Collimation System Research Articles

Operational results on the fully automatic LHC collimator alignment

The Large Hadron Collider has a complex collimation system installed to protect its sensitive equipment from normal and abnormal beam losses. The collimators are set around the beam following a multistage transverse setting hierarchy. The insertion position of each collimator is established using beam-based alignment techniques to determine the local beam position and rms beam size at each collimator. During previous years, collimator alignments were performed semiautomatically, with collimation experts present to oversee and control the alignment. During run II, a new fully automatic alignment tool was developed and used for collimator alignments throughout 2018. This paper discusses the improvements on the alignment software to automate it using machine learning, whilst focusing on the operational results obtained when testing the new software in the LHC. The alignment tests were conducted with both proton and ion beams, and angular alignments were performed with proton beams. This upgraded software successfully decreased the alignment time by a factor of 3 and made the results more reproducible, which is particularly important when performing angular alignments.

Concept of a Facility of Neutron Radiography and Tomography at the Research Reactor WWR-K in Almaty, Kazakhstan

At the research reactor WWR-K (Institute of Nuclear Physics, Ministry of Energy, Almaty, Kazakhstan) work on the creation of a new experimental facility for research using neutron radiography and tomography method has begun. It is planned that the facility will form a neutron beam with a cross section of 200 × 200 mm, with a characteristic parameter L/D of 130, using a vacuum collimator system. To obtain neutron radiographic images, a specially designed detector system based on a 6LiF/ZnS scintillation screen and a high-resolution high-sensitivity video camera with a rotated mirror will be used. To protect the video camera from radiation, a two-mirror optical system is proposed. The paper presents a schematic diagram and description of the main components of the experimental setup.

Data-driven cross-talk modeling of beam losses in LHC collimators

The Large Hadron Collider at CERN is equipped with a collimation system to intercept potentially dangerous beam halo particles before they damage its sensitive equipment. The collimator settings are determined following a beam-based alignment procedure, in which the collimator jaws are moved towards the beam until losses appear in the beam loss monitors. When the collimator reaches the beam envelope, beam losses propagate mainly in the direction of the beam and are, therefore, also observed by other nearby beam loss monitors. This phenomenon is known as cross talk. Due to this, collimators are aligned sequentially to be able to identify which losses are generated by which collimator, such that any cross talk across beam loss monitors positioned close to each other is avoided. This paper seeks to quantify the levels of cross-talk observed by beam loss monitors when multiple collimators are moving, to be able to determine the actual beam loss signals generated by their corresponding collimators. The results obtained successfully predicted the amount of cross-talk observed for each of the cases tested in this study. This was then extended to predict loss map case studies and the proton impacts at each collimator by comparing them to simulations.

Collimation of laser diode beams for free space optical communications

Based on a far-field asymptotic expression of diode laser beams, the collimation characteristics of a diode laser beam are investigated. In this paper we propose a method for designing a collimating system of a laser diode and do experiment to confirm it. It is shown that the collimated beam divergence angles in the fast- and slow-axis directions are different, if the beam sizes in the two orthogonal directions near the collimating system are the same. It is also shown that a circular beam spot could be obtained in the far field zone, only if the beam divergence angles have the same value in the two orthogonal directions, although the beam is not circularized near the collimating system.

Quench Protection Study of a 11 T Nb3Sn Model Dipole for the High Luminosity LHC

The planned upgrade of the Large Hadron Collider collimation system requires the installation of 11 T Nb 3 Sn dipole magnets in the dispersion suppressor areas. Due to the large stored energy density and the low copper stabilizer section, the quench protection of these magnets is particularly challenging. The baseline protection scheme after installation in the main dipole circuit is based on quench heaters and a bypass diode. The maximum allowable temperature during quench has a primary importance. In one of the latest short model magnets, full protection studies were performed up to a quench integral and hot spot temperatures well beyond the design value in order to understand the limits. Measurements are compared to electro-transient and thermo-mechanical models to evaluate quench propagation, temperature rise in the conductor, and thermal stress due to temperature gradients in the coil and surrounding structure.

