Biomedical subjects

G J Kramer

Publications and source records attributed to G J Kramer.

At least 19 recordsLinked to original sources

PubMed2006

Radial structure of Alfvén eigenmodes in the DIII-D tokamak through electron-cyclotron-emission measurements.

The spatial structure of toroidal Alfvén eigenmodes and reversed shear Alfvén eigenmodes in DIII-D is obtained from electron-cyclotron-emission measurements. Peak measured temperature perturbations are of similar magnitude for both toroidal Alfvén eigenmodes and reversed shear Alfvén eigenmodes and found to be deltaT(e)/T(e) approximately equal to 0.5%. Simultaneous measurements of density fluctuations using beam-emission spectroscopy indicate deltan(e)/n(e) approximately equal to 0.25%. Predictions of the measured temperature and density perturbation profiles as well as deltaT(e)/deltan(e) from the ideal magnetohydrodynamic code NOVA are in close agreement with experiment.

Journal Article↗

PubMed2006

Multitude of core-localized shear Alfvén waves in a high-temperature fusion plasma.

Evidence is presented for a multitude of discrete frequency Alfvén waves in the core of magnetically confined high-temperature fusion plasmas. Multiple diagnostic instruments confirm wave excitation over a wide spatial range from the device size at the longest wavelengths down to the thermal ion Larmor radius. At the shortest scales, the poloidal wavelengths are comparable to the scale length of electrostatic drift wave turbulence. Theoretical analysis confirms a dominant interaction of the modes with particles in the thermal ion distribution traveling well below the Alfvén velocity.

Journal Article↗

PubMed2005

Role of bremsstrahlung radiation in limiting the energy of runaway electrons in tokamaks.

Bremsstrahlung radiation of runaway electrons is found to be an energy limit for runaway electrons in tokamaks. The minimum and maximum energy of runaway electron beams is shown to be limited by collisions and bremsstrahlung radiation, respectively. It is also found that a massive injection of a high-Z gas such as xenon can terminate a disruption-generated runaway current before the runaway electrons hit the walls.

Journal Article↗

PubMed2005

Measurement of turbulence decorrelation during transport barrier evolution in a high-temperature fusion plasma.

A low power polychromatic beam of microwaves is used to diagnose the behavior of turbulent fluctuations in the core of the JT-60U tokamak during the evolution of the internal transport barrier. A continuous reduction in the size of turbulent structures is observed concomitant with the reduction of the density scale length during the evolution of the internal transport barrier. The density correlation length decreases to the order of the ion gyroradius, in contrast with the much longer scale lengths observed earlier in the discharge, while the density fluctuation level remain similar to the level before transport barrier formation.

Journal Article↗

PubMed2004

Monitoring Alfvén cascades with interferometry on the JET Tokamak.

A microwave interferometry technique is applied for the first time for detecting a discrete spectrum of Alfvén cascade (AC) eigenmodes excited with fast ions in reversed magnetic shear plasmas of the Joint European Torus. The interferometry measurements of plasma density perturbations associated with ACs show an unprecedented frequency and time resolution superior to that obtained with external magnetic coils. The measurements of ACs are used for monitoring the evolution of the safety factor and density of rational magnetic surfaces in the region of maximum plasma current.

Journal Article↗

PubMed2004

Atomistic mechanisms for the (1 x 1) <==> hex surface phase transformations of Pt(100).

The atomistic mechanisms and dynamics of the (1 x 1) --> hex surface reconstruction of the clean Pt(100) and its lifting upon CO adsorption have been studied by means of both density functional theory and molecular dynamics simulations based on the modified embedded-atom method. It was found that during the surface reconstruction from the square (1 x 1) to the approximately 20% more dense (quasi)hexagonal phase, the required extra atoms are extracted from the second layer, leaving highly mobile subsurface vacancies, even in the presence of adatom islands or steps. These vacancies will, by surface diffusion, coalesce to form steps. In contrast, during the reverse process--where the hex surface reconstruction is lifted upon adsorption of CO--the roughly 20% excess surface atoms are in a collective manner ejected to form chains of adatoms. In turn, these adatoms coalesce into islands and steps by surface diffusion. A result of the two totally different mechanisms is that the reconstruction process can be concluded not to be the reverse of the deconstruction process.

Journal Article↗

PubMed2004

Observation of odd toroidal Alfvén eigenmodes.

Experimental evidence is presented for the existence of the theoretically predicted odd toroidicity induced Alfvén eigenmode (TAE) from the simultaneous appearance of odd and even TAEs in a normal shear discharge of the joint European torus. The modes are observed in low central magnetic shear plasmas created by injecting lower hybrid current drive. A fast ion population was created by applying ion cyclotron heating at the high-field side to excite the TAEs. The odd TAEs were identified from their frequency, mode number, and timing relative to the even TAEs.

Journal Article↗

PubMed2003

New interpretation of alpha-particle-driven instabilities in deuterium-tritium experiments on the Tokamak Fusion Test Reactor.

