Demonstration of Tokamak Discharge Shutdown with Shell Pellet Payload Impurity Dispersal.

The first rapid tokamak discharge shutdown using dispersive core payload deposition with shell pellets has been achieved in the DIII-D tokamak. Shell pellets are being investigated as a possible new path toward achieving tokamak disruption mitigation with both low conducted wall heat loads and slow current quench. Conventional disruption mitigation injects radiating impurities into the outer edge of the tokamak plasma, which tends to result in poor impurity assimilation and creates a strong edge cooling and outward heat flow, thus requiring undesirable high-Z impurities to achieve low conducted heat loads. The shell pellet technique aims to produce a hollow temperature profile by using a thin, low-ablation shell surrounding a dispersive payload, giving a greatly increased impurity ablation (and radiation) rate when the payload is released in the plasma core. This principle was demonstrated successfully using 3.6 mm outer diameter, 40 µm thickness diamond shells holding boron powder. The pellets caused rapid (<10 ms) discharge shutdown with low conducted divertor heat fluence (∼0.1 MJ/m^{2}). Confirmation of massive release of the boron powder payload into the plasma core was obtained spectroscopically. Some evidence for the formation of a hollow temperature profile during the shutdown was observed. These first results open a new avenue for disruption mitigation research, hopefully enabling development of highly effective methods of avoiding disruption wall damage in future reactor-scale tokamaks.

First Direct Observation of Runaway-Electron-Driven Whistler Waves in Tokamaks.

DIII-D experiments at low density (n_{e}∼10^{19} m^{-3}) have directly measured whistler waves in the 100-200 MHz range excited by multi-MeV runaway electrons. Whistler activity is correlated with runaway intensity (hard x-ray emission level), occurs in novel discrete frequency bands, and exhibits nonlinear limit-cycle-like behavior. The measured frequencies scale with the magnetic field strength and electron density as expected from the whistler dispersion relation. The modes are stabilized with increasing magnetic field, which is consistent with wave-particle resonance mechanisms. The mode amplitudes show intermittent time variations correlated with changes in the electron cyclotron emission that follow predator-prey cycles. These can be interpreted as wave-induced pitch angle scattering of moderate energy runaways. The tokamak runaway-whistler mechanisms have parallels to whistler phenomena in ionospheric plasmas. The observations also open new directions for the modeling and active control of runaway electrons in tokamaks.

Spatiotemporal Evolution of Runaway Electron Momentum Distributions in Tokamaks.

Novel spatial, temporal, and energetically resolved measurements of bremsstrahlung hard-x-ray (HXR) emission from runaway electron (RE) populations in tokamaks reveal nonmonotonic RE distribution functions whose properties depend on the interplay of electric field acceleration with collisional and synchrotron damping. Measurements are consistent with theoretical predictions of momentum-space attractors that accumulate runaway electrons. RE distribution functions are measured to shift to a higher energy when the synchrotron force is reduced by decreasing the toroidal magnetic field strength. Increasing the collisional damping by increasing the electron density (at a fixed magnetic and electric field) reduces the energy of the nonmonotonic feature and reduces the HXR growth rate at all energies. Higher-energy HXR growth rates extrapolate to zero at the expected threshold electric field for RE sustainment, while low-energy REs are anomalously lost. The compilation of HXR emission from different sight lines into the plasma yields energy and pitch-angle-resolved RE distributions and demonstrates increasing pitch-angle and radial gradients with energy.

Observation of a multimode plasma response and its relationship to density pumpout and edge-localized mode suppression.

Density pumpout and edge-localized mode (ELM) suppression by applied n=2 magnetic fields in low-collisionality DIII-D plasmas are shown to be correlated with the magnitude of the plasma response driven on the high-field side (HFS) of the magnetic axis but not the low-field side (LFS) midplane. These distinct responses are a direct measurement of a multimodal magnetic plasma response, with each structure preferentially excited by a different n=2 applied spectrum and preferentially detected on the LFS or HFS. Ideal and resistive magneto-hydrodynamic (MHD) calculations find that the LFS measurement is primarily sensitive to the excitation of stable kink modes, while the HFS measurement is primarily sensitive to resonant currents (whether fully shielding or partially penetrated). The resonant currents are themselves strongly modified by kink excitation, with the optimal applied field pitch for pumpout and ELM suppression significantly differing from equilibrium field alignment.