Characteristics of popular photon beam collimators

Purpose: To compare the physical and dosimetric aspects of radiation beam collimation systems that may affect the stereotactic radiosurgery and radiotherapy of small lesions. Methods: Gamma knife (GK) cones, Cyber-knife Iris/InCise collimators, popular Varian and Elekta multi-leaf collimators (MLCs) were theoretically analyzed. Leaf-edge effects (Inter- and intra-leaf leakages, field penumbras, and dosimetric leaf gap - DLG) and transmission through some collimators were measured with ion chambers and films in phantoms and electronic portal imagers in an extended distance. GK plans and IMRT/VMAT plans were generated for ten patients and conformity index (CI) and dose drop (DD) from target to 5-mm ring were compared. Results: New MLCs have improved field shaping conformity, reduced transmission but not field penumbra and DLG. The measured DLG changed with time of usage and varied across the field. GK plans had better CI and DD for small lesions of < 1 cm while MLC-based VMAT/IMRT plans improved CI for large lesions of > 2 cm. Conclusions: New thinner and taller MLCs have improved the field shaping conformity and transmission but not leaf edge effects. Cone-based irradiation is still the best to small lesions of < 1 cm while VMAT/IMRT using new MLCs provide more conformal dose to irregular target usually with size of > 2 cm.

Analysis of divergence angles of truncated and untruncated beams in collimation system

This research is performed to evaluate the divergence angle of far-field distribution of the collimation system. A concise method that can analyze the divergence performance of a collimation system is put forward. Simulation methods for beam truncated systems and beam untruncated systems are proposed. For truncated beams, a proper parabolic distribution at z=0 is used to approximate the beam with “bell-shaped” EFD (equivalent field distribution). Results show that the flat-top beam (h=1) has the minimal divergence angle. The parabolic function is easier to process than the Gaussian function. For untruncated beams, the calculation results indicate that the Lorentzian beam has a minimum divergence angle. It is foreseeable that this study will play a positive role in driving the design and evaluation of the collimation system.

Plan Quality and Secondary Cancer Risk Assessment in Patients with Benign Intracranial Lesions after Radiosurgery using the CyberKnife M6 Robotic Radiosurgery System

This study was performed to examine the quality of planning and treatment modality using a CyberKnife (CK) robotic radiosurgery system with multileaf collimator (MLC)-based plans and IRIS (variable aperture collimator system)-based plans in relation to the dose–response of secondary cancer risk (SCR) in patients with benign intracranial tumors. The study population consisted of 15 patients with benign intracranial lesions after curative treatment using a CyberKnife M6 robotic radiosurgery system. Each patient had a single tumor with a median volume of 6.43 cm3 (range, 0.33–29.72 cm3). The IRIS-based plan quality and MLC-based plan quality were evaluated by comparing the dosimetric indices, taking into account the planning target volume (PTV) coverage, the conformity index (CI), and the dose gradient (R10% and R50%). The dose–response SCR with sarcoma/carcinoma induction was calculated using the concept of the organ equivalent dose (OED). Analyses of sarcoma/carcinoma induction were performed using excess absolute risk (EAR) and various OED models of dose–response type/lifetime attributable risk (LAR). Moreover, analyses were performed using the BEIR VII model. PTV coverage using both IRIS-based plans and MLC-based plans was identical, although the CI values obtained using the MLC-based plans showed greater statistical significance. In comparison with the IRIS-based plans, the MLC-based plans showed better dose falloff for R10% and R50% evaluation. The estimated difference between Schneider’s model and BEIR VII in linear-no-threshold (Lnt) cumulative EAR was about twofold. The average values of LAR/EAR for carcinoma, for the IRIS-based plans, were 25% higher than those for the MLC-based plans using four SCR models; for sarcoma, they were 15% better in Schneider’s SCR models. MLC-based plans showed slightly better conformity, dose gradients, and SCR reduction. There was a slight increase in SCR with IRIS-based plans in comparison with MLC-based plans. EAR analyses did not show any significant difference between PTV and brainstem analyses, regardless of the tumor volume. Nevertheless, an increase in target volume led to an increase in the probability of SCR. EAR showed statistically significant differences in the soft tissue according to tumor volume (1–10 cc and ≥10 cc).

A method for measuring angular bearing errors in the “antenna - radome” system in the phased-array beam scanning region

The paper introduces a method for measuring angular bearing errors in an antenna-radome system in a two-dimensional angular region of scanning of electronically steerable antenna, i.e. phased array antenna, active phased array antenna. The measurements were based on an antenna measuring collimator system - “compact testing ground”. The mathematical expressions used in processing the obtained data are given. To make a matrix of angular bearing errors, measurements are carried out at different angles of the heel of the antenna - radome system when the phased array antenna beam is deflected in oblique planes. To perfect the method at the initial stage, we used a quick-release model of the radome, which has the ability to introduce angular bearing errors

Physical and biological impacts of collimator-scattered protons in spot-scanning proton therapy.

