Beam Pipe Radius Research Articles

Performance of the ALICE upgraded inner tracking system

Major upgrades of the ALICE (A Large Ion Collider Experiment) detector are underway and will be completed during the LHC long shutdown 2 in order to start operation in 2022 for LHC run 3. One key part of this upgrade is the new Inner Tracking System (ITS2), a full silicon-pixel detector constructed entirely with CMOS monolithic active pixel sensors. The upgraded ITS2 detector consists of three inner layers (50 μm thick sensors) and four outer layers (100 μm thick sensors) covering 10 m2 and containing 12.5 billion pixels with a pixel pitch of 27 μm × 29 μm. Compared with the silicon tracking system used during the LHC run 1 and run 2, the increased granularity, the very low material budget (0.35% X 0/layer in the inner barrel) as well as a smaller beam pipe radius, will result in a significant improvement of impact-parameter resolution and tracking efficiency. The assembly of the detector and services finished in December 2019, and the detector was fully installed in the ALICE experiment in May 2021. A comprehensive commissioning phase (on the surface) was completed in December 2020 to validate the detector performance. In this paper, an overview of the design and construction, as well as the performance of the ITS2 studied from the on-surface commissioning will be discussed in detail.

Stability limits with Landau damping in the FCC-hh

The resistive wall impedance is one of the expected main drivers of transverse beam instabilities in the proposed Future Circular Collider hadron-hadron option (FCC-hh). We obtain the resistive wall impedance for the FCC-hh beam screen from a two-dimensional finite element solver. The impedances and resulting growth rates are compared to the LHC, using similar models for the resistivity of the copper layer. Similar to the LHC and in addition to active feedback, dedicated octupole magnets together with a finite chromaticity should be employed for Landau damping, as a cure against transverse beam instabilities. The stability boundary provided by an LHC-like octupoles configuration in combination with an electron lens is obtained from a dispersion relation including the two-dimensional tune spreads. The prediction from the simple dispersion relation are compared to the corresponding beam transfer function and to the stability boundaries reconstructed using particle tracking with an effective impedance. The electron cloud induced tune spreads and their scaling with higher energy and smaller beam pipe radius are estimated. Besides the important estimation of growth rates and stability threshold for FCC-hh we also try to improve the understanding of the scaling of coherent instabilities and their thresholds with energy towards a possible highest-collider limit, using the example of two high-energy colliders, the existing LHC and the proposed FCC-hh.

RF ANTENNA DESIGN FOR BUTTON-TYPE BEAM POSITION MONITORS USING BIO-INSPIRED OPTIMIZATION METHODS

Accelerator based facilities are in a leading position for crafting many scientific and technical innovations for a wide range of application from aviation to medicine. Beam Position Monitors (BPMs) are critical diagnostics tools for such facilities. This study presents bio-inspired methods known as Particle Swarm Optimization and Evolutionary Algorithms in order to design RF antennas for button-type BPMs. Our results show that the antenna parameters obtained using this multiple objective approaches present suitable SNR and linearity values for signal processing. It is found that using an antenna radius of 5.5 mm and beam-pipe radius of 17.5 mm, we can obtain SNR values around 40 dB which can be electronically processed.

Production and integration of the ATLAS Insertable B-Layer

During the shutdown of the CERN Large Hadron Collider in 2013-2014, an additional pixel layer was installed between the existing Pixel detector of the ATLAS experiment and a new, smaller radius beam pipe. The motivation for this new pixel layer, the Insertable B-Layer (IBL), was to maintain or improve the robustness and performance of the ATLAS tracking system, given the higher instantaneous and integrated luminosities realised following the shutdown. Because of the extreme radiation and collision rate environment, several new radiation-tolerant sensor and electronic technologies were utilised for this layer. This paper reports on the IBL construction and integration prior to its operation in the ATLAS detector.

The ATLAS IBL CO2 cooling system

The ATLAS Pixel detector has been equipped with an extra pixel layer in the space obtained by a smaller radius beam pipe. This new pixel layer called the Insertable B-Layer (IBL) was installed in 2014 and is operational in the current ATLAS data taking. The IBL detector is cooled with evaporative CO2 and is the first of its kind in ATLAS. The ATLAS IBL CO2 cooling system is designed for lower temperature operation (< −35̂C) than the previous developed CO2 cooling systems in High Energy Physics experiments. The cold temperatures are required to protect the pixel sensors for the expected high radiation dose received at an integrated luminosity of 550 fb1. This paper describes the design, development, construction and commissioning of the IBL CO2 cooling system. It describes the challenges overcome and the important lessons learned for the development of future systems which are now under design for the Phase-II upgrade detectors.

