Transverse Beam Position Research Articles

Monitoring Carbon Ion Beams Transverse Position Detecting Charged Secondary Fragments: Results From Patient Treatment Performed at CNAO.

Particle therapy in which deep seated tumours are treated using 12C ions (Carbon Ions RadioTherapy or CIRT) exploits the high conformity in the dose release, the high relative biological effectiveness and low oxygen enhancement ratio of such projectiles. The advantages of CIRT are driving a rapid increase in the number of centres that are trying to implement such technique. To fully profit from the ballistic precision achievable in delivering the dose to the target volume an online range verification system would be needed, but currently missing. The 12C ions beams range could only be monitored by looking at the secondary radiation emitted by the primary beam interaction with the patient tissues and no technical solution capable of the needed precision has been adopted in the clinical centres yet. The detection of charged secondary fragments, mainly protons, emitted by the patient is a promising approach, and is currently being explored in clinical trials at CNAO. Charged particles are easy to detect and can be back-tracked to the emission point with high efficiency in an almost background-free environment. These fragments are the product of projectiles fragmentation, and are hence mainly produced along the beam path inside the patient. This experimental signature can be used to monitor the beam position in the plane orthogonal to its flight direction, providing an online feedback to the beam transverse position monitor chambers used in the clinical centres. This information could be used to cross-check, validate and calibrate, whenever needed, the information provided by the ion chambers already implemented in most clinical centres as beam control detectors. In this paper we study the feasibility of such strategy in the clinical routine, analysing the data collected during the clinical trial performed at the CNAO facility on patients treated using 12C ions and monitored using the Dose Profiler (DP) detector developed within the INSIDE project. On the basis of the data collected monitoring three patients, the technique potential and limitations will be discussed.

Optical Modulation of Electron Beams in Free Space.

We exploit free-space interactions between electron beams and tailored light fields to imprint on-demand phase profiles on the electron wave functions. Through rigorous semiclassical theory involving a quantum description of the electrons, we show that monochromatic optical fields focused in vacuum can be used to correct electron beam aberrations and produce selected focal shapes. Stimulated elastic Compton scattering is exploited to imprint the required electron phase, which is proportional to the integral of the optical field intensity along the electron path and depends on the transverse beam position. The required light intensities are attainable in currently available ultrafast electron microscope setups, thus opening the field of free-space optical manipulation of electron beams.

Towards zero radiation isocentre size: minimising radiation beam isocentricity on Elekta linear accelerators by means of optimising look-up tables.

The most important geometric characteristics of SRS/SBRT treatments are precise target localisation and precise aiming of the radiation beam at the target. The AAPM-RSS Medical Physics Practice Guideline 9.a. for SRS/SBRT recommends that the radiation isocentricity (i.e. beam deviation from the isocentre) should not exceed 1mm for SRS and 1.5mm for SBRT. Minimising the beam deviations from the treatment target, largely due to the gantry sag, can improve the accuracy of radiosurgery and stereotactic treatments and commonly beam steering parameters are optimised to achieve this objective. This study aims to investigate, as a proof of concept, if it is possible to eliminate the beam deviations on Elekta linear accelerators altogether by optimising gantry angle dependent beam steering parameters, as stored in look-up tables. The investigation used the EPID-based Winston-Lutz test at 13 gantry angles separated every 30° (from - 180° to + 180°). Elekta linacs have two look-up tables that can be customised explicitly for radial beam angle and transverse beam position. Modifications of the radial look-up table were limited by the radial beam asymmetry inhibit of more than 5%, as measured by the linac in-built ionisation chamber. Therefore, only small radial beam deviation reductions of 0.1mm were achieved (on average from 0.37 to 0.26mm) while radial beam symmetry changed significantly by up to ± 7%, depending on the gantry angle as measured by the IC Profiler™. The optimised transverse look-up table resulted in reduction of transverse beam deviations to almost zero (on average from 0.20 to 0.03mm), however, that changed the transverse beam symmetry by almost a constant value of 1%, as measured by the IC Profiler™. Ideally, two additional look-up tables are needed for effective beam steering, one for radial beam position and one for transverse beam angle. Four look-up tables in total would enable customising beam centre position and beam symmetry at any gantry angle that would minimize radiation isocentre size without compromising beam symmetry.