The original description of alpha particle driven instabilities in the Tokamak Fusion Test Reactor in terms of toroidal Alfvén eigenmodes (TAEs) remained inconsistent with three fundamental characteristics of the observations: (i) the variation of the mode frequency with toroidal mode number, (ii) the chirping of the mode frequency for a given toroidal mode number, and (iii) the antiballooning density perturbation of the modes. It is now shown that these characteristics can be explained by observing that cylindrical-like modes can exist in the weak magnetic shear region of the plasma that then make a transition to TAEs as the central safety factor decreases in time.

Journal Article↗

PubMed2003

Mechanism and dynamics of the CO-induced lifting of the Pt(100) surface reconstruction.

The first atomistic simulations of the CO-induced lifting of the Pt(100)-hex reconstruction have been performed. During this phase transformation the surface changes back to bulk-terminated Pt(100)-(1 x 1), whereby the surface atom density decreases by approximately 20%. The simulations reveal a mechanism collective in nature, indicating that restructuring proceeds through ejection of chains of Pt atoms. These chains explain the anisotropy as seen in scanning tunneling microscopy experiments. The restructuring rate depends nonlinearly on the CO coverage, but the absence of local clustering of CO excludes an explanation in terms of elementary reaction kinetics as proposed previously.

Journal Article↗

PubMed1998

Effects of early treatment on maxillary arch development in BCLP. A study on dental casts between 0 and 4 years of age.

The present investigation analyses longitudinally the effects of early orthopaedic and/or surgical treatment on maxillary alveolar arch development in 30 children with a complete bilateral cleft lip and palate (BCLP). Palatal arch dimensions were measured on dental casts and their growth velocities during different treatment periods were calculated. Differences in growth velocities between consecutive treatment periods were examined and tested statistically. Furthermore, growth velocities were compared with those of 80 non-cleft children. Before lip closure, growth of the intercanine width of children with a BCLP and non-cleft children was comparable. Only for arch length significantly was less growth observed in BCLP patients in comparison with the control group. After lip closure, intercanine width, arch length and segmental angle diminished. During the intersurgical period, arch form seemed to adapt to a new muscular balance. Immediately after soft palate surgery, growth of the intercanine width and intertuberosity width was restricted. This negative growth was compensated in the postsurgical period, where a catch-up growth of intertuberosity width was even observed.

Alveolar Process↗

PubMed1998

Maxillary arch dimensions in bilateral cleft lip and palate from birth until four years of age in boys.

OBJECTIVE: To describe the development of maxillary arch dimensions in children with bilateral cleft lip and palate (BCLP) during the first 4 years of life and to compare it with that in noncleft children. DESIGN: This was a retrospective, mixed-longitudinal study. SETTING: The study was conducted at the Cleft Palate Center of the University Hospital of Nijmegen. SUBJECTS: The sample consisted of 26 boys with BCLP who were born between 1976 and 1990 and treated at the University Hospital of Nijmegen. Data for a control group of 34 noncleft boys were collected at the University Hospital of Amsterdam. METHOD: Palatal arch dimensions were digitized on dental casts. A comparison between BCLP and noncleft dimensions was made at fixed time intervals. RESULTS: At birth, anterior and posterior arch widths as well as arch depths were significantly larger in children with BCLP. After 7 months (lip closure), anterior arch width and arch depth diminished considerably in the cleft group. After 12 months (palatoplasty), a slight decrease in posterior arch width was observed, and arch depths showed slight catch-up growth. At 4 years of age, anterior arch width was significantly narrower and anterior arch depth was shorter in children with BCLP than in control subjects. Posterior arch width was significantly wider. CONCLUSIONS: During the first 4 years of life, maxillary arch dimensions in children with BCLP show a unique development that is significantly different from that in noncleft children.

Cephalometry↗

PubMed1997

Incidence of dental caries in the primary dentition in children with a cleft lip and/or palate.

The incidence of dental caries in the primary dentition was determined in Dutch cleft lip and/or palate children (n = 81) and in children without a congenital malformation (n = 77). In the oral cleft group the incidence of dental caries (0.037; 95% CI 0.031-0.046) was significantly higher than in the control group (0.004; 95% CI 0.002-0.007) with a crude incidence rate ratio of 9.3. Adjusted for oral hygiene, oral cleft yielded a rate ratio (relative risk) of approximately 3.5 (95% CI 1.35-9.28) in the multivariate analysis. Dental caries occurred in 25 children with an oral cleft (30.9%) and in 5 control children (6.5%). All types of teeth were affected in the oral cleft group while in the control group dental caries only occurred in maxillary incisors and second molars. The highest incidence of dental caries was found in the teeth beside the oral cleft and in the primary molars of the oral cleft group.

Case-Control Studies↗

PubMed1996

Early palatal changes after initial palatal surgery in children with cleft lip and palate.

