Beam Symmetry Research Articles

Radiation From a Corrugated Circular Waveguide With a Flange

—Electromagnetic radiation from a corrugated waveguide with a flange is considered with the eigenvector expansion approach. The continuous and discrete eigenvectors are used to match the boundary conditions and a system of simultaneous equations for the discrete modal coefficients is formulated. A rigorous solution is presented in convergent series and its behavior is illustrated in terms of radiation pattern, beam axial symmetry, and return loss. The corrugation cut on the inner wall is found to focus an input beam into the central region of the waveguide, yielding desirable characteristics such as axial beam symmetry, low sidelobe level, and low cross-polarization level.

Improved delivery efficiency for step and shoot intensity modulated radiotherapy using a fast-tuning magnetron

The delivery efficiency of step and shoot intensity modulated radiotherapy (IMRT) has been improved by the installation of fast-tuning magnetrons into three travelling wave linear accelerators. The IMRT delivery efficiency and the beam start-up performance have been compared before and after installation. Start-up and inter sub-field times were reduced by an average of 3.0 s. A typical start-up time from depression of the start button to beam on is now around 4 s. Delivery efficiency for a variety of clinical and quality control prescriptions was improved by an average of 30.7% (range 7.4–60.9%), depending on a complex combination of the number of sub-fields, distance moved by leaves and dose rate. For the oldest accelerator (7 years old), dosimetric accuracy was significantly improved for low dose sub-fields. The dose output was within 2% for a 1 monitor unit (MU) sub-field and 1% for a 2 MU sub-field. The two newer accelerators displayed similar or better dose characteristics even before fast-tuning magnetron installation. Beam symmetries and flatnesses were acceptable at all energies and dose rates, and showed no obvious degradation in low dose sub-fields. It is recommended that fast-tuning magnetrons are adopted for accelerators of this design performing step and shoot IMRT.

Electromagnetic field in a slab of photonic crystal by BPM

The finite-difference vector beam propagation method is used to simulate propagation of Gaussian light beam in a finite slab of photonic crystal. A possibility to control light flow by the change of symmetry and polarisation of the incident light beam is demonstrated.

Monitor unit settings for intensity modulated beams delivered using a step‐and‐shoot approach

Two linear accelerators have been commissioned for delivering IMRT treatments using a step-and-shoot approach. To assess beam startup stability for 6 and 18 MV x-ray beams, dose delivered per monitor unit (MU), beam flatness, and beam symmetry were measured as a function of the total number of MU delivered at a clinical dose rate of 400 MU per minute. Relative to a 100 MU exposure, the dose delivered per MU by both linear accelerators was found to be within +/-2% for exposures larger than 4 MU. Beam flatness and symmetry also met accepted quality assurance standards for a minimum exposure of 4 MU. We have found that the performance of the two machines under study is well suited to the delivery of step-and-shoot IMRT. A system of dose calculation has also been commissioned for applying head scatter corrections to fields as small as 1x1 cm2. The accuracy and precision of the relative output calculations in water was validated for small fields and fields offset from the axis of collimator rotation. For both 6 and 18 MV x-ray beams, the dose per MU calculated in a water phantom agrees with measured data to within 1% on average, with a maximum deviation of 2.5%. The largest output factor discrepancies were seen when the actual radiation field size deviated from the set field size. The measured output in water can vary by as much 16% for 1x1 cm2 fields, when the measured field size deviates from the set field size by 2 mm. For a 1 mm deviation, this discrepancy was reduced to 8%. Steps should be taken to ensure collimator precision is tightly controlled when using such small fields. If this is not possible, very small fields should not contribute to a significant portion of the treatment, or uncertainties in the collimator position may effect the accuracy of the dose delivered.

Evaluation of a radiotherapy electron contamination deflecting system

The combination of a clinically useable neodymium iron boron rare earth magnetic deflector with a clinically viable thin walled helium bag which could be placed over a patients skin has shown that it could be very effective in reducing unwanted skin dose to radiotherapy patients by the removal of unwanted lepton contamination from the entry beam while not affecting dose at depth or beam symmetry. Results have shown that up to 70% of existing dose applied to the patients skin which causes damage could be removed by the tested devices. The magnetic deflectors characteristics for interference with linear accelerator electronic components and field strengths that a patient would be exposed to are also tested.

Performance and beam characteristics of the Siemens Primus linear accelerator.