Design and fabrication of stripline BPM for the ESS MEBT

Beam Position Monitors (BPM) are non-destructive diagnostics devices used in particle accelerators to measure the transverse beam position (beam center of mass) and phase of the accelerated beam. There are various ways of realization of the BPMs in accelerators having different measurement requirements, particle type or energy. Several types of BPM pickups have been studied including the button type, grounded striplines and matched striplines. The choice and final design is based on a matched stripline to accommodate the signal transmission to electronics and provide relatively higher signal level for low velocity (β=0.088) proton beam within MEBT. Due to mechanical space limits, all the BPMs are embedded inside quadrupoles; hence it is required to use non-magnetic materials in the BPM sensor, in particular the feedthroughs. The BPM sensor is composed of several components, which are separately machined with tolerances of 10μm, and then welded together by the use of electron beam welding. The analysis show for long bunches, the multi bunch transfer impedance is different than of single bunch due to bunch signals overlapping. The design, fabrication and tests of the BPM sensors were performed at ESS-Bilbao and their installation and the electrical checks at ESS-Lund. Prior to installation in the ESS accelerator tunnel, one BPM stripline was installed and the BPM system tested with the CEA-IPHI beam. This paper gives an overview of the electromagnetic and mechanical design and the fabrication of BPM striplines. Furthermore, the results of the RF measurements are compared with the simulations.

A scintillator-based range telescope for particle therapy

The commissioning and operation of a particle therapy centre requires an extensive set of detectors for measuring various parameters of the treatment beam. Among the key devices are detectors for beam range quality assurance. In this work, a novel range telescope based on a plastic scintillator and read out by a large-scale CMOS sensor is presented. The detector is made of a stack of 49 plastic scintillator sheets with a thickness of 2–3 mm and an active area of 100 × 100 mm2, resulting in a total physical stack thickness of 124.2 mm. This compact design avoids optical artefacts that are common in other scintillation detectors. The range of a proton beam is reconstructed using a novel Bragg curve model that incorporates scintillator quenching effects. Measurements to characterise the performance of the detector were carried out at the Heidelberger Ionenstrahl-Therapiezentrum (HIT, Heidelberg, GER) and the Clatterbridge Cancer Centre (CCC, Bebington, UK). The maximum difference between the measured range and the reference range was found to be 0.41 mm at a proton beam range of 310 mm and was dominated by detector alignment uncertainties. With the new detector prototype, the water-equivalent thickness of PMMA degrader blocks has been reconstructed within ± 0.1 mm. An evaluation of the radiation hardness proves that the range reconstruction algorithm is robust following the deposition of 6,300 Gy peak dose into the detector. Furthermore, small variations in the beam spot size and transverse beam position are shown to have a negligible effect on the range reconstruction accuracy. The potential for range measurements of ion beams is also investigated.

Study on the influence of beam transverse position on the cavity bunch length measurement

Monopole modes in the resonant cavities are wildly used to obtain the beam current and the bunch length, while dipole modes are used to measure the beam transverse position. It is generally recognized that the monopole modes are independent of the beam transverse offset. In this paper, the influence of beam transverse offset on the bunch length measurement using monopole modes is analyzed. The simulation results show that the relative error of the bunch length measurement is less than 1 % when the beam offset is within 1 mm.

