Accurate Shaping Research Articles

A 12-Pulse AC–DC Rectifier With High-Quality Input/Output Waveforms

This paper describes an ac-dc rectifier, which draws sinusoidal current waveforms from the utility. Also, it can supply either a ripple-free current or voltage to the dc load by using a dc filter. The technology involves an accurate shaping of the dc current by using two forced-commutated switches (IGBTs). The shaping of the dc current is reflected back into the shaping of the input currents, which become pure sine waves. The resulting system is reliable and able to handle rapid load variations with a simple and robust control system. It also operates under rectification and inversion. Detailed analysis and a complete set of experimental results from a 380-V 20-kVA rectifier system are provided.

Volume definition in radiotherapy planning for lung cancer: how the radiologist can help

Effective treatment for carcinoma of the lung remains one of the biggest challenges in oncology. Radical radiotherapy may be a curative option for patients who are unsuitable for radical surgery either because of disease stage or because of co-morbidity. Long-term disease control with radical radiotherapy is disappointing with only about 6% of patients treated being alive at 5 years . Technological advances involved in the planning and delivery of radiotherapy may improve this. The advent of conformal radiotherapy, utilizing computed tomography and three-dimensional planning systems, allows much more accurate shaping of the radiation fields. This greater accuracy of target volume definition facilitates a reduction in the radiation dose to normal tissues, allowing for dose escalation to the tumour. Delineation of the target volume can be problematic. Conventional CT has limitations in term of distinguishing between benign and malignant tissues, e.g. the size criteria for involved lymph nodes. The oncologist uses a combination of radiological and clinical information when defining the target volume but their radiological interpretation of imaging is inferior to that of a radiologist. The Royal College of Radiologists (RCR) issued guidance in 2004 on the optimal imaging strategies for common cancers. These guidelines address issues regarding the localisation and staging of cancers and treatment planning, and also reporting and training. They recommend the development of closer links between radiologists and oncologists to optimise the interpretation of imaging and target volume definition. This article aims to briefly explain the planning process involved in irradiating lung cancers, highlight problematic areas and suggest ways in which co-operation with radiologists may improve the delivery of radiotherapy and therefore the treatment outcomes for this group of patients.

Leaky-bucket shaper design based on time interval grouping

A new shaper design based on the concept of time interval grouping, which targets accurate shaping for large numbers of variable length packet flows is presented in this letter. The proposed design can implement leaky bucket shaping with low complexity and very high speed of operation, features highly desirable in high-speed networking applications. Its performance, as shown by computer simulations, combines high link bandwidth utilization with adequate accuracy in the control of the produced traffic profile compared to other solutions published in the literature.

Leaky-bucket shaper design based on time interval grouping

A new shaper design based on the concept of time interval grouping, which targets accurate shaping for large numbers of variable length packet flows is presented in this letter. The proposed design can implement leaky bucket shaping with low complexity and very high speed of operation, features highly desirable in high-speed networking applications. Its performance, as shown by computer simulations, combines high link bandwidth utilization with adequate accuracy in the control of the produced traffic profile compared to other solutions published in the literature.

Feedback Control for Accurate Shaping of Ultrashort Optical Pulses prior to Chirped Pulse Amplification

Through experiments, we demonstrate accurate pulse shaping of femtosecond optical pulses in both amplitude and phase, prior to chirped pulse amplification. The nonlinear transfer function of the amplifier was compensated with feedback control referring to the amplified pulse shape measured by frequency resolved optical gating (FROG) or temporal analysis, by dispersing a pair of light e fields (TADPOLE). In FROG-referring control, the pulse shaper is adaptively controlled using a control index calculated from the two-dimensional FROG trace image, while in TADPOLE-referring control, the shaping phase-mask is corrected only a few times using the reconstructed spectrum phase of the output pulse. The performance and limitations of pre-shaping systems are discussed by comparing two pulse shaper devices, namely a liquid-crystal spatial light modulator and an acoustic optical phase dispersion filter, two pulse measurement techniques, and two feedback control schemes.