To improve the penumbra of low‐energy beams used in spot‐scanning proton therapy, various collimation systems have been proposed and used in clinics. In this paper, focused on patient‐specific brass collimators, the collimator‐scattered protons' physical and biological effects were investigated. The Geant4 Monte Carlo code was used to model the collimators mounted on the scanning nozzle of the Hokkaido University Hospital. A systematic survey was performed in water phantom with various‐sized rectangular targets; range (5–20 cm), spread‐out Bragg peak (SOBP) (5–10 cm), and field size (2 × 2–16 × 16 cm2). It revealed that both the range and SOBP dependences of the physical dose increase had similar trends to passive scattering methods, that is, it increased largely with the range and slightly with the SOBP. The physical impact was maximized at the surface (3%–22% for the tested geometries) and decreased with depth. In contrast, the field size (FS) dependence differed from that observed in passive scattering: the increase was high for both small and large FSs. This may be attributed to the different phase‐space shapes at the target boundary between the two dose delivery methods. Next, the biological impact was estimated based on the increase in dose‐averaged linear energy transfer (LET d) and relative biological effectiveness (RBE). The LET d of the collimator‐scattered protons were several keV/μm higher than that of unscattered ones; however, since this large increase was observed only at the positions receiving a small scattered dose, the overall LET d increase was negligible. As a consequence, the RBE increase did not exceed 0.05. Finally, the effects on patient geometries were estimated by testing two patient plans, and a negligible RBE increase (0.9% at most in the critical organs at surface) was observed in both cases. Therefore, the impact of collimator‐scattered protons is almost entirely attributed to the physical dose increase, while the RBE increase is negligible.

Using branch-and-price to determine optimal treatment plans for volumetric modulated arc therapy (VMAT)

Volumetric Modulated Arc Therapy (VMAT) is the state-of-the-art technique for external radiation therapy treatment. In this method, radiation can be delivered continuously on one or more arcs during the rotation of the gantry of the linear accelerator. This property makes VMAT powerful in obtaining high conformal plans in terms of dose distribution within short treatment times. However, the apertures composed by the leaves of the multileaf collimator (MLC) system that shapes continuously the radiation are interdependent, which makes treatment planning hard. We propose a mixed integer linear programming model for VMAT planning problem and exact branch-and-price algorithms to solve it. The objective of the model is to minimize total radiation that is delivered to the patient, and pricing subproblem is decomposable by rows of the MLC and can be solved as a shortest path problem. We generate a large set of test instances from a real data set and evaluate the performance of the proposed branch-and-price algorithm. Computational results reveal that new algorithms are efficient and capable of finding optimal solutions for large problem instances.

Thermal neutron profile inside J-PARC main ring tunnel

Abstract To estimate the beam losses and activation in the J-PARC main ring (MR) tunnel, thermal neutron distribution was measured using gold foil activation detectors with the Cd difference method. Hundred pairs of bare and Cd-covered gold foils were prepared to cover the entire area of the MR tunnel. The relative and absolute activities of the gold foils were measured using an imaging plate and an HP-Ge detector for some reference foils, respectively. A total of 97% of the thermal neutrons were detected in the injection section, where a beam collimation system was installed to localize the beam loss in the MR tunnel. The thermal neutron fluxes were compared with the calculation results from a Monte Carlo simulation code, PHITS. The calculation results were generally in good agreement with the experimental thermal neutron flux distribution. Furthermore, we investigated a simple empirical formula, ϕ th = C Q ∕ S , to estimate the thermal neutron flux in the accelerator room, where Q is the neutron source intensity and S is the total surface area of the room. A coefficient C of 1.61 ± 0.15 was deduced in the MR tunnel, which is within the error range of the previous value measured in the CERN CHARM facility using a 24 GeV/c proton beam despite the different projectile energies and room shapes. The γ dose rates at the same positions as the gold foils were measured as well. A strong correlation was observed between the thermal neutron fluxes and γ dose rates with respect to the surrounding concrete types. This result suggests that the dose rate on the concrete wall can be used to estimate the distribution of the thermal neutron fluxes and the amount of beam loss in the MR.