Pedestal bifurcation and resonant field penetration at the threshold of edge-localized mode suppression in the DIII-D Tokamak.

Rapid bifurcations in the plasma response to slowly varying n=2 magnetic fields are observed as the plasma transitions into and out of edge-localized mode (ELM) suppression. The rapid transition to ELM suppression is characterized by an increase in the toroidal rotation and a reduction in the electron pressure gradient at the top of the pedestal that reduces the perpendicular electron flow there to near zero. These events occur simultaneously with an increase in the inner-wall magnetic response. These observations are consistent with strong resonant field penetration of n=2 fields at the onset of ELM suppression, based on extended MHD simulations using measured plasma profiles. Spontaneous transitions into (and out of) ELM suppression with a static applied n=2 field indicate competing mechanisms of screening and penetration of resonant fields near threshold conditions. Magnetic measurements reveal evidence for the unlocking and rotation of tearinglike structures as the plasma transitions out of ELM suppression.

Reduction of edge-localized mode intensity using high-repetition-rate pellet injection in tokamak H-mode plasmas.

High repetition rate injection of deuterium pellets from the low-field side (LFS) of the DIII-D tokamak is shown to trigger high-frequency edge-localized modes (ELMs) at up to 12× the low natural ELM frequency in H-mode deuterium plasmas designed to match the ITER baseline configuration in shape, normalized beta, and input power just above the H-mode threshold. The pellet size, velocity, and injection location were chosen to limit penetration to the outer 10% of the plasma. The resulting perturbations to the plasma density and energy confinement time are thus minimal (<10%). The triggered ELMs occur at much lower normalized pedestal pressure than the natural ELMs, suggesting that the pellet injection excites a localized high-n instability. Triggered ELMs produce up to 12× lower energy and particle fluxes to the divertor, and result in a strong decrease in plasma core impurity density. These results show for the first time that shallow, LFS pellet injection can dramatically accelerate the ELM cycle and reduce ELM energy fluxes on plasma facing components, and is a viable technique for real-time control of ELMs in ITER.

Experimental investigation of the role of fluid turbulent stresses and edge plasma flows for intrinsic rotation generation in DIII-D H-mode plasmas.

The first measurements of turbulent stresses and flows inside the separatrix of a tokamak H-mode plasma are reported, using a reciprocating multitip Langmuir probe at the DIII-D tokamak. A strong co-current rotation layer at the separatrix is found to precede intrinsic rotation development in the core. The measured fluid turbulent stresses transport toroidal momentum outward against the velocity gradient and thus try to sustain the edge layer. However, large kinetic stresses must exist to explain the net inward momentum transport leading to co-current core plasma rotation. The importance of such kinetic stresses is corroborated by the success of a simple orbit loss model, representing a purely kinetic mechanism, in the prediction of features of the edge corotation layer.

Resonant pedestal pressure reduction induced by a thermal transport enhancement due to stochastic magnetic boundary layers in high temperature plasmas.

Good alignment of the magnetic field line pitch angle with the mode structure of an external resonant magnetic perturbation (RMP) field is shown to induce modulation of the pedestal electron pressure p(e) in high confinement high rotation plasmas at the DIII-D tokamak with a shape similar to ITER, the next step tokamak experiment. This is caused by an edge safety factor q95 resonant enhancement of the thermal transport, while in contrast, the RMP induced particle pump out does not show a significant resonance. The measured p(e) reduction correlates to an increase in the modeled stochastic layer width during pitch angle variations matching results from resistive low rotation plasmas at the TEXTOR tokamak. These findings suggest a field line pitch angle resonant formation of a stochastic magnetic edge layer as an explanation for the q95 resonant character of type-I edge localized mode suppression by RMPs.

Imaging divertor strike point splitting in RMP ELM suppression experiments in the DIII-D tokamak.