Transverse diagnostics based on dipole mode signal fitting method in TESLA-type accelerating cavities at the free-electron laser FLASH

TESLA cavities are used at the free-electron laser in Hamburg (FLASH) to accelerate electron bunches for generating intense free-electron laser beams. Two specially designed couplers mounted at both ends of each cavity damp the higher order modes (HOMs) excited by the beam in these cavities. By using a specific dipole mode signal extracted from these couplers, one can determine the transverse beam position as well as the dipole mode polarizations and center in each cavity. We introduce a new method based on fitting the dipole mode signal to correlate the dipole mode amplitude and phase to the measured beam position. With this method, the TESLA cavities can be used as HOM-based beam position monitors (HOMBPMs), delivering consistent results over several months, with a resolution better than $10\text{ }\text{ }\ensuremath{\mu}\mathrm{m}\text{ }$ RMS. We implemented this method for a whole accelerating module for beam orbit monitoring. The method also delivers the polarization axes and the center of the dipole mode for all eight cavities in this module. Our measurements show that the cavities did not move inside the module within almost one year.

Design and simulation of cavity BPM for BEPCII

In the linac of Beijing Electron Positron Collider II (BEPCII), the resolution of existing BPMs is several hundred microns. But the requirement of transverse position resolution of BPM for future top-up injection needs to be more precise. This paper is mainly concerned about the design and simulation of the cavity beam position monitor (CBPM) for BEPCII linac and free electron laser (FEL). The requirement of beam transverse position resolution for linac is several microns, even a few nanometers for FEL. When the beam bunch goes through the CBPM, it can excite different electromagnetic field modes in resonator. In these modes, TM110 is the most sensitive to beam transverse position offset. The design of CBPM is aim to analyze the relationship between TM110 and beam transverse position by CST Studio and Ansys HFSS. In this research, we design a pair of rectangular slots for position cavity to revise the direction of electromagnetic field. The simulation result of theoretical resolution is able to reach 15 nm@1nC with the operating frequency 2500 MHz. It is obviously that CBPM has more excellent resolution than all of operating BPM in BEPCII. Meanwhile, the research of CBPM improves the application of beam detection technology in BEPCII and accumulates technology for future design of HEPS and FEL. The mechanical design of the prototype is being fabricated.

Development of compact highly sensitive beam position monitors for storage rings

The history of the search for electric dipole moment (EDM)takes us back in time for more than five decades. EDMs have gained interest due to the fact that they violate the CP symmetry. Hence, they potentially provide additional sources of CP violation and possibly contribute to the resolution of one of the greatest puzzles of cosmology; mainly to explain the matter abundance in the universe. The JEDI collaboration is currently preparing for measuring the Deuteron EDM in the COoler SYnchrotron (COSY). One of the major challenges is the precise knowledge about the beam position along the storage ring. Transverse beam positions largely control the systematic errors. Thus the development of compact and highly sensitive Beam Position Monitors (BPMs) is particularly important for precision experiment like the EDM searches. This paper describes the development of the latest set of Rogowski coils as compact beam position monitors. It also describes some of the preliminary results towards optimizing the sensitivity of these coils by controlling the noise sources and approaching improved signal to noise ratio (SNR).

Detuning related coupler kick variation of a superconducting nine-cell 1.3 GHz cavity

Superconducting TESLA-type cavities are widely used to accelerate electrons in long bunch trains, such as in high repetition rate free electron lasers. The TESLA cavity is equipped with two higher order mode couplers and a fundamental power coupler (FPC), which break the axial symmetry of the cavity. The passing electrons therefore experience axially asymmetrical coupler kicks, which depend on the transverse beam position at the couplers and the rf phase. The resulting emittance dilution has been studied in detail in the literature. However, the kick induced by the FPC depends explicitly on the ratio of the forward to the backward traveling waves at the coupler, which has received little attention. The intention of this paper is to present the concept of discrete coupler kicks with a novel approach of separating the field disturbances related to the standing wave and a reflection dependent part. Particular attention is directed to the role of the penetration depth of the FPC antenna, which determines the loaded quality factor of the cavity. The developed beam transport model is compared to dedicated experiments at FLASH and European XFEL. Both the observed transverse coupling and detuning related coupler kick variations are in good agreement with the model. Finally, the expected trajectory variations due to coupler kick variations at European XFEL are investigated and results of numerical studies are presented.

