Search

Nanoparticle (NP) display potentially offers a new way to both stabilize and, in many cases, enhance enzyme activity over that seen for native protein in solution. However, the large, globular and sometimes multimeric nature of many enzymes limits their ability to attach directly to the surface of NPs, especially when the latter are colloidally stabilized with bulky PEGylated ligands. Engineering extended protein linkers into the enzymes to achieve direct attachment through the PEG surface often detrimentally alters the enzymes catalytic ability. Here, we demonstrate an alternate, hybrid biomaterials-based approach to achieving directed enzyme assembly on PEGylated NPs. We self-assemble a unique architecture consisting of a central semiconductor quantum dot (QD) scaffold displaying controlled ratios of extended peptide-DNA linkers which penetrate through the PEG surface to directly couple enzymes to the QD surface. As a test case, we utilize phosphotriesterase (PTE), an enzyme of bio-defense interest due to its ability to hydrolyze organophosphate nerve agents. Moreover, this unique approach still allows PTE to maintain enhanced activity while also suggesting the ability of DNA to enhance enzyme activity in and of itself.

Above-threshold ionization (ATI) electron energy spectra of argon with 800 nm, 40 fs pulses display a contrast which goes through a sharp minimum around 6×1013 W cm-2. The contrast change encompasses the whole ATI spectrum including the two plateaux. The results of a full numerical solution of the time-dependent Schrodinger equation and the integration over the interaction volume show that in the low-energy part of the spectrum this effect is due to the closing of the 12-photon ionization channel and the correlated opening of the possibility for 12-photon resonances with a different parity. However, the calculation suggests a different origin for the substructure observed in the plateau.

Passive charge exchange (PCX) emission between neutralized background ions entering the plasma from the walls, and fully ionized light impurities in a tokamak fusion plasma have been investigated. The incentive was to improve the evaluation of active charge exchange (ACX) spectra, leading to ion temperature, impurity density and plasma rotation. The reconstruction of synthetic line-of-sight-integrated PCX emission spectra is based on a modelled neutral density profile as derived from the FRANTIC code, local emission rates for D0(1s) and D0(2s) donor states and finally local impurity ion densities (C6+, He2+) from CX analysis. As a result of the PCX modelling the experimental errors in ion temperature values can be reduced and the range of accessible PCX spectra extended from magnetic axis to separatrix. A comparison between the modelled intensity of the synthetic spectra and experimental PCX data also allows a consistency check of neutral density and its radial distribution.

On the basis of experiments carried out in JET in D:T mixtures varying from 100:0 to 5:95 and those carried out in hydrogen plasmas, the isotopic mass dependence of ELM parameters and the edge pedestal pressure in NBI and ICRF heated H mode plasmas is presented. The ELM frequency is found to decrease with the atomic mass number in both ICRH and NBI discharges. However, the frequency in the case of ICRH is about 8-10 times higher than that in the case of NBI. Assuming that ELMs occur at a critical edge pressure gradient, limited by the ballooning instability, the scaling of the maximum edge pressure is most consistent with the assumption that the width of the transport barrier scales as the ion poloidal Larmor radius governed by the average energy of fast ions at the edge. The critical edge pressure in NBI heated discharges increases with the isotopic mass, which is consistent with the higher deduced width of the edge transport barrier in tritium than those in deuterium and hydrogen. The critical edge pressure in ICRH discharges is smaller, presumably, due to the smaller fast ion contribution to the edge region. As a consequence of the edge pressure scaling with isotopic mass, the edge operational space in an ne-Te diagram increases with operation in tritium. If the evidence that the edge pedestal width is governed by the average energy of fast ions in the edge prevails, the pedestal in ITER would be controlled by the slowing down energy spectrum of alpha particles in the edge.

An overview of JET experimental results in DT plasmas directly relevant to ITER modes of operation is presented. Experiments in D:T mixtures varying from 100:0 to 10:90 and those carried out in hydrogen plasmas show that the H mode threshold power has an approximately inverse isotope mass dependence. Matching some of the key dimensionless parameters to the ITER values, the ITER similarity experiments with ITER shape and safety factor q show that the global energy confinement time is practically independent of isotopic mass (~A0.03±0.08), where A is the atomic mass of the hydrogenic species. Subtracting the edge pedestal energy (which scales as ~A0.57±0.2) from the total stored energy leads to a ~A-0.17±0.1 dependence of confinement in the plasma core, very similar to that expected from the gyro-Bohm transport (~ A-0.2) model. The observed scaling of the edge pedestal energy is consistent with a model in which the edge pressure gradient saturates at the ballooning limit over a region of width that scales as the ion poloidal Larmor radius governed by the average energy of the fast ions in the edge. The steady state total stored energy for a given input power in both ICRH and NBI discharges is the same despite the lower edge pedestal in the ICRH case, which is compensated for by more peaked power deposition profiles in ICRH. The ELM frequency is smaller with NBI; it decreases with isotopic mass in both NBI and ICRH discharges. A steady state, type I ELMy H mode discharge with ITER shape and q at 3.8 T/3.8 MA with an input power of 22 MW produced a Q ≈ 0.18 for 3.5 s and extrapolates well to ignition with ITER parameters. Here, Q is the ratio of fusion output power to input power. The thermal ELMy H mode confinement in both deuterium and tritium gas fuelled plasmas decreases significantly when the plasma density exceeds 0.75 of the Greenwald (nGW) limit, and the maximum density achieved is 0.85nGW. In L mode, the density limit decreases with increasing isotope mass roughly in accordance with code predictions. ITER reference ICRH scenarios have been evaluated. Second harmonic heating of tritium at the densities available in JET produces strong tails and heats electrons predominantly as expected. The 3He minority in 50:50 D:T and tritium dominated plasmas showed strong bulk ion heating leading to ion temperatures up to 13 keV with ICRH alone. Deuterium minority ion cyclotron heating in tritium plasmas at a power level of 6 MW produced steady state record values of Q ≈ 0.22 for more than 2.5 s.

