Explaining Cold-Pulse Dynamics in Tokamak Plasmas Using Local Turbulent Transport Models.

A long-standing enigma in plasma transport has been resolved by modeling of cold-pulse experiments conducted on the Alcator C-Mod tokamak. Controlled edge cooling of fusion plasmas triggers core electron heating on time scales faster than an energy confinement time, which has long been interpreted as strong evidence of nonlocal transport. This Letter shows that the steady-state profiles, the cold-pulse rise time, and disappearance at higher density as measured in these experiments are successfully captured by a recent local quasilinear turbulent transport model, demonstrating that the existence of nonlocal transport phenomena is not necessary for explaining the behavior and time scales of cold-pulse experiments in tokamak plasmas.

Observation of Efficient Lower Hybrid Current Drive at High Density in Diverted Plasmas on the Alcator C-Mod Tokamak.

Efficient lower hybrid current drive (LHCD) is demonstrated at densities up to n[over ¯]_{e}≈1.5×10^{20} m^{-3} in diverted plasmas on the Alcator C-Mod tokamak by operating at increased plasma current-and therefore reduced Greenwald density fraction. This density exceeds the nominal "LH density limit" at n[over ¯]_{e}≈1.0×10^{20} m^{-3} reported previously, above which an anomalous loss of current drive efficiency was observed. The recovery of current drive efficiency to a level consistent with engineering scalings is correlated with a reduction in density shoulders and turbulence levels in the far scrape-off layer. Concurrently, rf wave interaction with the edge and/or scrape-off-layer plasma is reduced, as indicated by a minimal broadening of the wave frequency spectrum measured at the plasma edge. These results have important implications for sustaining steady-state tokamak operation and indicate a pathway forward for implementing efficient LHCD in a reactor.

Turbulence Nonlinearities Shed Light on Geometric Asymmetry in Tokamak Confinement Transitions.

A comprehensive study of fully frequency-resolved nonlinear kinetic energy transfer has been performed for the first time in a diverted tokamak, providing new insight into the parametric dependences of edge turbulence transitions. Measurements using gas puff imaging in the turbulent L-mode state illuminate the source of the long known but as yet unexplained "favorable-unfavorable" geometric asymmetry of the power threshold for transition to the turbulence-suppressed H mode. Results from the recently discovered I mode point to a competition between zonal flow (ZF) and geodesic-acoustic modes (GAM) for turbulent energy, while showing new evidence that the I-to-H transition is still dominated by ZFs. The availability of nonlinear drive for the GAM against net heat flux through the edge corresponds very well to empirical scalings found experimentally for accessing the I mode.

Fast Low-to-High Confinement Mode Bifurcation Dynamics in a Tokamak Edge Plasma Gyrokinetic Simulation.

Transport barrier formation and its relation to sheared flows in fluids and plasmas are of fundamental interest in various natural and laboratory observations and of critical importance in achieving an economical energy production in a magnetic fusion device. Here we report the first observation of an edge transport barrier formation event in an electrostatic gyrokinetic simulation carried out in a realistic diverted tokamak edge geometry under strong forcing by a high rate of heat deposition. The results show that turbulent Reynolds-stress-driven sheared E×B flows act in concert with neoclassical orbit loss to quench turbulent transport and form a transport barrier just inside the last closed magnetic flux surface.

Effects of magnetic shear on toroidal rotation in tokamak plasmas with lower hybrid current drive.

Application of lower hybrid (LH) current drive in tokamak plasmas can induce both co- and countercurrent directed changes in toroidal rotation, depending on the core q profile. For discharges with q(0) <1, rotation increments in the countercurrent direction are observed. If the LH-driven current is sufficient to suppress sawteeth and increase q(0) above unity, the core toroidal rotation change is in the cocurrent direction. This change in sign of the rotation increment is consistent with a change in sign of the residual stress (the divergence of which constitutes an intrinsic torque that drives the flow) through its dependence on magnetic shear.

Edge temperature gradient as intrinsic rotation drive in Alcator C-Mod tokamak plasmas.

Intrinsic rotation has been observed in I-mode plasmas from the C-Mod tokamak, and is found to be similar to that in H mode, both in its edge origin and in the scaling with global pressure. Since both plasmas have similar edge ∇T, but completely different edge ∇n, it may be concluded that the drive of the intrinsic rotation is the edge ∇T rather than ∇P. Evidence suggests that the connection between gradients and rotation is the residual stress, and a scaling for the rotation from conversion of free energy to macroscopic flow is calculated.

Alternating current delivered into the scala media alters sound pressure at the eardrum.

Alternating current delivered into the scala media of the gerbil cochlea modulates the amplitude of a test tone measured near the eardrum. Variations in the electromechanical effect with acoustic stimulus parameters and observed physiological vulnerability suggest that cochlear hair cells are the biophysical origin of the process. Cochlear hair cells have traditionally been thought of as passive receptor cells, but they may play an active role in cochlear micromechanics.

Physics design of the in-vessel collection optics for the ITER electron cyclotron emission diagnostic.

