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Notes: Spatially (0.1 cm3) and temporally (1 μs) resolved magnetic field measurement techniques using a heavy ion beam probe as a test particle source are described. The measurement of both steady-state and time-varying fields is discussed. The plasma flux function can be determined by measuring the toroidal velocity of the beam ion in an axisymmetric device, because the canonical angular momentum of a particle, Pφ=qψ+MνφR, is conserved in an axisymmetric system. Corrections due to nonaxisymmetry can be significant in tokamaks and must be taken into account for the current profile and fluctuation measurements. The requirements and design of a toroidal velocity detector are discussed. The signals expected in experiments using the Texas Experimental Tokamak (TEXT) heavy ion beam probe with a velocity detector have been calculated, and they are at least two orders of magnitude higher than the amplifier noise for dc measurements of poloidal and ergodic magnetic limiter fields and for sawtooth and MHD oscillations. Low-level turbulence is expected to produce signals below the noise level.

Notes: 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 measure a coherent density fluctuation on a hollow cathode arc in 1969, and soon afterward to measure the space potential. Since then beam probes have been used to measure the space potential and fluctuations in both density and space potential for plasmas with varying magnetic geometries. There is continuing development work to study the feasibility of using beam probes to measure magnetic fluctuations and magnetic field structure. Sensitivity for measuring density and potential fluctuations is best demonstrated by what is the most sophisticated beam probe to date: the 500-keV system on TEXT. For broadband measurements (50–250 kHz), the TEXT beam probe has demonstrated a sensitivity to space potential fluctuations of 2 V (rms), and resolution for ñe/ne of 10−3.Recent measurements on both ISX-B and TEXT have demonstrated the capability of obtaining simultaneous ñ and cursive-phi˜ measurements at three separate locations in the plasma. For some locations the sample volumes are poloidally separated and S(kθ, w) can be estimated for both ñ and cursive-phi˜. This permits evaluation of the net electrostatic-fluctuation-induced particle flux. Some of the problems still being encountered with present beam probes are the nonideal behavior of the energy analyzer, cross talk between ñ and cursive-phi˜ for high-wave-number fluctuations, the effect of finite sample volume and sample volume spacing on the evaluation of k spectra, simultaneous measurement of two components of the k vector, probing of the complex 3-D magnetic fields, and extension of the measurements to higher-energy beam probe systems.

Notes: 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.

Notes: 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 ñ/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. Corresponding phase velocities, vph∼3.5×105 cm/s, are similar to drift wave velocities. © 2001 American Institute of Physics.

Notes: In this paper we present space potential profiles directly observed in a toroidal helical plasma of the Compact Helical System (CHS) [K. Matsuoka et al., Proceedings, 12th International Conference on Plasma Physics and Controlled Nuclear Fusion, Nice, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 2, p. 411], using a 200 keV heavy ion beam probe. The potential profiles exhibit widely varied characteristics, including positive and negative polarities for electron cyclotron and neutral beam-heated plasmas, respectively. The behavior of high-energy particles in the CHS plasmas are deduced from loss cone diagrams evaluated from the observed potential profiles. © 1997 American Institute of Physics.

Notes: Electron temperature and density fluctuations are measured in the core of the Texas Experimental Tokamak-Upgrade (TEXT-U) [P. H. Edmonds, E. R. Solano, and A. J. Wootton, in Proceedings of the 15th Symposium on Fusion Technology, Utrecht (Elsevier Science, Amsterdam, 1989), Vol. 1, p. 342] plasma across the poloidal cross section. The high spatial resolution of the heavy-ion beam probe (HIBP) and correlation radiometry of electron cyclotron emission (CRECE) reveal that both the density and temperature fluctuations are strongly poloidally asymmetric. Temperature fluctuation measurements indicate a broadband drift wave feature localized near the plasma equatorial plane on both the high- and low-field sides, which is consistent with density fluctuation measurements by far infrared (FIR) scattering. In contrast, the HIBP observes this feature localized only to the low-field side. Excellent spatial resolution allows us to investigate whether changes in the gradient affect the fluctuation amplitudes. We find that indeed, the temperature fluctuations increase with the electron temperature gradient. Results also link density fluctuations to changes in the density gradient. © 1996 American Institute of Physics.