Experimental plasma-edge gradients in JET during the edge-localized-mode (ELM) free H-mode are examined for evidence of the presence and location of the transport barrier region inside the magnetic separatrix. High spatial resolution data in electron density is available in- and outside the separatrix from an Li-beam diagnostic, and in electron temperature inside the separatrix from an ECE diagnostic, while outside the separatrix, a reciprocating probe provides electron density and temperature data in the scrape-off layer. Ion temperatures and densities are measured using an edge charge-exchange diagnostic. A comparison of observed widths and gradients of this edge region with each other and with theoretical expectations is made. Measurements show that ions and electrons form different barrier regions. Furthermore, the electron temperature barrier width (3-4 cm) is about twice that of electron density, in conflict with existing scaling laws. Suitable parametrization of the edge data enables an electron pressure gradient to be deduced for the first time at JET. It rises during the ELM-free phase to reach only about half the marginal pressure gradient expected from ballooning stability before the first ELM. Subsequent type I ELMs occur on a pressure gradient contour roughly consistent with both a constant barrier width model and a ballooning mode envelope model.

All major JET systems have been fully commissioned for D-T and the DTE1 series of experiments has started with the D-T fuel mixture and operating conditions foreseen for ITER. In the area of ITER physics, significant results have been produced in both D-D and D-T. In D-D, the LH threshold power database has been extended, the bounds on edge-electron temperature and density in ELMy H-modes have been defined and the advantages of Types I and III ELMy discharges have been compared. In D-T plasmas, the isotope effect on H-mode threshold power and transport has been determined so that a more accurate assessment can be made of the ignition margin and heating requirements for ITER. Trace tritium experiments have provided first particle transport measurements and an assessment of the ITER reference ion-cyclotron resonance-frequency heating scenarios has been started, In the area of fusion performance, record D-D neutron yields have been obtained by controlling the plasma and current profiles in hot ion ELM-free H-modes and optimized shear modes. In D-T, internal transport barriers have been readily established in optimized shear discharges and Alfven eigenmodes have been observed.

An analytical procedure is presented which enables a fast estimate of collision-energy-dependent cross-section effects on thermal charge exchange spectra. The model is based both on experimental evidence and numerical simulations showing that the observed charge exchange (CX) spectra are essentially Gaussian in their shape. The collision-energy-dependent emission rate leads effectively to a lineshift (apparent velocity), usually to a reduction in linewidth (apparent temperature), and to a change in the effective emission rate averaged over the entire thermal velocity distribution function. It is demonstrated that the cross-section effect can be treated analytically introducing an approximated emission rate factor which retains the characteristics of a Maxwellian velocity distribution function using an exponential expression with only linear and quadratic velocity terms in its exponent. An algebraic deconvolution procedure is described, which enables the reconstruction of true temperature, velocity and intensities from measured CX spectra. Examples taken from a recent JET experimental campaign are used to illustrate the cross-section effects on low-Z impurity CX spectra for a comprehensive variety of neutral beams (deuterium, tritium or helium), target densities, temperatures and toroidal rotation speeds. An overview is given of representative correction factors established for high-power, high-temperature plasmas, as well as for plasmas with combined neutral beam and radiofrequency heating, and for the case of locked modes.

An account is given of the high performance plasmas established by development of the H-mode regime in JET in the experimental campaigns up to 1992. High performance in this case is measured in terms of the confinement enhancement achieved over the L-mode scaling as measured using the plasma diamagnetism. Three JET H-mode regimes have achieved enhancement factors (H G DIA ) over Goldston L-mode scaling of 2.5 < H G DIA < 4.0. These are the Pellet Enhanced Performance (PEP) H-MODE, the high bootstrap fraction (high β POL ) H-mode and the Hot Ion (HI) H-mode. The phenomenology of these three regimes is reviewed and contrasts and common threads are elucidated

Active beam emission spectroscopy with high energy Li beams has been shown to be a successful technique for determining electron density profiles of low and medium density plasma edges of a tokamak. For high density plasma edges, however, the reconstruction algorithm necessary to extract the electron density profile from the Li beam emission profile at 671 nm encounters a singularity point at which numerical instabilities can occur. This behaviour is particularly pronounced for noisy input data. An 'integral method' has been developed at JET to replace the standard calculation method in the singularity region of the profile, and the 'normal' deconvolution method is then used for data in the high density region. Using this approach, routine deconvolution of emission profiles extending from low density regions well into high density regions of the plasma has been made possible. The sensitivity of the algorithm to noisy input data and errors on input plasma parameters has been examined for typical conditions expected at JET. The simulations show that even for unfavourable conditions electron density profiles can be recovered with an overall accuracy better than 25%.