Early palatal growth and development after primary palatal closure was studied in children with different types of cleft lip and palate (CLP). Palatal dimensions were measured on dental casts taken at fixed ages, from 9 months to 4 years of age. The results showed that soft and hard palatal closure in one stage had a significant impeding influence on posterior sagittal palatal growth compared to closure of the soft palate only. Timing of surgery possibly had a small temporary restrictive effect on posterior transverse palatal growth and development. Type and severity of the oral cleft had a significant effect on transverse palatal development and anterior sagittal dimensions. Anterior arch width was reduced in children with a complete unilateral (U) CLP or bilateral (B) CLP. The palates of the latter children had consistently larger anterior arch depths. Compared to normal children, palates of cleft children changed anteriorly from wider at 9 months of age to narrower at 4 years of age. Arch depths were smaller in cleft children except for anterior arch depths in children with complete BCLP.

Age Factors↗

PubMed1996

Prediction of early palatal growth and development in children with cleft lip and palate.

With the use of multilevel modeling of longitudinal data, the relationship between the developmental status of the palate at 4 years of age and early palatal growth and development from birth to 1.5 years of age was studied. A model to predict palatal depths and widths at 4 years of age was developed and tested. The sample consisted of 130 children with cleft lip and/or palate. They were divided into three groups according to surgical treatment; (1) no palatal surgery, (2) soft palatal surgery, and (3) soft and hard palatal surgery. Results showed that in groups 1 and 2, the total arch depth at 4 years of age is strongly related to the initial arch depth at birth. No or low correlations were found between palatal depth at 4 years of age and the growth velocity at birth. The anterior arch width in groups 2 and 3 at 4 years of age appeared to be strongly related to the early growth velocity. In group 1, a moderate correlation with the palatal width and growth velocity at birth was observed. The results of the individual predictions of arch depths and arch widths were reasonable to fair. It was concluded that prediction of individual arch variables at 4 years of age is possible, although within limits. Clinically this means that extreme development can be discriminated from more average development.

Age Factors↗

PubMed1994

Palatal changes after lip surgery in different types of cleft lip and palate.

This study concerns palatal development during 6 months following primary lip closure. The sample consisted of 75 children with different forms of cleft lip and palate and 51 noncleft children. The palate was measured at 3 months of age, just before lip surgery, after surgery at 6 months, and again at 9 months of age. The results showed that lip closure has a strong effect in the anterior alveolar region. This effect was restricted to 3 months after surgery. The changes in complete clefts were more explicit than in incomplete cleft forms. Furthermore, the data showed that arch depth reduction due to lip surgery was compensated for by continued anteroposterior palatal growth. Early orthopedics appeared to prevent major palatal collapse immediately after lip surgery. Finally simultaneous closure of the alveolar cleft at the nasal side resulted in continued reduction of anterior cleft width.

Case-Control Studies↗

PubMed1993

Emergence of the deciduous canines and molars in CLP children.

The emergence of the deciduous canines and molars was investigated as part of a longitudinal study into the emergence of the deciduous dentition in Dutch cleft lip and/or palate children (CLP) (Kramer et al., 1989). The study comprised a group of 71 CLP children and 74 normal children that served as a control. Mean emergence ages of the deciduous upper and lower canines and molars were studied. Emergence of the deciduous primary molar on the cleft side was delayed 2 months in the upper jaw and 1 month in the lower jaw in children with an unilateral cleft lip and palate. The deciduous primary molar on the non-cleft side was not delayed as compared with normal children. Local anatomical relationships could be a possible explanation for these findings. The deciduous canines and secondary molars in both jaws showed no significant differences in mean emergence ages as compared with the control group, nor between the different cleft types. No sex differences were found in any of the groups. A table is presented containing the mean emergence ages of all the teeth of the deciduous dentition for the various CLP-groups, divided in cleft and non-cleft side, and the control group.

Analysis of Variance↗

PubMed1992

Early palatal changes in complete and incomplete cleft lip and/or palate.

Early palatal development in various complete and incomplete forms of cleft lip and/or palate (CLP) was studied from birth to 3 months of age by means of dental casts. Palatal morphology (shape) and dimensions--based on reproducible reference points--were determined in a group of 128 CLP children and 68 normal children who served as controls. Substantial normal palatal growth during the first 3 months of life was observed. Round arch forms changed into oval arch forms. Growth mainly takes place in the sagittal direction (+4 mm) (transverse: +1 mm). Palates of CLP children differed significantly dependent on the type of cleft and whether the cleft was complete or incomplete. Cleft lip and alveolus children and bilateral cleft lip and palate children had more elongated palatal arches, whereas unilateral cleft lip and palate children and cleft palate children had wider palatal arches than the control group. Incomplete clefts differed from the control group in the same direction as their complete cleft forms, though less distinctly. Preoperative orthopedics used in CLP patients does not stimulate growth. On the contrary, it even restricts growth.

Cleft Lip↗

PubMed1992

Water, water everywhere, but not a drop to wash in.

Could you continue operating under mandatory water restrictions? During a drought, textile rental operations can expect mandatory water cuts of 10% to 20% or more. If you're already running a tight ship and not wasting water, you may think you have no choice but to reduce the poundage of fabric processed. Not so. Here are some simple procedures to help you conserve water every day.

Conservation of Natural Resources↗