Library

Notes: For the first time, a heavy ion beam probe has been used to measure the poloidal magnetic flux in a tokamak. In this measurement, first proposed over 20 years ago, the toroidal displacement of the heavy ion beam probe particles is caused by the force of the poloidal magnetic field on the ion beam probe particles. In a nearly toroidally symmetric device such as the Texas Experimental Tokamak, the toroidal position of the ions at the detector consists of a part proportional to the poloidal magnetic flux at the sample volume and an integrated contribution of the poloidal magnetic flux along the trajectory. The local part in the relation between beam position and magnetic flux is used as a correction term in an iterative algorithm that calculates the poloidal magnetic flux. The q profile and the current density profile can be derived from the measured poloidal flux. Errors of the measurement can be due to uncertainties in the analyzer position and also in the knowledge of the location of the sample volume. Also uncertainties in the plasma current and position contribute to the error of the poloidal flux measurement. It is essential to know the magnetic field outside the plasma. The calculation of the outside field is difficult due to the presence of the iron core. The error in the measurement of the poloidal flux is on the order of 1%. The current profiles and q profiles were measured for discharges with on axis and off axis electron cyclotron resonance heating (ECRH), and with impurity injection. As expected, all current profiles peak in the center. With central ECRH, the measured current profiles are consistent with a prediction based on Spitzer resistivity and the measured electron temperature profile. In these plasmas, the safety factor has a central value of qc=0.95±0.1 and near the center of the plasma, the poloidal flux is observed to increase with time. The current profiles for off axis ECRH exhibit a larger current density in the wings of the distribution than comparable centrally heated ECRH plasmas. Results from plasmas with impurity injection yield qc=0.6 which is significantly lower than expected since these plasmas have m=2 activity. © 1998 American Institute of Physics.

Notes: 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. © 1995 American Institute of Physics.

Notes: The displacement out of the toroidal plane of the heavy ions of the beam probe diagnostic is caused by forces due to the poloidal magnetic field. In a nearly toroidally symmetric device such as the TEXT tokamak, the toroidal position of the ions at the detector consists of a part proportional to the poloidal magnetic flux at the sample volume and an integrated contribution of the poloidal magnetic flux along the trajectory. The local part in the relation between beam position and magnetic flux is used in an iterative algorithm that calculates the trajectories in the magnetic field of the tokamak. The q profile can be derived from the measured poloidal flux. Errors of the measurements can be due to uncertainties in the analyzer position and also in the knowledge of the location of the sample volume. Also, uncertainties in the plasma current and position contribute to the error in the q measurement. It is essential to know the magnetic field outside the plasma. The calculation of the outside field is difficult due to the presence of the iron core and is still a matter of investigation, since it has a strong effect on the measurement. A description of the measurement and an error analysis is presented with examples of q profiles for discharges with ECH heating and strong magnetic fluctuations. © 1995 American Institute of Physics.

Notes: A 160 kV HIBP system was used to make fluctuation measurements on ATF. Most of the fluctuation data were taken with low density ECH heated plasmas at 0.95 T or less. However, there is a limited amount of data taken at less than optimal conditions during plasmas with neutral beam injection. Details of the diagnostic equipment and analysis methods will be presented along with the results of the fluctuation data analysis. The results will be compared with the available data from other ATF fluctuation diagnostics. © 1995 American Institute of Physics.

Notes: 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 plasma makes determination of measurement locations exclusively from trajectory calculations difficult. Initial operation results have shown that the magnetic field model we are using to calculate our ion trajectories has an inaccuracy of about 10%, and thus subsequent development of improved confining field models is important. Secondary signals have been detected, and the levels are smaller than that from the UV induced noises. Methods to increase the signal levels are discussed. A very rough estimation of the potential at a typical MST core location is 0.8–2 kV. Fluctuations in the frequency range 100–20 kHz have also been observed. © 2001 American Institute of Physics.

Notes: 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. © 2001 American Institute of Physics.

Notes: Presently, experiments are underway at the Plasma Dynamics Laboratory at Rensselaer Polytechnic Institute to demonstrate that the techniques developed for heavy ion beam probe diagnostics (HIBP) can be used to measure radio frequency (rf) fluctuations in plasmas. We hope to measure fluctuations in plasma density and magnetic and electric fields. This will provide a direct measurement of the electric and magnetic fields in the plasma during ICRF heating and thereby improve understanding of heating deposition and wave physics. In addition, the field and the density measurements will be used to determine the plasma reaction to the heating experiments. It is expected that the density measurements will be easiest to interpret, while the electric field measurement will be the most difficult to interpret. The diagnostic issues that will be important in taking data at rf frequencies include faster electronics, signal levels, and path effects. We have used a current to voltage amplifier design to measure 0–500 kHz fluctuations in several previous experiments. By reducing the gain and changing some components, a very similar design is capable of operation at rf frequencies. The modified circuit has been tested up to 15 MHz and worked well. The number of beam ions striking the detector plate in one rf period will be too small to obtain good enough statistics for fluctuation measurements, and therefore, averages over many cycles will be required. We expect to be able to achieve millisecond time resolution in the experiments. The global nature of the modes will tend to make path effects important in the HIBP signals. On the other hand, since the beam will take more than one period to cross the plasma, phase shifts may cancel some of these effects. In addition, a path effect term due to dA/dt will be much more important relative to the electric potential than in lower frequency experiments. The initial experimental plan is to do a series of measurements in which a lithium ion beam passes through an argon helicon plasma. The helicon plasma was chosen because its high density (of order 1019 m−3) will produce a larger HIBP signal than can be obtained from other small plasmas. The helicon plasma is formed within a solenoidal magnetic field of 1 kG on axis. The plasma is excited by an rf antenna that is a modification of the type used in Boswell's experiments.1 The rf power source is presently a 500 W, 13.56 MHz generator. From calculation of final trajectories we have determined that 16–29 keV Li ions can be used to probe a plasma with 1 kG magnetic field on axis. If the signal levels with a lithium beam are too small, a molecular hydrogen source will be used. For testing the basic operation of the ion beam probe we will use a simple plate detector mounted on the output flange. These preliminary experiments will be used to determine the feasibility of measuring density and magnetic field fluctuations. A second set of experiments using a more traditional HIBP energy analyzer as a detector is also planned. This detector will also be able to measure electric field effects on the probing ions. It will also be less sensitive to UV noise from the plasma. © 2001 American Institute of Physics.

Notes: 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 ñe(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 MST is determined by plasma conditions, some effort has been made in the design of the MST beam probe to make it possible to characterize B before, during, and after the plasma discharge. © 1999 American Institute of Physics.

Notes: During the last operational period of ATF, a heavy ion beam probe was used to make measurements of the plasma potential, potential fluctuations, and electron density fluctuations. Due to the limited operational time and resources these measurements were made primarily on ECH plasmas, although some neutral beam heated plasma data were taken. The general results of these measurements and a discussion of the possible mechanisms which could cause uncertainties in the results will be given. The diagnostic method will be outlined with an emphasis on issues related to the operation of a beam probe in a stellarator environment.

Notes: 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.