Fast visible imaging of the lower divertor from above is used to study the structure and dynamics of lobes induced by resonant magnetic perturbations (RMPs) in Edge-Localized Mode (ELM) suppression experiments in DIII-D. The best compromise between the amount of light and sharp imaging was obtained using emission at 601 nm from Fulcher band molecular deuterium. Multiple spatially resolved peaks in the D2 emission, taken as a proxy for the particle flux, are readily resolved during RMPs, in contrast to the heat flux measured by infrared cameras, which shows little spatial structure in ITER-like conditions. The 25 mm objective lens provides high spatial resolution (2-4 mm/pixel) from the centerpost to the outer shelf over 40° toroidally that overlaps the field of view of the IRTV that measures the divertor heat flux, allowing direct comparison in non-axisymmetric discharges. The image is coupled to a Phantom 7.3 camera using a Schott wound fiber bundle, providing high temporal resolution that allows the lobe dynamics to be resolved between ELMs and across ELM suppression onset. These measurements are used to study the heat and particle flux in 3D magnetic fields and to validate models for the plasma response to RMPs.

Converging collimation and a large-field-of-view scintillation camera.

Three low-energy multihole converging collimators were evaluated in reference to accepted imaging parameters using a prototype large-field-of-view scintillation camera. A substantial improvement in resolution and/or sensitivity with depth was observed. Because of the large detector, the inherent reduction in field size due to convergence was not detrimental to satisfactory imaging of most target volumes. Such collimation should prove to be of significant clinical value.

Testing for isometry during reconstruction of the anterior cruciate ligament. Anatomical and biomechanical considerations.

Instrumented tibiofemoral (bone-to-bone) excursion wires were implanted in the mid-substance of the anteromedial, central, and posterior fiber-regions of the anterior cruciate ligament through limited anterior and posterior arthrotomies in eight fresh knees from cadavera. The change in the distance of linear separation between each pair of osseous fiber-insertion sites was measured and was plotted against the angle of flexion of the knee as the knee was cycled through a 120-degree range of motion. Testing conditions likely to be present during intraoperative testing for isometry were used (anterior cruciate fibers transected, quadriceps relaxed, femur stabilized with the patient in the supine position and the leg freely dependent, and motion of the knee induced in neutral rotation by force applied at the level of the foot). In no instance did the insertion-site centers of any fiber-region exhibit isometric behavior (change in the distance of linear separation of 1.0 millimeter or less). The least deviations from isometry (range, 1.4 to 3.1 millimeters) were observed for the anteromedial sites, under conditions when the gravitational dependency of the lower leg was constrained. When the leg hung in a dependent manner during passive motion, the deviation from isometry of the anteromedial sites of insertion increased significantly (range, 2.8 to 5.6 millimeters). The central sites of insertion were generally less isometric than the anteromedial sites, and the posterior sites were the least isometric, regardless of testing conditions.

Delayed repair of a ruptured pectoralis major muscle. A case report.

Repair of a pectoralis major muscle rupture at the tendinous insertion into the humerus was successfully performed 13 years after the initial injury. Repair was possible only because the ruptured sternal portion of the muscle was scarred to the intact clavicular portion and therefore had not retracted. The attachment of the avulsed sternal head to the intact clavicular head enabled successful restoration of strength and function, as well as normal contour and appearance of the pectoralis major muscle complex.

Anatomy and kinematics of the lateral collateral ligament of the knee.

The anatomy and kinematics of the lateral collateral ligament were studied in 10 unembalmed limbs and 20 isolated femurs and fibulas. The ligament's average overall length was 66 mm (range, 59 to 74) and the average greatest dimension of its thin middle portion was the anteroposterior dimension of 3.4 mm (range, 3 to 4). The center of the femoral attachment site was 3.7 mm posterior to the ridge of the lateral epicondyle, not at it apex. A potential radiographic technique for operatively locating the femoral attachment site to within 3 mm is described. During knee flexion in neutral rotation the distance between the femoral and fibular attachment sites of the lateral collateral ligament decreased to 88% of its value in full extension. With 6.5 N x m of applied external rotation force, beyond 30 degrees of flexion the attachment sites rapidly approximated. With the same internal rotation force, beyond 15 degrees of flexion the attachment sites separated. From 60 degrees to 105 degrees they were greater than 100% of the value in full extension, suggesting significant distraction between the attachment sites. These changes correlated well with the ligament's change from an 11 degrees posterior slope in extension to a 19 degrees anterior slope in flexion with no applied rotation.

Association of burners with cervical canal and foraminal stenosis.