Automatic spike detection in beam loss signals for LHC collimator alignment

Abstract A collimation system is installed in the Large Hadron Collider to protect its super-conducting magnets and sensitive equipment from potentially dangerous beam halo particles. The collimator settings are determined following an alignment procedure, whereby collimator jaws are moved towards the beam until a suitable spike pattern, consisting of a sharp rise followed by a slow decay, is observed in nearby beam loss monitors. This indicates the collimator jaw is aligned to the beam. The current method for aligning collimators is semi-automated whereby an operator must continuously observe the loss signals to determine whether the jaw has touched the beam, or if some other perturbation in the beam caused the losses. The human element in this procedure can result in errors and is a major bottleneck in automating and speeding up the alignment. This paper proposes to automate the human task of spike detection by using machine learning. A data set was formed from previous alignment campaigns, from which fourteen manually engineered features were extracted and six machine learning models were trained, analysed in-depth and thoroughly tested. The suitability of using machine learning in LHC operation was confirmed during collimator alignments performed in 2018, which significantly benefited from the models trained through machine learning in this study.

A New Precision Goniometer Mechanism for Direct Angle Measurement by a Rotary Encoder

In order to perform optical tests of an optical device such as a camera, a collimator system is needed which is aligned to a precision two axes rotary system (gimbal) where the optical device (Unit Under Test, UUT) is mounted. By the help of gimbal system, precision positioning of the UUT can be achieved and optical tests can be performed with a high accuracy. However, it is hard to position elevation axis accurately for relatively heavier UUT because of direct moment applied by UUT to the elevation axis of the gimbal system. Therefore, directly driven elevation axis is replaced by a goniometer system. Due to gear coupling clearance in goniometer systems, precision positioning is not achievable. In order to overcome this problem, a new mechanism, which is driven by a power screw, is used for the goniometer and virtual center of rotation is moved within the goniometer for direct angle measurement by a rotary encoder. Validation test is performed by autocollimator and precise mirror calibrators to check the accuracy of the new goniometer system.

Thermal Study of the Ironless Inductive Position Sensors Installed on the LHC Collimators

The safe operation of the European Organization for Nuclear Research Large Hadron Collider relies on a complex system for equipment protection. An essential part of machine protection is the collimation system since it is used to protect the equipment from beam losses. This is achieved by using two jaws to intercept the particles making up the beam halo. Since the jaws are in close proximity to the beam, they must be positioned with micrometer precision from it. An ironless inductive position sensor (I2PS) is a relatively new linear position sensor used to measure the distance of the jaws with respect to the beam. During the operation, it is noted that the position read by the I2PS could be affected by temperature variation causing position-reading errors. This paper presents an experimental study conducted on the I2PS to determine the relationship between the position read by this transducer and the ambient temperature. Furthermore, this paper aims at determining the impact of the temperature changes in the collimator body, on which the I2PS is installed, and the position read by the sensor. The results show that the uneven thermal distribution along the body of the sensor leads to high position readings error. These results also show that, depending on the moving coil position, the sensor behaves differently, and hence, it is very difficult to compensate for the effect. Finally, this paper presents the results of a possible modification tested on the same setup to reduce the thermal effects.

Development of a fiber laser diode module in the spectral range of 445–450 nm with an output optical power of more than 100 W

This study considers the possibility of developing a laser diode module with a fiber output, having an output power more than 100 W in the spectral range of 445–450 nm, for processing metals with a low reflectance in this wavelength range, such as gold, silver, and copper. As sources of radiation, 64 blue laser diodes were used in a standard TO-18 case with a diameter of 5.6 mm and an output optical power of 2 W each, combined into a single 8×8 array. The developed optical system for collimation, spatial beam conversion, and radiation focusing on the end face of the optical fiber makes it possible to input optical power of more than 116 W into a fiber with an efficiency of more than 90%.

Characterization of the HalcyonTM multileaf collimator system.

PurposeTo characterize the stacked and staggered dual‐layer multileaf collimator (MLC) on the HalcyonTM system.MethodsThe novel MLC assembly was reviewed and compared to the widely used MillenniumTM 120‐leaf MLC system. We investigated the MLC positioning stability over 70 days using Machine Performance Check (MPC) data. We evaluated the leaf transmission, penumbra, leaf end effect, and leaf edge effect. Leaf transmission through distal, proximal, and both MLC layers was measured with a Farmer chamber, by comparing an open and a closed field. Leaf penumbra was measured using film for three different MLC‐defined field sizes. The leaf end effect was measured with sweeping gap fields of varying gap sizes defined by the distal MLC. The leaf edge effect was evaluated using the Electronic Portal Imaging Device (EPID) for the different banks, gantry positions, and collimator angles. Point dose measurements for 10 test plans were compared to dose predictions of two dose calculation model versions.ResultsFrom MPC data, the largest measured MLC positioning accuracy deviation was within 0.1 mm. The proximal MLC exhibited greater deviations compared to the distal MLC. The distal‐and‐proximal‐combination had reduced inter‐leaf and intra‐leaf transmission compared to delivery with distal‐only. The measured leaf transmission was 0.41% for distal‐only, 0.40% for proximal‐only, and negligible for distal‐and‐proximal‐combination. The leaf end penumbra was wider compared to the leaf edge penumbra. The leaf end effect was measured to be −0.2 mm. The leaf edge effect showed minimal bank, gantry position, and collimator angle dependence. However, a systematic deviation between measurements and treatment planning system handling of the leaf edge effect was observed. The discrepancy between the measured and predicted dose in the 10 test plans improved with the latest version of the dose calculation algorithm.ConclusionThe characteristics of the stacked and staggered dual‐layer MLC on the HalcyonTM system were presented.

