S. C. Aceto

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.

The ATF heavy ion beam probe (invited)

A heavy ion beam probe (HIBP) has been implemented on 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 presented a diagnostic environment more challenging than that found on previous beam probe systems. Particular attention has therefore been given to in situ system alignment and control capabilities. Measurements of electric potential profiles, electron density profiles, electron density fluctuations, and electric potential fluctuations have now been made with this system. Most of the data obtained were for ECH heated discharges, but we were also able to make measurements of a few NBI heated plasmas. In addition to our calibration techniques, we were able to establish a reasonable confidence level for the data obtained since we could identify the most important potential profile characteristics predicted by theory and ...

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.

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.

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.

Measurements of the space potential of electron cyclotron heated plasmas in the Advanced Toroidal Facility

A heavy ion beam probe has been used to measure the space potential of electron cyclotron heated plasmas in the Advanced Toroidal Facility. The results of the measured potential profiles are presented along with the radial electric field strength profiles derived from these measurements. The measured potential profiles have been compared to those predicted by a simple model of the plasma based upon an ambipolarity constraint on the ion and electron particle fluxes in steady state. The trends of the measured and modeled profiles are in agreement, although the strength of the electric field predicted by the model is much greater than that inferred from the potential profile measurement. When an oxygen-like impurity ion species is added to the model, the predicted electric field strengths are altered significantly and become much closer to the measured values. >

Electron density profile measurement with a heavy ion beam probe

The feasibility of electron density profile measurements using a heavy ion beam probe in high‐temperature plasmas has been demonstrated earlier [J. Schwelberger et al., Bull. Am. Phys. Soc. 36, 2292 (1991); Yu. N. Dnestrovskij et al., Sov. J. Plasma Phys. 12, 130 (1986)]. Two algorithms were developed to obtain density profiles from the heavy ion beam probe on the Advanced Toroidal Facility (ATF). A comparison of the algorithms is presented with a detailed study of the errors involved in the measurements. The errors can be due to uncertainties in cross sections, electron temperature, the line average density measurement, and the ion trajectory calculations. The heavy ion beam probe density profile measurement is not very susceptible to errors as long as the electron temperature stays above 30 eV. If the electron temperature is below this value, a small uncertainty in the temperature introduces a large error in the density. Also, important for a good density profile measurement is the calculation of the co...