Compensation for a Transfer Function of a Regenerative Amplifier to Generate Accurately Shaped Ultrashort Pulses in Both the Amplitude and Phase

We demonstrated accurate pulse shaping of amplified femtosecond optical pulses by modulating the amplitude and phase prior to regenerative chirped-pulse amplification. We measured the amplitude and phase of amplified pulses using a spectral interferometer with a well-defined reference pulse. Based on these measurements, the pulse shaper was adjusted only a few times to compensate for a transfer function of the amplifier consisting of the gain narrowing and self-phase modulation.

Adaptive pulse shaping of phase and amplitude of an amplified femtosecond pulse laser by direct reference to frequency-resolved optical gating traces

By adaptive control of both spectral phase and amplitude of seed pulses with an acousto-optic programmable dispersion filter located before a pulse stretcher, accurate pulse shaping was achieved for ultrashort laser pulses delivered through a regenerative amplifier. Two-dimensional patterns obtained from frequency-resolved optical gating were used for calculating cost functions in an adaptive control algorithm without pulse shape reconstruction during the control of pulse shape.

A soft-switched current-controlled converter for induction machine drives

The paper proposes a current controlled inverter operating in zero voltage switching (ZVS) mode for an induction machine drive. Operation with no voltage stress in the DC link bus is achieved. Together with the soft switching operation, a fixed frequency bang-bang current control technique is also implemented to allow for an accurate shaping of sinusoidal currents to feed the motor. As a result, a ripple free torque profile in steady state operation is achieved. With the soft switching technique it is possible to operate conventional IGBTs at 40 kHz. A detailed analysis of the circuit operation is presented. The feasibility of the proposed scheme is experimentally verified on a prototype.

Design of a command-triggered plasma opening switch for terawatt applications

Inductive energy storage systems can have high-energy density, leading to smaller, less-expensive systems. The crucial element of an inductive energy storage system is the opening switch. This switch must conduct current while energy is stored in an inductor, then open quickly to transfer this energy to a load. Plasma can perform this function. The plasma opening switch (POS) has been studied for more than two decades. Though advances have been made, success with the conventional plasma opening switch has been limited. The plasma opening switch concept described here has the potential to increase the power achieved with inductive energy storage pulsed-power systems. This system is also command-triggered. Major improvements in performance and command-triggering both require control of the plasma conductor. This paper shows the design of a system that uses magnetic fields to control the plasma. An externally applied magnetic field clears plasma from the transmission line, For maximum power, efficiency, and accurate pulse shaping, the plasma must be cleared from most of the transmission line conductor spacing. The goal of the system described here is to clear more than 60% of the gap. This is much more than present performance levels. This paper discusses the operation of plasma opening switches, and the ways this switch design overcomes some of the limitations. Because of the importance of magnetic field penetration into the plasma, a model for field penetration is presented. The design of a terawatt-level system is shown, and the mechanism to reduce the trigger power is described. Particle-in-cell simulations of the triggered opening are shown.

Modulated beam conformal therapy for head and neck tumors

Purpose : The goal of modulated-beam conformal therapy is to reduce the dose to healthy tissue and sensitive structures around a uniformly irradiated target volume. Multiple intensity-modulated fields offer improved tissuesparing dose distributions. New computer-based systems for planning and delivering such treatments may soon be available from different commercial sources that will make the formulation of an intensity-modulated treatment plan and its execution widely available at any treatment facility that has the resources to acquire the necessary equipment. This work reports on a study of the integration of two such systems. Methods and Materials : Treatment planning was done using a commercially available inverse planning algorithm based on simulated annealing. The plans arbitrarily assumed nine coplanar x-ray beams at nonopposed gantry angles. Intensity modulation was computed for each beam. The modulated field at each gantry angle was broken down into a series of uniform (nonmodulated) subfields, which could be delivered as a sequence to produce the desired dose distribution. Because a large number of subfields was delivered, a multileaf collimator (MLC) was used for field shaping. This allowed rapid and accurate field shaping for treatments made up of several hundred sublields. Computer control of the MLC and linear accelerator allowed delivery of d less than .01 Gy per subfield. Treatment was delivered on a prototype, computer-controlled accelerator and MLC system. Resulting dose distributions were analyzed using film and an anatomically specific, homogeneous phantom. Results : The treatment plans were evaluated using dose-volume histogram analysis. The plans provided acceptably uniform irradiation of the target volume without exceeding dose tolerances for nearby critical structures. The plans were successfully delivered by a prototype dynamic MLC. The time needed to deliver a sequence of subfields at one gantry angle ranged from 0.7 to 2.0 min. Isodoses from film agreed reasonably well with planned isodose distributions. Conclusions : It is feasible to plan and deliver fixed gantry, modulated-beam conformal therapy for head and neck tumors with systems being developed commercially. The planned dose distributions exhibit significant potential for sparing closely spaced normal tissue structures in the head and neck.

