FIG 3 - uploaded by Marija Cauchi
Content may be subject to copyright.
Source publication
The correct functioning of a collimation system is crucial to safely and successfully operate high-energy particle accelerators, such as the Large Hadron Collider (LHC). However, the requirements to handle high- intensity beams can be demanding, and accident scenarios must be well studied in order to assess if the collimator design is robust agains...
Context in source publication
Context 1
... there may be machine conditions [16,17] that expose the TCTs and thus put them at risk of damage. This paper evaluates the effectiveness of operating with tilted collimator jaws in case of a direct impact of one full intensity bunch on TCTs, as a consequence of an asynchronous beam dump accident. The TCT design is described, and its thermal and mechanical response, when impacted by 3.5 and 7 TeV bunches of 1 . 3 × 10 11 protons, is analyzed through a finite element method (FEM) approach. In particular, we investigate the behavior of the TCTs in case their planar collimating surface is either exactly parallel to the beam or is tilted by − 1 or Æ 5 mrad. In all cases, the beam impact parameter to the point of first contact with the collimator is set to 0.5 mm. During the LHC Run 1, 38 collimators had -alloy jaws. These are the TCLAs and TCTs described in Sec. I above. This study focuses on TCTs as they are the non-CFC collimation system elements that are mostly at risk of damage in case of an asynchronous beam abort. A TCT collimator consists of two parallel jaws contained in a vacuum tank, with the beam passing through the center of the jaw gap (Fig. 3). For optimal performance, the jaws have to be centered around the actual beam orbit. Each TCT jaw has a total length of 1.2 m (1 m active length þ 0 . 1 m tapering at the upstream and downstream parts of the jaw) and consists of five inserts made of INERMET® 180. These insert blocks are screwed to a copper housing, which is cooled by 27 °C water flowing at 25 l = min through an array of square-shaped Cu-Ni tubes brazed to the back side of the housing (Fig. 4). Two stepping motors per jaw allow independent adjustment of jaw tilt and jaw position relative to the beam center [18]. Being in proximity to the beam, the collimator jaws are continuously exposed to direct interaction with high-energy particles. In normal operation, the time constant, which describes variations in thermal load due to particle loss on the collimator jaws, ranges from seconds to hours. On the other hand, in an asynchronous beam abort accident, the relevant time scale for energy deposition in the jaw material is on the order of μ s or ns. This very fast energy deposition provokes a thermodynamic response of the hit material, including the development of shock waves within the collimator structure. As the material of the collimator jaw cannot respond to the rapid increase in temperature [19] caused by the hadronic shower, structural deformations can occur [20]. This study investigates collimator damage under such conditions. The probability that an asynchronous beam dump event occurs was originally estimated to be once per year [14]. However, the probability that a TCT is hit directly by a full intensity bunch is lower, as other error conditions [21] must be simultaneously present for this to happen. When an asynchronous abort is detected, the remaining horizontal extraction kickers are fired within 0 . 9 μ s, and only one bunch should escape the beam dump system. In this context, the present work focuses on the admittedly low probability case of the impact of one bunch on a TCT jaw. If such an accident happens during physics or collimation beam-based alignment setups, it can have serious consequences. Different jaw error cases have been identified, taking into account conditions when the planar collimating surface of the TCT is either exactly parallel to the beam, or has a slight inclination of a few mrad due to misalignment errors of the collimator installation at the beam line (Figs. 5 and 6). We focus on accidents involving horizontal TCTs due to the fact that a miskick accident can only act on the horizontal ...
Similar publications
The American Association of Physicists in Medicine Task Group 36 (AAPM TG-36) data can be used to estimate peripheral dose (PD) distributions outside the primary radiation field. However, the report data does not apply to linear accelerators equipped with a multileaf collimator (MLC) and universal wedge (UW). Tertiary multileaf collimators have bee...
Citations
... So we focus on heating problems to evaluate the tolerable loss, which needs the finite element method (FEM) analysis. FLUKA calculations by using the USRBIN card can provide the inputs of the heat generation rate for the FEM approach [46], and the numerical FEM program ANSYS® is adopted in this simulation. Heat conduction and thermal radiation are assumed as the cooling effects in the FEM calculations. ...
Beam loss detection is essential for the machine protection and the fine-tuning of the accelerator to reduce the induced radioactivity. Monte Carlo simulations are also vital for the choice of beam loss monitor (BLM) type, for predicting and understanding the BLM response to beam losses. At the China Spallation Neutron Source (CSNS), the cylindrical ionization chamber (IC) filled with Ar/N2 gas mixture is the main type of the BLM to detect the beam losses. This paper presents the detailed beam loss experiments and fluka simulations for the CSNS BLM system. It includes the response functions of the BLM detectors by taking into account the contributions of different secondary particles to the energy deposition in the sensitive volume of the BLM. Dedicated experiments were compared with fluka simulations. It was found that in the low energy section of the linac the usual ion chamber based BLMs are not sensitive enough. A BLM based on BF3 enclosed by a high-density polyethylene (PE) moderator is effective to detect the beam losses through detecting thermal neutrons. Its signal is about 3 orders of magnitude higher than that of Ar/N2 BLM for a beam energy of 15 MeV. The simulations provide the results of the intrinsic delay time for the BF3 monitor and the thickness optimization for PE to reduce the delay. Finally, the simulated spatial resolution of loss location by detection of neutrons is also evaluated for a beam energy of 15 MeV, which presents a resolution of ∼2 m in our experimental configuration.
