P. E. McLaren

Recent advances in heavy ion beam probe diagnostics (invited)

Heavy ion beam probes (HIBPs) have proven to be a unique tool for measuring fluctuations and particle transport in tokamaks. They have been used to measure fluctuations in density, electric potential, and magnetic vector potential. The density and potential fluctuation measurements have determined the particle flux due to electrostatic turbulence in the TEXT and ISX‐B tokamaks. In these measurements, the frequency spectra (0–500 kHz) of the phase between density and potential, the wave numbers of the fluctuations, and the fluctuation level are obtained. Three topics are discussed in this paper. We present measurements of magnetic fluctuations during MHD activity using the TEXT HIBP. Analysis of these measurements indicates that the diagnostic is primarily sensitive to the local value of Aφ in the sample volume unless the local Aφ is small. In addition, we discuss instrumental effects associated with wave number measurements. We will discuss the effects of sample volume size on the wave number measurements...

Electric field studies of a 2 MeV electrostatic energy analyzer

An energy analyzer based on the Proca and Green parallel‐plate design is being developed for use with the 2 MeV heavy ion beam probe on TEXT. In a departure from the conventional configuration, guard ring electrodes will not be used. Instead, a shaped top plate will provide for comparable, or improved, uniformity of the analyzer electric field region. To quantify this effect, and to characterize the electrostatic field, numerical solution methods have been utilized. Simulations have included effects of top plate shape, wire screens, vacuum chamber design, and dielectric support structures. The modeling has permitted us to design an analyzer electrode structure that is an integral part of a uniquely shaped vacuum vessel. The design electric field is 20 kV/cm with less than 1% error in uniformity within the parallel plate region. To examine the electric field structure experimentally, a quarter‐scale prototype analyzer has been constructed and tested. The electric field characteristics are examined by varying the path of a heavy ion beam through the analyzer and examining the resulting analyzer performance. A simulated vacuum wall can be positioned to examine the effects of different vessel configurations and to determine the sensitivity of the analyzer to this boundary condition. The experimental results show excellent agreement with the numerically predicted fields and confirm the validity of the shaped top plate electrode concept.

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...

Energy analyzer for the ATF heavy ion beam probe

The energy analyzer for the ATF heavy ion beam probe has been built and tested. Because the analyzer will be required to operate in fields as high as 550 G on ATF and since calibration of the analyzer in the absence of a field will not be possible once it is installed, a special emphasis has been made to characterize its performance on a test stand. In order to ensure accurate knowledge of the analyzer geometric parameters, it was assembled with the aid of a coordinate axis measuring machine. The results of these tests are presented and a comparison to ideal analyzer performance is made.

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.

Text‐upgrade 2 MeV heavy ion beam probe (abstract)

A unique 2 MeV heavy ion beam probe diagnostic system, presently under construction, with two injection lines and two energy analyzers will be used to measure the space potential as well as density, electrostatic (and possibly magnetic) fluctuations, and average wave numbers in the TEXT upgrade tokamak. The capabilities of the 2 MeV beam include a good penetration of the TEXT‐U magnetic fields (up to 2.2 T) and access to most of the plasma cross section (90%) including all null points (during divertor operations), the top, bottom, and outside edges. We plan to use parallel plate electrostatic energy analyzers which require that 400 kV be held on a large electrode across a 20 cm vacuum gap. Feasibility tests of such analyzers are under way. Progress of construction and testing of different parts of the system are reported. A description of the 2 MeV heavy ion beam probe system and a discussion on the measurements to be performed will be presented. This work was supported by DOE.

Initial operation of the TEXT‐Upgrade 2‐MeV heavy ion beam probe (abstract)

The new heavy ion beam probe diagnostic system on TEXT is presently operational at reduced energy. Thallium and cesium ion beams with energy up to 590 keV have been obtained using a single injection beamline and an energy analyzer that can be operated with low‐energy beams only (≤590 keV). At full energy (up to 2 MeV) two injection beamlines and two energy analyzers (presently under construction) will allow access to 90% of the plasma cross section. Stable 2‐MeV test beams with currents up to 10 μA have been obtained but not injected into the tokamak. The capacitive liner feedback control system allowed stable operations with ripple less than 100 V. Results of initial operations and preliminary measurements of density fluctuations are presented. This work is supported by the U. S. Department of Energy.

Evolution of the 2‐MeV electrostatic energy analyzer for TEXT‐Upgrade (abstract)

The development of an electrostatic energy analyzer to be used with the 2‐MeV heavy ion beam probe on the TEXT‐Upgrade tokamak has presented several very interesting voltage breakdown phenomena. In order to maintain a 400‐kV potential on large surface area plates (46.2 cm×152 cm) various configurations of support dielectrics, voltage feedthrough, plate materials, and plate geometry have been evaluated. Experimental evaluation of several designs and the resulting modifications leading to a working analyzer will be presented. This work was supported by the U.S. Department of Energy.

Electric field and plasma potential measurements on TEXT-U using the 2 MeV heavy ion beam probe (HIBP)

Summary form only given, as follows. The 2 MeV HIBP on TEXT-Upgrade has been used to measure the plasma potential for a variety of discharge conditions. Small scale, multiple point measurements throughout a cross section of the plasma give variations in electric field with a resolution of better than 10 V/cm. Scan lines through the plasma produce a potential profile along a radial path from plasma center to edge. Changes in electric field and potential are monitored during impurity injection and with 500 kW of ECH heating. Fluctuations in plasma potential have been observed deep within the plasma with 40 V rms sensitivity.

Plasma potential measurements on text‐upgrade with a 2 MeV heavy ion beam probe (abstract)a)

The plasma potential is measured in TEXT‐upgrade tokamak by injection and detection of high energy heavy ions (thallium and cesium with a single charge) using a 2 MeV accelerator and a parallel plate energy analyzer. The change in beam energy, as it crosses the plasma, gives the local plasma potential at the measurement volume. Recent results of high energy beam operations are presented.