Refinements in mandible reconstruction

The basic approach to free flap mandible reconstruction is well established. Superior functional and aesthetic results are possible with appropriate preoperative planning and meticulous attention to detail during execution of the procedure. The most important factors under the surgeon's control that contribute to a high-quality result are accurate graft shaping and insetting, proper donor site selection, optimal incision planning, and a simplified approach to performing the microvascular anastomoses.

Controlled bending of cranial bone grafts: a simple surgical technique

The cranium serves as a primary bone graft donor site in reconstructive craniofacial surgery. However, the mechanical properties of cranial bone predispose to fragmentation when excessive bending forces are applied, or to a return to the original shape when bending forces are insufficient. The precise and sustained contouring of cranial bone therefore remains a practical clinical problem. This paper describes a simple surgical technique which relies upon the application of a miniplate to the cortical surface of the cranial bone segment prior to bending. The miniplate serves to reinforce the segment, allowing accurate shaping of cranial bone and maintenance of the newly acquired contour.

Fabrication of an unusual mosaic lens

The design philosophy and the construction techniques to make a novel mosaic lens to collect Čerenkov radiation are described. The lens blanks were rough cut into segments by a water jet. The segments were cut to the final inner and outer diameters by ultrasonically assisted machining, an unusual new technique developed at the Los Alamos National Laboratory. The technology will soon be available to the public through Sonic Mill, Inc., 3820 Academy Parkway North, NE, Albuquerque, N.M. 87109. The advantage of ultrasonically assisted machining lies in rapid, accurate shaping of brittle materials such as ceramics and glass, which can, by this technique, be shaped about as fast as aluminum.

Vacu‐Blast ‘High‐Tech Hammer’ Shapes Ariane Body Panels

THE Belgian aerospace company, SABCA, which manufactures a number of components under licence for Aerospatiale's ‘Ariane’ programme, is using a special peen‐forming technique for the accurate shaping of the external housings of the liquid fuel boosters (P.A.L.) fitted to Ariane IV.

Contouring of a solid silicone block: a new use for the Shaw scalpel.

The use of the Shaw hemostatic scalpel eliminates the frustration and physical fatigue formerly associated with the carving of solid silicone implants from blocks of silicone of all grades. It enables very accurate shaping and saves operating and anesthesia time. The smooth end product may, in many cases, eliminate the need for factory-made implants. This method of shaping silicone block is highly recommended. It is not suggested that the instrument should be purchased for this purpose alone, but if purchase of the scalpel is being considered for other purposes, this may be a significant factor in the decision.

Synthetics in Head and Neck Surgery: With Particular Reference to Silicone

I PRESENT this entire subject with slight misgivings in that we have long been advocates of the use of autogenous implants. These implants remain the most physiologically acceptable but are not without shortcomings in their own right, particularly in regard to availability, difficulty in accurate shaping and absorption or resorption by the body. I am on record as recently as 1963 as expressing concern about the use of the new synthetic materials, particularly as only time could prove the final outcome of their use and acceptance by the body. In the last 21/2 years we have used silicone in various forms with considerable caution and some precautions which I would like to include in this presentation. In the statement made in 1963 referred to above I further stated, My wide experience with synthetic implant materials is in the removal rather than the insertion of these implants. I have had a