Notes: The response of a tokamak discharge to a sharp drop in edge temperature differs significantly from that expected from typical local transport models in several important respects. Laser ablation of carbon induces large (ΔT/T≤70%), rapid (〈200 μs) electron temperature drops in the outermost region of the plasma, r/a≥0.9. This cold pulse proceeds through the outer plasma (r/a≥0.75), rapidly compared with power balance or sawtooth predictions. However, the pulse shrinks markedly thereafter, disappearing near r/a∼0.5. Within r/a∼0.3, the temperature rises promptly. The results are inconsistent with conventional local transport models; a nonlocal phenomenology, in which transport coefficients increase in the edge and decrease in the core, is suggested. The turbulence levels measured with a heavy ion beam probe increase near the edge but are unchanged in the core. © 1995 American Institute of Physics.

Notes: Broad bandwidth (60 kHz ≤f≤500 kHz), long wavelength (k¯θ≤2 cm−1) density fluctuation measurements were made using a heavy ion beam probe on the Texas Experimental Tokamak (TEXT) [K. W. Gentle, Nucl. Technol./Fusion 1, 479 (1981)]. Fluctuation amplitudes and correlation length scalings do not correspond to linear or quasilinear drift wave theory. Power-weighted root-mean-square (rms) wave number estimates are much lower (〈k¯θ〉rmsρs≤0.1, ρs=[miTe]1/2/eB) than quasilinear theoretical expectations (k¯⊥ρs∼0.3). Core results indicate strong turbulence with a wave number spread, Δk∼k¯ at fixed frequency f, consistent with strong turbulence predictions, while edge results can be considered consistent with weak turbulence. In between the core and edge (0.6≤r/a≤0.8) is a transition region. © 1995 American Institute of Physics.

Notes: Fluctuations in electric potential, electron pressure, and their relative phase have been measured in the RENTOR tokamak [Phys. Rev. Lett. 55, 2417 (1985)] using a heavy ion beam probe. Between 70 and 200 kHz, pressure fluctuation levels of p˜/p∼0.04 and potential fluctuation levels of eφ˜/Te=0.08–3 have been observed. The phase angle between the pressure and potential fluctuations is approximately 25°. The heat transport by the observed fluctuations is not enough to account for the observed heat loss at the plasma edge. The RENTOR results are compared with measurements on the TEXT (J. Forster, Ph.D. thesis, Rensselaer Polytechnic Institute, 1987) and ISX-B [Phys. Rev. Lett. 59, 1301 (1987)] tokamaks and with drift wave and rippling mode theories.

Notes: Recent experiments on the Texas Experimental Tokamak (TEXT) [Plasma Phys. Controlled Fusion 27, 1335 (1985)] have focused on identifying the drives and transport significance of the turbulence in the interior of discharges in the neo-Alcator confinement regime. Far-infrared (FIR) scattering consistently observes density fluctuations indicative of electron drift waves, i.e., ωk∼ωe*+kθvE, where vE≡−Er/B (the electron diamagnetic frequency Doppler-shifted by the E×B poloidal plasma rotation) and an amplitude that scales inversely with the density scale length Ln. Although consistent with scattering on the power-weighted frequency ω¯, heavy-ion beam probe (HIBP) data typically indicate k¯θρs〈0.1 such that ω¯(very-much-greater-than)ω¯e*+k¯θvE and n¯/ne(very-much-less-than)1/k¯θLn. Experiments with a modulated gas feed and/or electron-cyclotron resonance heating (ECRH) seem to rule out ∇Te as the turbulent drive, although little evidence for ∇ne is apparent either. In fact, the interior fluctuations seen by the HIBP seem to depend more on edge conditions than local gradients. The quasilinear fluxes due to dissipative trapped electrons are found to be sufficient to account for the equilibrium fluxes in the region of low collisionality (νe*(very-much-less-than)1) for a drift-wave spectrum (k¯θρs∼0.3), but not for the low wave numbers seen by the HIBP. While the estimates with k¯θρs∼0.3 predict the scaling of the electron energy flux over a range of discharge conditions with νe*(very-much-less-than)1, the model seems unable to predict the magnitude (or phase) of the perturbed particle flux during modulated gas feed experiments. In the outer regions of the plasma where νe*(approximately-greater-than)1, the calculated quasilinear fluxes are inadequate, even including untrapped particles.