Measurement of the electron cyclotron emission (ECE) is one of the primary diagnostics for electron temperature in ITER. In-vessel, in-vacuum, and quasi-optical antennas capture sufficient ECE to achieve large signal to noise with microsecond temporal resolution and high spatial resolution while maintaining polarization fidelity. Two similar systems are required. One views the plasma radially. The other is an oblique view. Both views can be used to measure the electron temperature, while the oblique is also sensitive to non-thermal distortion in the bulk electron distribution. The in-vacuum optics for both systems are subject to degradation as they have a direct view of the ITER plasma and will not be accessible for cleaning or replacement for extended periods. Blackbody radiation sources are provided for in situ calibration.

Temperature gradient scale length measurement: A high accuracy application of electron cyclotron emission without calibration.

Calibration is a crucial procedure in electron temperature (Te) inference from a typical electron cyclotron emission (ECE) diagnostic on tokamaks. Although the calibration provides an important multiplying factor for an individual ECE channel, the parameter ΔTe/Te is independent of any calibration. Since an ECE channel measures the cyclotron emission for a particular flux surface, a non-perturbing change in toroidal magnetic field changes the view of that channel. Hence the calibration-free parameter is a measure of Te gradient. BT-jog technique is presented here which employs the parameter and the raw ECE signals for direct measurement of electron temperature gradient scale length.

Cerebrospinal fluid pharmacokinetics of cefpodoxime proxetil in piglets.

Cefpodoxime is an oral third-generation cephalosporin used for the treatment of acute upper-respiratory tract infections caused by susceptible bacteria in children. Although not indicated for the treatment of bacterial meningitis, it is used to treat other infections produced by organisms associated with meningitis and may obscure the result of cerebrospinal fluid (CSF) cultures in children who develop meningitis while receiving oral antibiotics if sufficient concentrations are achieved in the CSF. This study evaluated the disposition of cefpodoxime and penetration into CSF in piglets. Fifteen Landacre-Camborough cross piglets (10-20 days old) received cefpodoxime proxetil oral suspension (10 mg/kg). Repeated plasma and CSF samples were collected over 24 hours for quantitation of cefpodoxime by HPLC. Pharmacokinetic analysis was performed on both plasma and CSF data. The plasma concentration versus time data for cefpodoxime were best characterized using a one-compartment model with first-order absorption. The mean (+/- SD) pharmacokinetic parameters for Cmax, tmax, and AUC0-infinity were 23.3 +/- 12.9 mg/L, 3.9 +/- 1.4 h, and 237 +/- 129 mg/L.h, respectively. CSF/plasma ratios for AUC0-infinity demonstrated a mean cefpodoxime penetration of approximately 5%. CSF penetration of cefpodoxime was evident following a single oral dose of cefpodoxime proxetil suspension. Despite the small percentage of total cefpodoxime dose distributing into the CSF, the resultant concentrations approached or exceeded the MIC90 for many bacterial pathogens considered susceptible to cefpodoxime. Accordingly, clinicians should use caution in the interpretation of CSF cultures in patients who develop clinical signs and symptoms consistent with meningitis and who have been previously treated with cefpodoxime.

Surgeon-controlled ultrasound-guided core biopsies in the breast--a prospective study and a new use for surgeons in the clinic.

AIMS: This study was to assess the accuracy of a surgeon performing ultrasound-guided core biopsies of the breast. METHODS: A prospective audit was carried out of 555 patients who underwent an ultrasound-guided core biopsy for a discrete solid mass [under 30 mm maximum diameter] by a single surgeon. The surgeon controlled the core biopsy needle and an ultrasonographer or radiologist provided the imaging with ultrasound. RESULTS: The accuracy of the surgeon in sampling the lesions [n = 555 core biopsies] was independent of the size of the lesion. This saved 272 patients having unnecessary surgery for a benign lesion. CONCLUSION: The practical involvement by the surgeon in breast ultrasound and performing core biopsies has reduced pressure on the breast radiologists, reduced the number of diagnostic surgical open biopsies and made the clinic more interesting for the surgeon. With increased surgical confidence in breast ultrasound, most small and impalpable cancers have pre-operative skin marking rather than X-ray wire localization prior to undergoing wide local excision. Trainee breast surgeons should be encouraged to learn breast ultrasound and core biopsies.

Cochlear emissions simulated using one-dimensional model of cochlear hydrodynamics.

Stapes velocity was computed using a nonlinear, one-dimensional model of cochlear hydromechanics. The model's compliances and damping coefficients were mechanically nonlinear and instantaneously varying in proportion to simulated current injected into the cochlea. Experimental data showing the spectral content of the pressure waveform near the eardrum during the delivery of sound and current to the cochlea were compared with model results.

Rapid force production in the cochlea.

Electrical stimulation of the mammalian cochlea causes a mechanical response which produces acoustic signals at the frequency of the electrical current. These electrically-evoked acoustic emissions can be as large as 34 dB SPL. Concurrent acoustic stimuli can enhance the emission response. Comparison of the enhancement effect with the cochlear microphonic (CM) suggests that the emissions originate from the outer hair cells (OHC). Frequency response measurements indicate a rate-limiting time constant for the force-generating process which is less than 35 microseconds.

