Machine Aperture Research Articles

Emittance exchange in electron booster synchrotron by coupling resonance crossing

The new generation of storage-ring based synchrotron-radiation light sources imposes challenges on off-axis injection schemes owing to small dynamic apertures. In many upgrade projects of existing light sources, including the Swiss Light Source (SLS) upgrade, it is planned to reuse their injector booster synchrotrons with little to no modifications, although their emittances may not be suitable for the upgraded storage ring. A reduction of injection beam emittance relaxes the requirements for injection devices and machine aperture. Since the beam emittance in an electron synchrotron is much smaller in the vertical plane than in the horizontal, where injections are normally implemented, a significant improvement can be made by reducing the horizontal emittance by exchanging the transverse emittances through coupling resonance crossing. We find a meaningful scaling parameter for the quality of the emittance exchange, showing the relationship between coupling strength and crossing speed. Radiative effects are added to investigate the consequences for electron synchrotrons. Finally, we present the first experimental demonstration of transverse emittance exchange in the SLS booster synchrotron.

Strip-line kicker and fast pulser R&D for the HEPS on-axis injection system

The HEPS plans to adopt on-axis injection scheme because the dynamic aperture of machine is not large enough for off-axis injection for its baseline 7BA lattice design. A couple sets of strip-line kicker and fast pulser of ±15 kV amplitude, 15 ns pulse bottom width are needed for bunch spacing of 10 ns to minimize perturbation on adjacent bunches. To achieve these requirement, a multifaceted R&D program including the strip-line kicker and HV pulser, was initiated last 2 years. So far, the prototype development of a 750 mm long strip-line kicker and a DSRD pulser was completed and the preliminary test results show they can meet the baseline requirement of the HEPS.

Simulation and optimization of beam losses during continuous transfer extraction at the CERN Proton Synchrotron

The proton beams used for the fixed target physics at the Super Proton Synchrotron (SPS) are extracted from the Proton Synchrotron (PS) by a multiturn technique called continuous transfer (CT). During the CT extraction, large losses are observed in locations where the machine aperture should be large enough to accommodate the circulating beam. This limits the maximum intensity deliverable due to the induced stray radiation outside the PS tunnel. Scattered particles from the interaction with the electrostatic septum are identified as the possible source of these losses. This article presents a detailed study aiming to understand the origin of losses and propose possible cures. The simulations could reproduce accurately the beam loss pattern measured in real machine operation and determine the beam shaving, intrinsic to the extraction process, as the cause for the unexpected losses. Since these losses are unavoidable, the proposed solution implies a new optics scheme displacing the losses to a region with better shielding. New simulations demonstrate the satisfactory performance of the new extraction optics and its suitability to be implemented in the machine. Finally, beam loss measurements in these new operation conditions confirmed the previous simulation results.

Volumetric modulated arc therapy for delivery of hypofractionated stereotactic lung radiotherapy: A dosimetric and treatment efficiency analysis

Purpose/objective(s) Volumetric modulated arc therapy (VMAT) allows for intensity-modulated radiation delivery during gantry rotation with dynamic MLC motion, variable dose rates and gantry speed modulation. We compared VMAT plans with 3D-CRT for hypofractionated lung radiotherapy. Materials/methods Twenty-one 3D-CRT plans for Stage IA lung cancer previously treated stereotactically were selected. VMAT plans were generated by optimizing machine aperture shape and radiation intensity at 10° intervals. A partial arc range of 180° was manually selected to coincide with tumor location. The arc was resampled down to 5° intervals to ensure dose calculation accuracy. Identical planning objectives were used for VMAT/3D-CRT. Parameters assessed included dose to PTV and organs-at-risk (OAR), monitor units, and multiple conformity and homogeneity indices. Plans were delivered to a phantom for time comparison. Results Lung V 20/12.5/10/5 were less with VMAT (relative reduction 4.5%, p = .02; 3.2%, p = .01; 2.6%, p = .01; 4.2%, p = .03, respectively). Mean/maximum-doses to PTV, dose to additional OARs, 95% isodose line conformity, and target volume homogeneity were equivalent. VMAT improved conformity at both the 80% (1.87 vs. 1.93, p = .08) and 50% isodose lines (5.19 vs. 5.65, p = .01). Treatment times were reduced significantly with VMAT (mean 6.1 vs. 11.9 min, p < .01). Conclusions Single arc VMAT planning achieves highly conformal dose distributions while controlling dose to critical structures, including significant reduction in lung dose volume parameters. Employing a VMAT technique decreases treatment times by 37–63%, reducing the chance of error introduced by intrafraction variation. The quality and efficiency of VMAT is ideally suited for stereotactic lung radiotherapy delivery.

Stability of the Horizontal Curvature of the LHC Cryodipoles During Cold Tests

The LHC will be composed of 1232 horizontally curved, 15 meter long, superconducting dipole magnets cooled at 1.9 K. They are supported within their vacuum vessel by three Glass Fiber Reinforced Epoxy (GFRE) support posts. Each cryodipole is individually cold tested at CERN before its installation and interconnection in the LHC 27 km-circumference tunnel. As the magnet geometry under cryogenic operation is extremely important for the LHC machine aperture, a new method has been developed at CERN in order to monitor the magnet curvature change between warm and cold states. It enabled us to conclude that there is no permanent horizontal curvature change of the LHC dipole magnet between warm and cold states, although a systematic horizontal transient deformation during cool-down was detected. This deformation generates loads in the dipole supporting system; further investigation permitted us to infer this behavior to the asymmetric thermal contraction of the rigid magnet thermal shield during cool-down. Controlling the helium flow rate in the thermal shield of the cryomagnet enabled us to reduce the maximal deformation by a factor of approximately two, thus increasing significantly the mechanical safety margin of the supporting system during the CERN cold tests

Collective Effects and the Design of the SSC

Evaluation of possible designs for the Superconducting Supercollider requires consideration of a variety of collective phenomena. Of particular interest is the scaling of current thresholds and growth rates with such parameters as machine circumference and aperture, magnetic field strength, momentum spread and acceptance, cell length, and phase advance. In this paper the interplay of collective effects and machine design is discussed with special attention to those issues which may be helpful in choosing a particular design.

Direct sagittal and coronal CT scanning—an economical light beam source for patient positioning

With certain sagittal and coronal CT scanning techniques (Anderson & Koehler, 1979; Bluemm, 1982; Veiga-Pires & Kaiser, 1982), the patient is positioned directly in the machine aperture with benches, headrests or handgrips for support. All modern CT scanners have a light beam system for patient positioning, but, in most cases the required slice is chosen outside the aperture and the couch moved a preset distance inwards.

Antiproton Yield Optimization in the CERN Antiproton Accumulator

The setting-up and optimization of antiprotons injected into the Antiproton Accumulator was initially based upon the measurement of circulating antiprotons by integration of their Schottky signals. This has now been supplemented by amplitude distribution measurements using an internal target. Also, the more readily detectable pions, muons and electrons, injected along with the antiprotons, give information on machine aperture and orbits, which can serve as a monitor of injection conditions during antiproton stacking.