D. R. Demers

Zonal flow measurements using a heavy ion beam probe

Zonal flows are of keen interest in efforts to understand transport in magnetically confined plasma. Of the well-developed diagnostics, the heavy ion beam probe, HIBP, is most suited to measure the radial electric field, Er, associated with the flows in present medium to large devices. This paper discusses why the HIBP is capable of measuring Er and how to design a HIBP to optimize zonal flow measurements. The TEXT HIBP is used as an example of a typical system. The NSTX spherical torus is used in a design study for future work with emphasis on zonal flow measurements. The key diagnostic considerations are (1) sample volume size, (2) sample volume orientation, and (3) the ability to rapidly scan the sample volume in the radial direction. The measurement of principal interest here is Er but there is additional information in a nonlinear analysis of the fluctuations.

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...

Radial electrostatic flux inferred from core measurements of potential and density fluctuations

Broadband potential fluctuations and electrostatic fluctuation induced particle flux have been measured in the core of a medium size tokamak plasma for the first time. Density fluctuations and wave numbers were simultaneously measured. Measurements indicate that radial electrostatic fluctuation induced particle flux in the core region, at the normalized radii of 0.17<ρ<0.38, due to modes with wave numbers less than 4 cm−1 is small in magnitude, is likely directed inward, and cannot account for particle flux predicted by the continuity equation and particle source codes. Estimates of electrostatic energy flux are also significantly less than values predicted via power balance analysis. Asymmetries in coherent density and potential fluctuation levels on closed flux surfaces are evident. Relative fluctuation levels n/n and φ tend to increase with radius, and the fluctuations are Boltzmann-like in the region 0.17⩽ρ⩽0.38. The direction of mode propagation is in the electron diamagnetic drift direction. Corres...

A spatial detector array for measuring plasma turbulence with the heavy ion beam probe

The heavy ion beam probe (HIBP) has become a valuable diagnostic for measuring the plasma space potential and electron density in high-temperature plasmas. However, due to the limitations placed on the design of the HIBP by the size and complexity of the electrostatic energy analyzer, only limited information can be extracted from the HIBP’s fluctuation measurements. By extending the range and number of sample volumes simultaneously recorded by the beam probe, a better understanding of the turbulence distribution, S(k,ω), can be achieved. This can be accomplished by installing an array of secondary particle detectors close to the plasma edge. InterScience, Inc. has developed a particle detector capable of measuring the small signal levels required by the HIBP in the high background environment associated with operating near the plasma edge. In addition, this detector has been incorporated into the design of a 20-element, spatial array for the 2 MeV HIBP on TEXT. A flexible design has been developed such t...

Spectroscopic ion beam imaging for investigations into magnetic field mapping of a plasma

The trajectory of an ion beam as it passes through a magnetically confined plasma is determined by the ion mass, energy, and charge state, and the magnetic field structure. In undergraduate physics laboratories, students use a measure of beam deflection in a well-defined magnetic field to determine the charge-to-mass ratio of a particle. The complementary analysis is equally valid; the field may be determined given a known charge-to-mass ratio. Additional complexity is introduced in a spatially nonuniform, time-varying magnetic field, such as that of a plasma. The technique of field mapping via spectral imaging is being developed with a heavy ion beam probe on the Madison Symmetric Torus. Technical issues, such as choice of wavelengths, optics, viewing geometry, and imaging hardware, are being addressed. Calculations indicate that beam emission is brighter than background bremsstrahlung for several transitions of interest. However, a wavelength region containing lines from the beam ions, but free of atomi...

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.

Measurements of broadband fluctuations and plasma potential with the 2 MeV heavy ion beam probe on the TEXT-U Tokamak (oral)

Abstract Broadband density fluctuations and plasma potential profiles are measured with a 2 MeV heavy ion beam probe (HIBP) diagnostic system on the TEXT-U tokamak. Measurements are performed in various locations including the edge and the core, as well as the high and low field side regions of the plasma. Results show a poloidal asymmetry in the core density fluctuation power spectra in which a distinct mode in the frequency range 70–170 kHz is present in the low field side region only. Furthermore, the HIBP provides a direct measurement of the local plasma potential from which the electric field profile across the plasma is inferred. Results showing the poloidal asymmetry as well as the potential and electric field profiles are presented.

Development of magnetic field mapping via heavy ion beam spectral imaging

Mapping magnetic fields via heavy ion beam spectral imaging relies upon establishing a high quality ion beam, identifying beam emission at wavelengths favorable for imaging, and designing an appropriate imaging configuration. Identifying emission lines suitable for imaging is difficult due to intense, broadband radiation of the target reversed field pinch plasma. To compensate, we have worked to raise the beam emission intensity. Simulations of the beam optics and characteristics have led to a technique that achieves a narrower beam and increased ion current at the plasma. Additionally, we are developing computer vision tools to reconstruct beam trajectories based on various camera and system configurations. We simulate charge coupled device images of the vessel interior and beam trajectories, and reconstruct three dimensional trajectories from image pairs. Analysis of the simulated images will guide the system specifications. We present results of the beam optics and camera simulations, surveys of radiat...

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.