Responses of auditory-nerve fibers to characteristic-frequency tones and low-frequency suppressors.

Responses of single auditory-nerve fibers to combinations of low-frequency (suppressor) and characteristic frequency (CF) tones were recorded. The shapes of period histograms were observed as the level of the suppressor was raised. For fibers with CFs above 5.0 kHz, at the lowest suppressor levels used, the phase of the suppressor tone which caused instantaneous rate to increase when presented alone caused a rate decrease when added to a CF tone. For suppressor frequencies of 500 and 100 Hz, as suppressor level is increased, two peaks appear in the period histograms. Further increase in level causes first one, then the other of these peaks to disappear and the histogram takes on the shape of the histogram to the suppressor alone. A similar progression of histogram shape follows a decrease in CF tone level for fixed suppressor level.

Haircell forward and reverse transduction: differential suppression and enhancement.

Cochlear outer haircells are believed to play a significant role in an amplification process which greatly enhances inner ear sensitivity. Haircell forward (mechanical-to-electrical) and reverse (electrical-to-mechanical) transduction may be involved. We have produced decreases in cochlear microphonic and increases in electrically-evoked cochlear emissions using the drug, furosemide. The data indicate forward and reverse transduction are not a simple bi-directional process and suggest that the outer haircells are part of a negative feedback system.

Acoustic enhancement of electrically-evoked otoacoustic emissions reflects basilar membrane tuning: experiment results.

Acoustic enhancement of the electrically-evoked otoacoustic emissions (EEOEs) was investigated by systematically varying acoustic frequency and intensity. The results demonstrated that simultaneous acoustic stimulation at frequencies around the characteristic frequency of the electrical current injection place was most effective in enhancing low-frequency EEOEs. Moreover, it was demonstrated that the enhancement was tuned and graded. The enhancement threshold tuning curves (defined as sound pressure level needed to achieve 1 dB of enhancement) resembled basilar membrane tuning at high sound pressure levels. The data suggest that the emissions were generated from a cochlear region near the electrode place, and the magnitude of the enhancement depends on the magnitude of the basilar membrane response to the acoustic stimulus.

Acoustic enhancement of electrically evoked otoacoustic emissions reflects basilar membrane tuning: a model.

A simple model for the acoustic enhancement of electrically evoked otoacoustic emissions (EEOEs) is presented in this paper. The model is based on the assumption that the enhancement is a result of the local interaction between the electrical current spreading in the scala media and the basilar membrane (BM) response to acoustic input. The analytical, steady-state response of the 1-dimensional linear cable to sinusoidal current injection is derived and is used to predict the current spreading in the cochlea. Acoustic enhancement at an emission generator is modeled as a magnitude change that is a sigmoid function of the local BM motion. The model results are in good agreement with the experimental findings and support our interpretation that the acoustic enhancement of EEOEs reflects BM tuning.

Nonlinear characteristics of electrically evoked otoacoustic emissions.

To further our knowledge of outer hair cell nonlinearities, we measured the dependence of the electrically-evoked otoacoustic emissions (EEOEs) on current level for a wide range of electrical frequencies. Alternating electrical current was delivered into the scala media of the gerbil cochlea while the EEOE was measured with a probe-tube microphone. While the EEOE scaled linearly with current level for many frequencies and current levels, notable exceptions occurred. For frequencies below 300 Hz and currents above 20-30 microA(peak), the gain (primary EEOE magnitude divided by the current level) increased abruptly. For higher frequencies, the gain often increased slightly with increasing current of up to 30-50 microA(peak), but decreased at even higher current levels. We also investigated the enhancement of the EEOE due to simultaneous acoustic stimulation. The enhancement of the EEOE was relatively insensitive to current level with little change in enhancement for current levels up to 20 microA(peak). For current levels above approximately 40 microA(peak), the enhancement decreased slightly.

Voltage-dependent elements are involved in the generation of the cochlear microphonic and the sound-induced resistance changes measured in scala media of the guinea pig.

The injection of d.c. current into scale media alters both the cochlear microphonic (CM) and the acoustically synchronized changing resistance (CR) measured in scala media. Positive current increases the CM and decreases the CR. The effect on the CM is greatest at high sound pressure level (SPL), whereas the effect on CR is greatest at low SPL. Negative current has a similar but opposite effect on both the CM and the CR. The results suggest that a voltage-dependent nonlinear element exists in cochlear hair cells.

Stimulation of efferents alters the cochlear microphonic and the sound-induced resistance changes measured in scale media of the guinea pig.

Electrical stimulation of the crossed olivo-cochlear bundle (COCB) increases both the cochlear microphonic and the acoustically synchronized changing resistance (CR) and it causes a decrease in the electrical impedance of scala media of the guinea pig. The similarity between the change in CR due to COCB stimulation and the change in CR due to negative d.c. polarization (Mountain, D.C., Hubbard, A.E. and Geisler, C.D. (1980): Hearing Res. 3, 215-229) suggests that the CR is dependent on the hair cell membrane potential measured with respect to scale tympani.