Imaging the LHC beams with silicon and scintillating fibre vertex detectors

The LHCb Vertex Locator (VELO) is used to reconstruct beam–gas interaction vertices which allows one to obtain precise profiles of the LHC beams. In LHCb, this information is combined with the profile of the reconstructed beam–beam collisions and with the LHC beam currents to perform precise measurements of the luminosity. This beam–gas imaging (BGI) method also allows one to study the transverse beam shapes, beam positions and angles in real time. Therefore, a demonstrator beam–gas vertex detector (BGV) based on scintillating fibre modules has been built and installed in LHC Ring 2 at point 4.

Long bunch trains measured using a prototype cavity beam position monitor for the Compact Linear Collider

The Compact Linear Collider (CLIC) requires beam position monitors (BPMs) with 50 nm spatial resolution for alignment of the beam line elements in the main linac and beam delivery system. Furthermore, the BPMs must be able to make multiple independent measurements within a single 156 ns long bunch train. A prototype cavity BPM for CLIC has been manufactured and tested on the probe beam line at the 3rd CLIC Test Facility (CTF3) at CERN. The transverse beam position is determined from the electromagnetic resonant modes excited by the beam in the two cavities of the pickup, the position cavity and the reference cavity. The mode that is measured in each cavity resonates at 15 GHz and has a loaded quality factor that is below 200. Analytical expressions for the amplitude, phase and total energy of signals from long trains of bunches have been derived and the main conclusions are discussed. The results of the beam tests are presented. The variable gain of the receiver electronics has been characterized using beam excited signals and the form of the signals for different beam pulse lengths with the $2/3\text{ }\text{ }\mathrm{ns}$ bunch spacing has been observed. The sensitivity of the reference cavity signal to charge and the horizontal position signal to beam offset have been measured and are compared with theoretical predictions based on laboratory measurements of the BPM pickup and the form of the resonant cavity modes as determined by numerical simulation. Finally, the BPM was calibrated so that the beam position jitter at the BPM location could be measured. It is expected that the beam jitter scales linearly with the beam size and so the results are compared to predicted values for the latter.

Cumulative HOM Excitation and Transition Effects in LCLS-II

LCLS-II is a proposed upgrade of the most powerful X-ray laser in the world built at SLAC National Accelerator Laboratory. When complete, multiple free electron laser (FEL) sections will be powered by a continuous wave superconducting radio frequency (SRF) electron linac. The vital parameters for FEL radiation quality are beam emittance and beam transverse position stability. Excitation of high order modes (HOM) in SRF niobium cavities leads to additional beam power dissipation through incoherent and coherent losses. Energy stored in HOM may cause cumulative effects — transverse and longitudinal position displacement of the center of bunches, which leads to transverse and longitudinal emittance dilution. Cumulative effects due to dipole HOM excitation in LCLS-II are analyzed. Transition HOM effects are caused by sudden changes (periodic or single case) in the beam bunch structure. Bunch center position disturbance due to transition HOM effects is estimated.

Optical beam transport to a remote location for low jitter pump-probe experiments with a free electron laser

In this paper we propose a scheme that allows a strong reduction of the timing jitter between the pulses of a free electron laser (FEL) and external laser pulses delivered simultaneously at the FEL experimental stations for pump-probe--type experiments. The technique, applicable to all seeding-based FEL schemes, relies on the free-space optical transport of a portion of the seed laser pulse from its optical table to the experimental stations. The results presented here demonstrate that a carefully designed laser beam transport, incorporating also a transverse beam position stabilization, allows one to keep the timing fluctuations, added by as much as 150 m of free space propagation and a number of beam folding mirrors, to less than 4 femtoseconds rms. By its nature our scheme removes the major common timing jitter sources, so the overall jitter in pump-probe measurements done in this way will be below 10 fs (with a margin to be lowered to below 5 fs), much better than the best results reported previously in the literature amounting to 33 fs rms.