Technical Note: Optimization of spot and trimmer position during dynamically collimated proton therapy.

To demonstrate a novel theoretical optimization design which considers beam spot and trimmer positioning in addition to beamlet weighting for dynamically collimated proton therapy (DCPT) treatments. Prior to this, the previous methods of plan optimization used to study this emerging technology relied upon an intuitive selection criterion to fix the trimmers blades for a uniform grid of beam spots before determining the individual beamlet weights. To evaluate the potential benefit from this new optimization design, a treatment planning optimization study was performed in order to compare the algorithm's functionality against the existing methods of plan optimization. A direct parameter optimization (DPO) method was developed to determine beam spot and trimmer positions cohesively with beamlet weighting for DCPT treatment plans. Gradients were numerically determined from applying small adjustments to the aforementioned parameters and quantifying the resulting impact on an objective function. This technique was compared to the conventional trimmer selection algorithm (TSA) which does not optimize spot position concurrently with trimmer position. Both planning methods were used to optimize a set of brain treatment plans, and the resulting dose distributions were compared with dose-volume histogram quantities in addition to target coverage, homogeneity, and conformity metrics. An overall improvement to the target conformity and healthy tissue sparing was achieved with DPO over TSA while maintaining an equivalent planning target volume (PTV) coverage index for the three brain patients evaluated in this study. On average, the conformity index improved by 5.5% when utilizing DPO. A similar improvement in reducing the dose to several organs at risk was also noted. Both the TSA and DPO planning methods can achieve highly conformal treatments with the dynamic collimation system (DCS) technology. However, an improvement in the target conformity and healthy tissue sparing was achieved by simultaneously optimizing beam spot position, trimmer location, and beamlet weights using DPO in comparison to the TSA technique.

C-mount packaged laser diode end-pumped passively Q-switched laser with a microlens as pump coupling component

A microlens has been optimum simulated and designed for an 808 nm laser C-mount packaged semiconductor laser rectangle beam collimation. By designing the suitable focal length, the fast axis light is compressed. After the compression, the fast axis and the slow axis have the same divergence angle, which increases the matching degree between the pump light and the fundamental light. The design for cylinder microlenses collimation system has been discussed in detail. In the laser we designed, the pump source is a C-mount packaged laser diode with controlling the duty cycle of electric pulse. We demonstrate a 63.1 mW, 1 kHz, 779 ps end-pumped Nd:YAG/Cr4+:YAG laser at 1064 nm using the above designed microlens capable of compressing fast axis light.

Collimation method studies for next-generation hadron colliders

In order to handle extremely-high stored energy in future proton-proton colliders, an extremely high-efficiency collimation system is required for safe operation. At LHC, the major limiting locations in terms of particle losses on superconducting (SC) magnets are the dispersion suppressors (DS) downstream of the transverse collimation insertion. These losses are due to the protons experiencing single diffractive interactions in the primary collimators. How to solve this problem is very important for future proton-proton colliders, such as the FCC-hh and SPPC. In this article, a novel method is proposed, which arranges both the transverse and momentum collimation in the same long straight section. In this way, the momentum collimation system can clean those particles related to the single diffractive effect. The effectiveness of the method has been confirmed by multi-particle simulations. In addition, SC quadrupoles with special designs such as enlarged aperture and good shielding are adopted to enhance the phase advance in the transverse collimation section, so that tertiary collimators can be arranged to clean off the tertiary halo which emerges from the secondary collimators and improve the collimation efficiency. With one more collimation stage in the transverse collimation, the beam losses in both the momentum collimation section and the experimental regions can be largely reduced. Multi-particle simulation results with the MERLIN code confirm the effectiveness of the collimation method. At last, we provide a protection scheme of the SC magnets in the collimation section. The FLUKA simulations show that by adding some special protective collimators in front of the magnets, the maximum power deposition in the SC coils is reduced dramatically, which is proven to be valid for protecting the SC magnets from quenching.