Siemens Primus is a small footprint, klystron driven medical linear accelerator incorporating a compact solid state modulator. A double focused multileaf collimator (MLC) replaces the lower jaw. The first Primus in the world was installed at St. Jude Children's Research Hospital in early 1997 with x-ray energies of 6 and 15 MV and electron energies of 8, 10, 12, 15, 18, and 21 MeV. The 10 cm depth dose for a 100 cm SSD 10 X 10 cm2 beam is 68% and 77% for 6 and 15 MV x rays, respectively. For both x-ray energies, beam flatness is slightly better than the manufacturers specification of 3% and beam symmetry is considerably better than 1%. The double focus design of the MLC produces a sharp penumbra (5-7 mm at 6 MV and 6-8 mm at 15 MV), increasing modestly with beam size. MLC leaf leakage is less than 1.25%. The depths of the 80% depth dose for the six electron energies of 8, 10, 12, 15, 18, and 21 MeV are 2.6, 3.2, 4.0, 4.9, 6.0, and 7.4 cm, respectively. Beam flatness is typically 2%-3% for all electron energies except 21 MeV, where it reaches 4% for a 25 X 25 cm2 cone. Electron beam symmetry is better than 1% for all energies except 21 MeV, where it is equal to 1%. The results are stored electronically and may be retrieved using anonymous ftp from the American Institute of Physics, Physics Auxiliary Publication Service.

Application of a video-optical beam imaging system for quality assurance of medical accelerators

Method validation techniques were developed and experiments were carried out using a beam imaging system (BIS, Wellhöfer Dosimetrie, Schwarzenbruck, Germany) for routine quality assurance of medical accelerators. The routine quality assurance tasks include x-ray beam flatness and symmetry check, light/radiation field congruence test, beam energy constancy for electrons and mechanical checks for couch and collimator rotations. Comparisons were made between the BIS application and conventional quality assurance methods that use radiographic films or detector arrays. In this work, we have demonstrated efficiency and accuracy of the BIS to perform some of the routine quality assurance tasks for medical linear accelerators.

Space-charge-neutralized ion induction linacs with cusp insulation

The space charge of ions limits the current that can be accelerated in a conventional ion linear accelerator (linac). Furthermore, the accelerating electric field must be kept low enough to avoid the generation and acceleration of counterstreaming electrons. These limitations have resulted in heavy ion fusion accelerator designs that employ long beam lines and would be expensive to build. Space charge neutralization and magnetic insulation of the acceleration gaps could substantially reduce these two limitations, but at the expense of increasing the complexity of the beam physics. We consider it necessary to keep all structures out of the beam path to maintain beam symmetry and to avoid beam-induced outgassing. Using only field coils outside of the beam results in a cusp field to insulate the accelerating gaps and a solenoidal field between acceleration gaps, which provides a containment force. Electrons can enter the beam region along the magnetic field lines and provide fairly good space charge neutralization. We have performed three-dimensional particle-in-cell simulations, which indicate that electrons remain tied to magnetic field lines, despite the development of electromagnetic fluctuations. Thus, the center of the beam is not neutralized, resulting in a non-linear net focusing force. We have developed a simple model to determine the beam emittance induced by the non-linearity of the focussing force. We discuss the constraints this model puts on the choice of ion and the number of stages required for the accelerator.

New UCSF proton ocular beam facility at the Crocker Nuclear Laboratory Cyclotron (UC Davis)

A new facility has been constructed at the Crocker Nuclear Laboratory at University of California Davis for the purpose of treating ocular tumors using the 67.5 MeV protons from the 76-in. isochronous cyclotron. Beam line design, commissioning, control system, beam characteristics, dosimetry, patient positioner and system performance are discussed. The unmodulated Bragg peak has a penetration of 29 mm in tissue at the isocenter with a peak to plateau ratio of 3.8:1 and a width of 5 mm (FWHM measured in water). The delivered dose is monitored by two transmission ionization chambers which are calibrated against a thimble ionization chamber with an NIST-traceable 60Co calibration factor. The Bragg peak is spread across the target volume by the use of range modulators. The residual range is varied by means of a variable water column. Daily variation in patient dosimetry is within ±3%. The beam penumbra (defined here as the distance between the 90% and 10% isodose levels) is 1.5 mm for a range-modulated beam at a collimator-to-isocenter distance of 50 mm. The beam flatness in a 25 mm diameter beam is within ±2% and the beam symmetry is ±1%. In the first 18 months, 50 patients have been treated with an average field size of 16.8 × 16.6 mm 2. The residual range varied between 13.0 mm to 29.2 mm with an average value of 22.4 mm, and the range modulation varied between 16 mm to 24 mm with an average value of 20 mm. The tumor thickness (height) ranged between 1.2 to 11.5 mm with mean of 5.2 mm. The age of the patients ranged from 25 to 88 yr.