Electron energy spectra from multiphoton ionization of krypton with 617 nm, 70 fs pulses have been recorded over a large intensity range. Near the appearance intensity, the spectra display Stark-induced resonances with s and d states. Up to about 8*1013 W cm-2, the spectra broaden and the structures are progressively washed out. Above this intensity, a strong resonance emerges, corresponding to the 4f state for which the static detuning is 2.94 eV, a value larger than the photon energy. The influence of a large static detuning and populations of excited states with shifts different from the threshold shift are discussed.

A method combining laser-induced fluorescence and injection of a neutral beam was employed to determine a time-varying electron-density in a conical theta-pinch plasma. The method consists of measuring the electron excitation rate relative to the spontaneous emission coefficient. An expression for the electron density in terms of the measured fluorescence and collision-induced emission signals has been formulated, taking into account the appreciable population of the excited level, as well as incomplete laser saturation. Neutral atomic beams (Li and Mg) were produced using laser ablation. A low-density hydrogen plasma was produced using a conical theta-pinch to simulate tokamak edge-plasma conditions. Using a Langmuir double-probe and emission spectroscopic methods, a time-varying electron density and an electron temperature. Measuring the electron excitation of the atomic beams with laser-induced fluorescence (LIF), the rapidly varying electron density could be deduced using either the Li 670 nm or the Mg 285 nm transition.

Photoelectron energy spectra from multiphoton ionization of magnesium reveal that direct nine or ten-photon transitions to 4p and possibly 5p thresholds are taking place around 586 nm and about 1012 W cm-2. Lineshapes indicate that such transitions are enhanced by the intermediate four-photon resonance with the 3p2 1S two-electron state, above the ionization threshold.

Measurements of the Stark widths of twelve Ar IV lines in the near UV, emitted by a gas-liner pinch plasma (electron density 2.4*1018 cm-3, electron temperature 7 eV), are reported, and compared with calculations of electron impact broadening. Good overall agreement is obtained with the semiclassical Gaunt factor approximation derived earlier by two of the present authors. When experimental factors are accounted for, no evidence is found for deviations of the Stark profiles from symmetrical Lorentzians. The spectral lines are also found to be practically unshifted. The importance of taking careful account of the structure of the radiating ion in the calculation is shown. The applicability of the impact approximation to such a dense but relatively cool plasma is also considered.

Multi-photon-double ionization of xenon by Ti:Sapphire laser pulses combined with their 25th harmonic has been studied by means of a momentum imaging spectrometer. The determination of the momenta of the emitted photoelectron pair and of its energy and angular correlation gives insight into the various mechanisms leading to double ionization. Although the conditions for non sequential direct multi-photon double ionization are met in the experiment, it is found that two-step sequential processes prevail.

Amplitude variability is a common feature in δ Scuti stars. These variations can be explained by beating of close frequencies or by true amplitude variability. Observations have shown that the occurrence of amplitude variability follows regularities in the frequency spectra. In the present work we compute the expected incidence of close frequency pairs in pulsation models of a selected sample of δ Scuti stars. Different stellar evolutionary stages are considered. We estimate how many close frequency pairs are predicted and examine the regularities of close frequency pairs in the frequency spectra.

Photo-double ionization (PDI) of xenon by the Ti:Sapphire laser fundamental frequency combined to its 19th and 25th harmonics has been studied by means of a momentum imaging spectrometer. The measurement of the two emitted photoelectrons' momenta and their energy and angular correlations gives insight into the various mechanisms leading to PDI. Although the conditions for non-sequential direct photo-double ionization are fulfilled in the experiment, it is found that the two-step processes dominate.

Single ionization of helium by a superposition of selected XUV high harmonics and infrared radiation has been studied by a momentum imaging technique. The measured angular distributions of photoelectrons are compared to numerical time-dependent calculations, showing very good agreement after average. The calculated angular distributions appear to depend critically on the delay between harmonic and infrared pulses on the attosecond scale, and on the relative phases and intensities of the harmonics.

93 hours of photoelectric photometry of FG Virginis were obtained during 1985 and 1986. Seven frequencies of pulsation with significant detections could be extracted from the photometry. The seven detected frequencies agree with those found in a larger, independent photometric study by Breger et al. carried out during 1993, in which ten frequencies were found. The present data also show the 8th and 9th frequencies found during 1993, but below the signal-to-noise limit of significance adopted in our studies. The y amplitudes determined from the present work and the 1993 data agree to ± 1.0 millimag, suggesting that no significant amplitude variations occurred during the eight years. For the dominant frequency of 12.72 c/d a phase shift of theta-b-y - theta-y = 7 degrees is found, compatible with a nonradial p mode of 1=1 and 2.