Numerical Design Analysis of Energy-Spread Monitor Using Multi-Stripline Electrodes

A new energy-spread monitor is under development in order to control the transverse positions and energy spread of a single-bunch positron beam with charges of 4 nC. This monitor is to be installed at a dispersion section of the beam transport line (E = 1.1 GeV) for the positron beam transporting from the linac to a damping ring towards the Super KEK B-factory. The author proposes the use of this new monitor consisting of multi-stripline electrodes in order to simultaneously measure not only the transverse beam positions but also the energy spread pulse-by-pulse. This report describes in detail the basic design of the energy-spread monitor, its detection sensitivity and its characteristics depending on the transverse positions and energy spread of the positron beam based on numerical simulations.

Simulation studies on laser pulse stability for Dalian coherent light source

Dalian Coherent Light Source will use a 300 MeV LINAC to produce fully coherent photon pulses in the wavelength range between 150–50 nm by high gain harmonic generation free electron laser (FEL) scheme. To generate stable FEL pulses, a stringent tolerance budget is required for the LINAC output parameters, such as the mean beam energy stability, electron bunch arrival time jitter, peak current variation and the transverse beam position offset. In order to provide guidance for the design of the Dalian Coherent Light Source, in this paper, the sensitivity of FEL pulse energy fluctuation to various error sources of the electron bunch was performed using intensive start-to-end FEL simulations.

褶皱管束流能量稳定器

Stringent tolerance budget is restricted on the linear accelerator (linac) parameters, such as the mean beam energy stability, peak current variation, electron bunch arrival time jitter, and the transverse beam position offset, in order to generate stable output pulses in modern linac driven free electron laser facility. In this paper, a corrugated pipe is used as a beam energy stabilizer. Analytical and numerical methods for designing such kind of stabilizer are described, and the simulation results show that the beam energy jitter can be reduced more than 10% after this structure is adopted for the case of Dalian Coherent Light Source (DCLS) linac.

Versatile Modes of Operation to Meet User Needs at SOLEIL

SOLEIL, a 2.75 GeV third-generation synchrotron light source [1], is presently delivering photons to 24 beamlines for users in five different filling patterns with top-up injection. The 430 mA multibunch (with three successive quarters filled; i.e., 312 out of 416 bunches) is complemented with time-resolved (hybrid multibunch, eight bunches and single bunch) and 4-2 ps bunch-length low-alpha modes. Even shorter bunches will be available at the end of 2013 with the forthcoming Femto-slicing project. With the continuous improvements of transverse beam position (200 nm RMS in the vertical plane) and the understanding of collective effects, the beam current and the stability provided to the beamlines have been steadily increased. New exotic modes of operation and upgrades of the optics to reduce the 3.9 nm.rad horizontal emittance are being studied.

Design and cold test of a rectangular cavity beam position monitor

The CBPM is a kind of monitor which is used for the measurement of beam transverse position. It is becoming increasingly popular due to its high potential in resolution performance. In theory, the resolution can reach about 1 nanometer. In this paper, a rectangular CBPM is designed for it has better X-Y isolation than a cylindrical one. It has been simulated and measured, and the results agree with each other very well. The procedures and results for the simulation and the cold test will be shown later and it will be proved that this is a reliable method for the CBPM design.

Statistical methods for transverse beam position diagnostics with higher order modes in third harmonic 3.9 GHz superconducting accelerating cavities at FLASH

Beam-excited higher order modes (HOMs) can be used to provide beam diagnostics. Here we focus on 3.9GHz superconducting accelerating cavities. In particular we study dipole mode excitation and its application to beam position determinations. In order to extract beam position information, linear regression can be used. Due to a large number of sampling points in the waveforms, statistical methods are used to effectively reduce the dimension of the system, such as singular value decomposition (SVD) and k-means clustering. These are compared with the direct linear regression (DLR) on the entire waveforms. A cross-validation technique is used to study the sample independent precisions of the position predictions given by these three methods. A RMS prediction error in the beam position of approximately 50μm can be achieved by DLR and SVD, while k-means clustering suggests 70μm.