Flatness characteristics for diagonal scans from Varian and Siemens linear accelerators

The advent of 3D treatment planning systems whose algorithms utilize diagonal scan data to perform dose calculations has made the collection of diagonal profile data essential. Manufacturers' specifications (MS) on beam flatness and symmetry apply to both the radial and transverse axes of all square field sizes from 10 × 10 cm 2 to the largest field available. Beam profile measurements were obtained for both diagonal axes over a range of field sizes and depths for two units, a Varian 2100C and a Siemens KD. In this note the International Electrotechnical Commission (IEC) flatness definition was used to characterize the diagonal flatness of each beam.

Quantitative assessment of beam perturbations caused by silicon diodes used for in vivo dosimetry

Purpose : To measure the effect of silicon diode detectors used for a in vivo dosimetry on beam characteristics and determine whether this effect is clinically significant. Methods and Materials : The beam characteristics were measured for 6- and 10-MV photon and 6–20 MeV electron energies from a Varian Clinac 1800 medical linear accelerator. Water was used for the medium, and measurements were made for various clinically common field sizes and depths. Results : Beam attenuations along the central axis were 10 and 7.5% for 6- and 10-MV photons respectively. Electron beam dose reductions were between 13 and 25% for 20-6-MeV electrons. Photon beam flatness varied up to 7% at different depths, but the symmetry was not affected much. Electron beam flatness and symmetry were siginificantly changed to as much as 18 and 6%, respectively. Conclusion : Use of diode detectors on central axis of photon and electron beams for in vivo dosimetry causes significant attenuation and alteration of the beam characteristics. The percentage of the volume affected is significant (e.g., 23% of the volume in a 4 × 4 field gets 10% less dose for a 6-MV photon beam), especially if these diodes are used for in vivo dosimetry on the central axis every day for every treatment, as is done in some clinics. Other beam parameters such as penumbra and skin dose are also affected. It is therefore recommended that the diodes be used only as needed.

Respiration gated radiotherapy treatment: a technical study

In order to optimize external-beam conformal radiotherapy, patient movement during treatment must be minimized. For treatment on the upper torso, the target organs are known to move substantially due to patient respiration. This paper deals with the technical aspects of gating the radiotherapy beam synchronously with respiration: the optimal respiration monitoring system, measurements of organ displacement and linear accelerator gating. Several respiration sensors including a thermistor, a thermocouple, a strain gauge and a pneumotachograph were examined to find the optimal sensor. The magnitude of breast, chest wall and lung motion were determined using playback of fluoroscopic x-ray images recorded on a VCR during routine radiotherapy simulation. Total dose, beam symmetry and beam uniformity were examined to determine any effects on the Varian 2100C linear accelerator due to gating.

Target diagnostics for intense lithium ion hohlraum experiments on Particle Beam Fusion Accelerator II

A review of the diagnostics used at Sandia National Laboratories to measure the parameters of intense lithium ion-beam hohlraum target experiments on Particle Beam Fusion Accelerator II will be presented. This diagnostic package contains an extensive suite of x-ray spectral and imaging diagnostics that enable measurements of target temperature and x-ray output. The x-ray diagnostics include time-integrated and time-resolved pinhole cameras, energy-resolved one-dimensional streaked imaging diagnostics, time-integrated and time-resolved grazing incidence spectrographs, a transmission grating spectrograph, an elliptical crystal spectrograph, a bolometer array, an 11- element x-ray diode array, and an 11-element PIN diode detector array. The incident Li beam symmetry and an estimate of incident Li beam power density can be measured from ion beam-induced characteristic x-ray line emission and neutron emission.

Bessel function modes and O(2)-symmetry breaking in diffractive optical pattern formation processes

Spontaneous two-dimensional transverse optical pattern formation is investigated in a single feedback mirror experiment using nematic liquid crystals as nonlinear optical media. It is shown that instead of theoretically predicted hexagons assuming plane wave input waves, the observed patterns are more correctly represented by spatial Bessel function modes if the symmetry of finite size laser beams is taken into account.

