppression structure. The UV flux appears to be the dominant cause of the remaining loading, which is substantial. The magnetic field being largely produced by the plas...

Suppression of plasma electrons in the diagnostic ports of the MST

The recent application of a heavy ion beam probe (HIBP) to the Madison Symmetric Torus (MST) has motivated the development of permanent magnet plasma suppression structures. Unconfined plasma at the MST diagnostic ports is free to flow out the ports and into adjoining diagnostic chambers. The HIBP system incorporates seven pairs of high voltage, electrostatic steering plates. Stray charged particles that exit the MST-HIBP ports are attracted to these biased steering plates, loading down the power supplies, and detrimentally affecting the desired operation of the plates. A second source of loading is electron current generated by UV light emitted from the MST plasma. Structures comprised of steel keepers and nickel plated magnets were designed to conform to the walls of the two HIBP diagnostic ports. The magnetic fields in the keeper aperture are able to suppress most of the plasma that would otherwise flow into the HIBP chambers. The fields external to the keeper structure are sufficiently small to avoid perturbing the confining fields at the plasma edge. Analysis indicates that electron current from UV radiation dominates the remaining loading of the HIBP steering plates.The recent application of a heavy ion beam probe (HIBP) to the Madison Symmetric Torus (MST) has motivated the development of permanent magnet plasma suppression structures. Unconfined plasma at the MST diagnostic ports is free to flow out the ports and into adjoining diagnostic chambers. The HIBP system incorporates seven pairs of high voltage, electrostatic steering plates. Stray charged particles that exit the MST-HIBP ports are attracted to these biased steering plates, loading down the power supplies, and detrimentally affecting the desired operation of the plates. A second source of loading is electron current generated by UV light emitted from the MST plasma. Structures comprised of steel keepers and nickel plated magnets were designed to conform to the walls of the two HIBP diagnostic ports. The magnetic fields in the keeper aperture are able to suppress most of the plasma that would otherwise flow into the HIBP chambers. The fields external to the keeper structure are sufficiently small to avoid ...

Initial Measurements of Plasma Potential in the Core of the MST Reversed Field Pinch with a Heavy Ion Beam Probe

Measurement of the plasma potential in the core of MST marks both the first interior potential measurements in an RFP, as well as the first measurements by a Heavy Ion Beam Probe (HIBP) in an RFP. The HIBP has operated with (20-110) keV sodium beams in plasmas with toroidal currents of (200-480) kA over a wide range of densities and magnetic equilibrium conditions. A positive plasma potential is measured in the core, consistent with the expectation of rapid electron transport by magnetic fluctuations and the formation of an outwardly directed ambipolar radial electric field. Comparison between the radial electric field and plasma flow is underway to determine the extent to which equilibrium flow is governed by E×B. Measurements of potential and density fluctuations are also in progress.Unlike HIBP applications in tokamak plasmas, the beam trajectories in MST (RFP) are both three-dimensional and temporally dynamic with magnetic equilibrium changes associated with sawteeth. This complication offers new opportunity for magnetic measurements via the Heavy Ion Beam Probe (HIBP). The ion orbit trajectories are included in a Grad-Shafranov toroidal equilibrium reconstruction, helping to measure the internal magnetic field and current profiles. Such reconstructions are essential to identifying the beam sample volume locations, and they are vital in MSTs mission to suppress MHD tearing modes using current profile control techniques. Measurement of the electric field may be accomplished by combining single point measurements from multiple discharges, or by varying the injection angle of the beam during single discharges.The application of an HIBP on MST has posed challenges resulting in additional diagnostic advances. The requirement to keep ports small to avoid introducing magnetic field perturbations has led to the design and successful implementation of cross-over sweep systems. High levels of ultraviolet radiation are driving alternative methods of sweep plate operation. While, substantial levels of plasma flux into the HIBP diagnostic chambers has led to the use of magnetic plasma suppression.

Analysis of heavy ion beam probe potential measurement errors in the Madison Symmetric Torus

The heavy ion beam probe on the Madison Symmetric Torus is capable of measuring the plasma potential at radial locations from about ρ=r/a=0.3u2002tou20020.75. Radial potential scans from two energy analyzer detectors have been used to assess measurement accuracy since they should produce identical profiles. The effects of analyzer characteristics, system alignment, sample volume locations and shapes, probing beam control, the quality of confining magnetic field information available, etc., have been assessed to determine the overall quality of the potential measurements. The accuracy of the measurements is found to be quite good relative to the potentials measured.

Heavy ion beam probe development

Summary form only given. We are conducting experiments with the ultimate goal of developing a non-perturbing diagnostic technique for measuring RF field and density fluctuations in fusion plasmas. In order to develop this technique, a helicon plasma with a heavy ion beam probe (HIBP) diagnostic is being constructed. The helicon plasma has a magnetic field of up to 1.5 kG produced by a set of circular coils. A 1 kW, 13.56 MHz RF generator will be used to drive helicon waves in the plasma. Diagnostic apparatus utilizes a 60 keV HIBP diagnostic beam and detector that were previously used on the Tokamak de Varenne. The detection electronics is being modified to operate at higher frequencies. Present status of the experiment as well as key issues in extending HIBP measurements to higher frequencies will be discussed. The HIBP proposed for LDX, based on the old EBT/TARA system, is intended to study convective cells in a simple magnetic dipole field, similar to that found in planetary magnetospheres. Both potential structures and the impact of flows around them as observed in fluctuation measurements can be characterized with such a system. An HIBP system has also been proposed for NSTX, based on the 500 kV TEXT HIBP. There is presently no plan for the 2 MeV TEXT Upgrade system.

Installation of the Madison Symmetric Torus heavy ion beam probe

Summary form only given. A heavy ion beam probe has been designed for application on the Madison Symmetric Torus with final construction and installation occurring in early 1999. The HIBP will be capable of making measurements of the plasma space potential /spl Phi/(r), simultaneous density and potential fluctuations, and magnetic measurements (both equilibrium fields and fluctuations). On MST, measurements of n/sub e/(t) and /spl Phi/(t) will take priority once the system is fully functional, with some effort also dedicated to equilibrium measurements, particularly of the confining magnetic field. The latter measurements will not be standard direct heavy ion beam probe measurements based on the detection of secondary ions produced by collisions between the probing beam and plasma electrons. Rather, information on the magnetic field will be inferred from overall ion trajectory characterization. While this basic set of measurements is being made, we will also be collecting information to determine the next plasma parameter to address. The prospects for these HIBP measurements are very exciting, since the information to be obtained has not previously been available from the core of a hot Reversed Field Pinch.

A heavy ion beam probe for Madison Symmetric Torus

Summary form only given. A heavy ion beam probe is being developed for application on the Madison Symmetric Torus (MST). It will be used to make measurements of the plasma potential, fluctuating plasma potential, electron density, and fluctuating electron density from the core to the edge region of the plasma in MST. While information on these quantities can and has been obtained with probes inserted in the surface region, none of the above measurements have been made in the core of a hot RFP. Potential and density measurements are the standard output from a beam probe system. Less well developed are the measurements of equilibrium and fluctuating magnetic fields. Because the confining field is determined by plasma conditions, some effort has made in the design of the MST beam probe to make it possible to characterize B before, during and after the plasma discharge. Also, magnetic fluctuations play a key role in RFP physics and, thus, this system will also be used to address fluctuating B once the other measurements have been established. Installation and calibration of the MSTH-HIBP will occur during the first six months of 1998. Regular operation is planned for the second half of the year.