The purpose of this study was to determine whether the burner phenomenon is associated with cervical canal and foraminal stenosis in a scholastic population. Lateral cervical radiographs were reviewed for 64 athletes, 15 to 18 years of age, who had sustained at least one burner. Controls consisted of age-matched athletes who had sustained head or neck trauma without evidence of the burner phenomenon (N = 32). Pavlov ratios were calculated for levels C-3 through C-6; both mean minimum and mean average ratios were determined. Available oblique radiographs from both the study (N = 31) and control (N = 15) groups were then used to calculate the foramen/vertebral body ratio--a measure of relative foraminal height. Significant differences were found between the burner and control groups for the mean minimum and mean average Pavlov ratios and foramen/vertebral body ratios. Scholastic athletes sustaining the burner phenomenon have an increased risk of cervical canal and foraminal stenosis as measured by the Pavlov and foramen/vertebral body ratios, respectively. The foramen/vertebral body ratio is an easily reproducible and reliable means of assessing foraminal dimensions from oblique radiographs and controls for x-ray magnification and rotation. Foraminal stenosis assessment may prove useful in predicting burner risk, especially in athletes with extension-compression injuries.

Injuries of the posterior cruciate ligament.

A review of the anatomy and biomechanics of the posterior cruciate ligament, and the systematic approach for the diagnosis and treatment of isolated posterior cruciate ligament injuries and posterior cruciate ligament insufficiency in combination with other ligamentous instabilities is discussed.

Biophysical Factors in Range-of-Motion Exercise.

In brief: Studies of the physical properties of connective tissue have helped determine improved techniques for therapeutic range-of-motion exercise. Laboratory studies indicate that the best way to permanently lengthen connective tissue structures without compromising their structural integrity is prolonged, low-intensity stretching at elevated tissue temperatures and cooling the tissue before releasing the tension. This article provides some practical therapeutic guidelines for physicians, trainers, and therapists who prescribe range-of-motion exercise for both rehabilitation and physical conditioning.

Status and characterization of the lithium beam diagnostic on DIII-D.

The 30 keV lithium beam diagnostic on DIII-D is suitable to measure both the radial electron density and poloidal magnetic field profiles in the pedestal. The refurbished system features a new setup to measure the Doppler shift allowing accurate alignment of the spectral filters. The injector has been optimized to generate a stable lithium neutral beam with a current of I = 15-20 mA and a diameter of 1.9 ± 0.1 cm measured by beam imaging. The typical temporal resolution is Δt = 1-10 ms and the radial resolution of ΔR = 5 mm is given by the optical setup. A new analysis technique based on fast Fourier transform avoids systematic error contributions from the digital lock-in analysis and accounts intrinsically for background light correction. Latest upgrades and a detailed characterization of the system are presented. Proof-of-principle measurements of the poloidal magnetic field with a statistical error of typically 2% show a fair agreement with the predictions modeled with the Grad-Shafranov equilibrium solver EFIT within 4%.

Restoring transmission of irradiated image fiber bundles.

Image fiber bundles are employed in fusion experiments and other high radiation environments where they are used to transmit an image from an unprotected objective lens to a radiation shielded camera. Due to their exposure to neutron and gamma radiation the transmission of these expensive image fiber bundles can rapidly degrade, especially at the shorter visible wavelengths, and require costly replacement. A cost-effective, non-destructive heat treatment process in which entire fiber bundles are heated gradually in air to 150°-200°C and held for tens of hours has been shown to recover much of the transmission lost due to the radiation induced absorption. The restoration process can be repeated multiple times without a loss in effectiveness, although some physical degradation of inter-fiber alignment has been observed. The results and the apparatus used for the successful restoration of the transmission of multiple image fiber bundles across their entire wavelength band will be presented.

Pharmacogenomics of the RNA world: structural RNA polymorphisms in drug therapy.

The use of pharmacogenomic biomarkers can enhance treatment outcomes. Regulatory polymorphisms are promising biomarkers that have proven difficult to uncover. They come in two flavors: those that affect transcription (regulatory single-nucleotide polymorphisms (rSNPs)) and those that affect RNA functions such as splicing, turnover, and translation (termed structural RNA SNPs (srSNPs)). This review focuses on the role of srSNPs in drug metabolism, transport, and response. An understanding of the nature and diversity of srSNPs and rSNPs enables clinical scientists to evaluate genetic biomarkers.