Beam symmetry preservation through a dipole with zero pole face rotations and uniform field

It is shown that the cylindrical symmetry of a charged particle beam can be preserved precisely while it is bent through a single dipole with zero pole face rotations and uniform field, although in one plane the beam is freely drifting whereas in the other plane the beam is focused. The beam conditions at the entrance to the dipole are derived and cast into two analytic formulas. It is shown that symmetry preservation calls for matched beam optical functions (β,α), that is, matched orientation and amplitude of the beam phase space ellipses into the dipole. The beam conditioning at the entrance to the dipole is discussed and formulated. Some illustrative cases are calculated, and an example design is presented. Finally, it is proved that such a symmetric matched beam may be defined for any system with equal diagonal R-matrix elements, and the results are applied to another specific case of a parallel sided magnet.

Transverse Patterns in Optical Reorientation of Nematic Liquid Crystals with a Single Feedback Mirror

Abstract Spontaneous transverse pattern formation is investigated in optical reorientation of nematic liquid crystals with a single feedback mirror experiment. The influence of the distance between the optically nonlinear nematic film and the mirror, the input beam symmetry and intensity upon the patterns is shown.

Ignition and high gain with ultrapowerful lasers*

Ultrahigh intensity lasers can potentially be used in conjunction with conventional fusion lasers to ignite inertial confinement fusion (ICF) capsules with a total energy of a few tens of kilojoules of laser light, and can possibly lead to high gain with as little as 100 kJ. A scheme is proposed with three phases. First, a capsule is imploded as in the conventional approach to inertial fusion to assemble a high-density fuel configuration. Second, a hole is bored through the capsule corona composed of ablated material, as the critical density is pushed close to the high-density core of the capsule by the ponderomotive force associated with high-intensity laser light. Finally, the fuel is ignited by suprathermal electrons, produced in the high-intensity laser–plasma interactions, which then propagate from critical density to this high-density core. This new scheme also drastically reduces the difficulty of the implosion, and thereby allows lower quality fabrication and less stringent beam quality and symmetry requirements from the implosion driver. The difficulty of the fusion scheme is transferred to the technological difficulty of producing the ultrahigh-intensity laser and of transporting this energy to the fuel.

Band-structure engineering in strained semiconductor lasers

The influence of both compressive and tensile strain on semiconductor lasers and optical amplifiers is reevaluated in the light of recent experimental and theoretical work. Strain reduces the three-dimensional symmetry of the lattice and helps match the wave functions of the holes to the one-dimensional symmetry of the laser beam. It can also decrease the density of states at the valence band maximum and so reduce the carrier density required to reach threshold. These two effects appear to adequately explain the TE and TM gain in compressive and tensile structures, including polarization-independent amplifiers, the behavior of visible lasers and the improved frequency characteristics of InGaAs/GaAs lasers. In 1.5 /spl mu/m InGaAsP/InP lasers phonon-assisted Auger recombination appears to remain the dominant current path and can explain why the temperature sensitivity parameter to remains >

Dependence of Bessel Beam Characteristics on Angular Spectrum Phase Variations

Abstract The Bessel beam propagation characteristics are investigated in the presence of azimuthally dependent phase variations of its angular components. The phase variations influence the beam symmetry and strongly reduce an axial intensity. An experimental interpretation of presented results is also discussed.

Twin-fiber interferometric method for measuring spatial coherence

A new method for measuring spatial coherence in which optical fibers replace the traditional slits in Young's experiment is developed. The beam is sampled by the fiber ends, and the phase difference that causes the fringes is introduced by translating one fiber along its long axis by a few wavelengths. The technique has several advantages over the slit method: The fibers can be moved to arbitrarily chosen points, as opposed to removing and replacing sets of slits, giving greater flexibility in measurement as well as easing alignment problems. This method also does not rely on diffraction, and therefore enables measurement of much dimmer beams and dimmer points on beams. One can take as many data points as desired, thus tracing the shape of the coherence function accurately, whereas with slits one is restricted to the number of slit sets available. Finally, one does not have to assume beam symmetry. Although the magnitude of the spatial coherence function is not accurately reproduced, this can be corrected for, and the ability to reproduce the shape of the beam remains a useful measurement tool. We report, in particular, measurements of spatial coherence of a single-stripe diode laser, as well as a HeNe laser.