J. F. Lewis

Heavy‐ion beam probe diagnostic systems (invited)

Heavy‐ion beam probing generally consists of passing a beam of 1+ ions through a plasma imbedded in a magnetic field. Secondary ions with higher ionization levels are produced by ionizing collisions with the plasma electrons. Detection of the secondary ions with a small‐aperture electrostatic energy analyzer allows continuous fluctuation measurements of the plasma density and space potential with both spatial and temporal resolution. Spatial resolution is the order of 0.1 cm3 and temporal resolution is presently electronics limited to ∼1 μs. The energy of the probing beam is determined primarily by the requirement that the secondary ion must escape from the plasma. Typical beam energies extend from 10 to 500 keV. The range of plasma densities that have been investigated is 1012 cm−3<ne<1014 cm−3. At the higher densities, beam attenuation becomes a serious problem. Higher beam energies provide better penetration of the magnetic field, and reduced beam attenuation. Heavy‐ion beam probes were first used to m...

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...

ATF heavy ion beam probe: Installation and initial operation

Installation of the HIBP on ATF began in the summer of 1988. All of the major hardware components have now been installed. The initial operation of the diagnostic has begun amid the final stages of testing and control system integration. The existence of significant magnetic fields and gradients outside of the main plasma volume and fully three‐dimensional particle trajectories have raised several interesting issues during the design, assembly, alignment, and operation of the beamline and analyzer. The diagnostic must function in a challenging environment. It must perform satisfactorily despite electrical interference from several nearby sources, pressure excursions caused by gas puffing, and UV/plasma loading.

In situ analyzer calibration methods for heavy ion beam probes installed on stellarator-like devices

An absolute calibration of an installed heavy ion beam probe (HIBP) energy analyzer is possible by detecting secondary ions that come from a source at a known location that is maintained at a known potential. It is also necessary that the magnetic field that exists during the calibration be the same magnetic field that exists when a plasma is present. These conditions can be met on stellarators, heliotrons, and torsatrons, which produce their magnetic configurations with external coil sets. Since no internal plasma current is required, suitable sources for producing secondary ions by interaction with the primary beam can be placed inside the vacuum vessels of these devices at known locations. Secondary ions can be produced by the interaction of the primary beam with thin films, gas box probes, electron beams, or neutral gas filling the vacuum vessel. Details of a spherical mesh probe that combines the advantages of several of these methods are given.

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.

Heavy-ion beam probe for the Advanced Toroidal Facility

Neoclassical theory predicts that radial electric fields develop in plasmas confined in nonaxisymmetric tori and that they strongly influence the confinement in such devices. A heavy‐ion beam probe has been planned for the ATF torsatron at Oak Ridge National Laboratory with the primary goal of providing direct measurements of the plasma potential radial profile and thus of the radial electric field. The complex ATF geometry and magnetic field structure present a diagnostic environment more challenging than that found on previous beam probe systems which have operated on tokamaks, mirrors, and bumpy tori. Particular attention has been given to system alignment and control capabilities. Since the probing ion trajectories remain essentially the same with or without the plasma, an in situ calibration scheme using suitable ionization targets is possible. Installation of the complete diagnostic system is planned for 1988.

Heavy ion beam probe diagnostic systems for stellarators

A heavy ion beam probe diagnostic is planned for ATF. The most significant device characteristics affecting the implementation of such a system on this complex magnetic geometry not encountered on tokamaks are the large toroidal separation (15°) of vertical and horizontal ports and substantial magnetic return fields in the region of the ion optics and energy analyzer electric fields. Procedures have been developed to make the application of beam probing possible in this environment.

Advances in heavy‐ion beam probing

Recent applications of heavy‐ion beam probes on such devices as the TEXT tokamak have shown the importance of increasing the capabilities of both the beam injection and detection systems. To this end we have investigated a new solid‐state ion detector that will substantially improve spatial resolution. The application of the diagnostic technique on substantially larger devices will also require new electrostatic energy analyzer geometries, which are also under investigation.

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.