... This is a common scenario when particle accelerator components are subject to high energy subatomic particle beams. Beam targets are one example, generally being slender rods directly hit by a beam in order to create a shower of secondary particles [5]. The rod is simply supported and free to expand at its extremities. ...
Analytical solutions detailing the propagation of longitudinal waves in slender rods subjected to a sudden increase of internal energy provide simple tools for the calculation of the temperature distribution in impacted rods as well as the resulting mechanical response. The topic is of great interest in particle accelerator technology, especially with regards to collimation systems, where beam intercepting devices can be generally approximated to one-dimensional (1D) elements potentially subjected, in accidental scenarios, to abrupt thermal energy depositions induced by the impacting particles. In this study, two finite element numerical models are presented and compared to the analytical solutions by Bertarelli, Dallocchio and Kurtyka, discussing the rapid temperature increase in slender rods due to particle beam impacts and the resulting dynamic longitudinal response. The first model is a sequentially coupled thermomechanical analysis; the second is based on a modal analysis to find the harmonic response of the system. The results indicate that phenomena neglected in analytical solutions, primarily dispersion of the longitudinal wave due to interactions with the free external surface of the rod, can be included in numerical models and can be observed in simulation results. The study further shows how numerical methods can be utilized to predict the frequencies and amplitudes of high-frequency disturbances in the longitudinal wave signal, and how these effects can be mitigated in preparation for experimental scenarios by fine-tuning the geometry of the rod and varying the duration of the pulse. This is especially useful with regards to experiments conducted in the HiRadMat facility at CERN, such as the recently conducted HRMT36 experiment, where high-frequency components can distort the signal to be observed.
... In addition, for the LHC or any future hadron collider various other phenomena or constraints may be important, such as the electron cloud produced by photoemission or beam-induced multipacting [18][19][20][21][22][23][24], beam instabilities [25][26][27][28], local beam losses and the efficiency of the collimation system [29][30][31][32][33][34][35][36], and, especially for heavyion operation, local magnet quenches induced by the collision products [37,38]. None of these potential additional limitations are included in our following analysis. ...
The integrated luminosity, a key figure of merit for any particle-physics collider, is closely linked to the peak luminosity and to the beam lifetime. The instantaneous peak luminosity of a collider is constrained by a number of boundary conditions, such as the available beam current, the maximum beam-beam tune shift with acceptable beam stability and reasonable luminosity lifetime (i.e., the empirical "beam-beam limit"), or the event pileup in the physics detectors. The beam lifetime at high-luminosity hadron colliders is largely determined by particle burn off in the collisions. In future highest-energy circular colliders synchrotron radiation provides a natural damping mechanism, which can be exploited for maximizing the integrated luminosity. In this article, we derive analytical expressions describing the optimized integrated luminosity, the corresponding optimum store length, and the time evolution of relevant beam parameters, without or with radiation damping, while respecting a fixed maximum value for the total beam-beam tune shift or for the event pileup in the detector. Our results are illustrated by examples for the proton-proton luminosity of the existing Large Hadron Collider (LHC) at its design parameters, of the High-Luminosity Large Hadron Collider (HL-LHC), and of the Future Circular Collider (FCC-hh).
... This study focuses on TCTs since they are the first metallic collimators exposed to high risks of damage in case of an asynchronous beam dump and they are essential for the protection of the downstream SC magnets. Similar studies have already been carried out for protons [16,17] and the developed finite element method (FEM) approach has been successfully validated in [18]. This paper presents a numerical FEM approach in order to evaluate and compare the thermomechanical response of TCTs in case of critical beam load cases involving proton and heavy ion beam impacts. ...
The CERN Large Hadron Collider (LHC) is designed to accelerate and bring into collision high-energy protons as well as heavy ions. Accidents involving direct beam impacts on collimators can happen in both cases. The LHC collimation system is designed to handle the demanding requirements of high-intensity proton beams. Although proton beams have 100 times higher beam power than the nominal LHC lead ion beams, specific problems might arise in case of ion losses due to different particle-collimator interaction mechanisms when compared to protons. This paper investigates and compares direct ion and proton beam impacts on collimators, in particular tertiary collimators (TCTs), made of the tungsten heavy alloy INERMET 180. Recent measurements of the mechanical behavior of this alloy under static and dynamic loading conditions at different temperatures have been done and used for realistic estimates of the collimator response to beam impact. Using these new measurements, a numerical finite element method (FEM) approach is presented in this paper. Sequential fast-transient thermostructural analyses are performed in the elastic-plastic domain in order to evaluate and compare the thermomechanical response of TCTs in case of critical beam load cases involving proton and heavy ion beam impacts.
This work presents a novel closed-loop control system for the detection and control of thermal deformations integrated in the secondary collimators of the Large Hadron Collider (LHC). Interactions between spurious particles lost transversally from the circulating beam core and the collimator jaws that make up the active area of the collimator will result in thermally induced deformations in those jaws. This interaction can push the jaw’s straightness out of the tolerance and force the jaw either into the beam core or away from it. This action can result in reductions in beam cleaning efficiency and increases in impedance on the beam. Whilst deformations in either direction are to be avoided, deformations into the beam are considered more of an issue as too much deformation can provoke beam dumps if beam losses are too high. To minimize this unavoidable thermal effect a novel adaptive closed-loop monitoring and actuation system, comprising of multiple intrinsic Fabry-Pérot interferometric (IFPI) optical sensors, and several integrated piezoelectric stack actuators, has been developed. When operating, this system can transiently monitor the jaws straightness and when required correct for deformations up to . In addition, subsequent steps to use this system as an active damper to reduce vibratory response, as experienced when the jaw undergoes a direct beam impact, are also discussed.