The Insertable B-Layer of the Atlas experiment for the Run-2 at the Large Hadron Collider

The Pixel Detector of the ATLAS experiment has shown excellent performance during the whole Run-1 of Lhc. Taking advantage of the long shutdown, the detector was extracted from the experiment and brought to surface, to equip it with new Service Quarter Panels, to repair modules and to ease installation of the Insertable B-Layer (Ibl). Iblis a fourth layer of pixel detectors and has been installed in May 2014 between the existing Pixel Detector and a new smaller radius beam pipe at a radial distance of 3.3cm from the beam axis. The realization of the Iblrequired the development of several new technologies and solutions in order to overcome the challenges introduced by the extreme environment and working conditions, such as the high radiation levels, the high pixel occupancy and the need of an exceptionally low material budget. Two silicon sensor technologies have been adopted for the Iblmodules: planar and 3D. Both of these are connected via bump-bonding to the new Fe-i4front-end read-out chip. Furthermore, the physics performance will be improved through the reduction of pixel size while, targeting for a low material budget, a new mechanical support using lightweight staves and a CO2 based cooling system have been adopted. An overview of the Iblproject as well as the experience in its construction is presented, focusing on adopted technologies, modules and staves production, qualification of assembly procedure, integration of staves around the beam pipe and commissioning of the detector.

Data driven search in the displaced $ b\overline{b} $ pair channel for a Higgs boson decaying to long-lived neutral particles

This article presents a proposal for a new search channel for the Higgs boson decaying to two long-lived neutral particles, each of which decays to $b\bar b$ at a displaced vertex. The decay length considered is such that the decay takes place within the LHC beampipe. We present a new data-driven analysis using jet substructure and properties of the tracks from the highly-displaced vertices. We consider a model with a 125 GeV Higgs boson with a significant branching fraction to decay via this mode, with the long-lived neutral particle having a mass in the range of 15--40 GeV and a decay length commensurate with the beam pipe radius. Such a signal can be readily observed with an integrated luminosity of 19.5 fb$^{-1}$ at 8TeV at the LHC.

Generalized Napoly integral to compute wake potentials in axisymmetric structure

The Napoly integral is the very useful method for calculations of wake potentials in structures where parts of the boundary extend below the beam pipe radius or the radii of the two beam pipes at both ends are unequal. It reduces CPU time a lot by deforming the integration path so that the integration contour is confined to the finite length over the gap of the structures. However, the original Napoly method cannot be applied to the transverse wake potentials in a structure where the two beam tubes on both sides have unequal radii . In this case, the integration path needed to be a straight line and the integration is stretched out to an infinite, in principle. We generalize the Napoly integrals so that integrals are always confined in a finite length even when the two beam tubes have unequal radii, for both longitudinal and transverse wake potential calculations. The extended method has been successfully implemented to the ABCI code.

EMITTANCE DEGRADATION DUE TO WAKE FIELDS IN A HIGH BRIGHTNESS PHOTOINJECTOR

Wake fields effects in addition to space charge forces may have an important impact during the emittance compensation process in a high brightness photo-injector. To study this effect we developed an upgraded version of the Homdyn code including off axis beam dynamics and wake fields. Homdyn describes a bunch as a uniformly charged cylinder, divided in cylindrical slices; in the upgraded version each slice's centroid can be transversally displaced from the nominal axis thus inducing wake fields. When the bunch is short as compared to the beam pipe radius, wake fields for a single cavity are calculated using methods of diffraction theory; instead we use, for a periodic collection of cavities, an asymptotic wake field obtained numerically at SLAC and then fitted to a simple function. As a first application we studied and verified a correction scheme for the SPARC photo-injector to control the bunch trajectory and angle at the entrance of the undulator. The correction scheme consists of a number of steering magnets and beam position monitors placed along the photo-injector. Two different steering approaches are analyzed and the emittance degradation is studied. The code demonstrates the steering positions and number do correct the bunch's orbit and angle and gives good results concerning the emittance degradation. The emittance and energy spread degradation due to wake fields in the emittance meter experiment is also discussed.

Electron cloud measurements in heavy-ion driver for HEDP and inertial fusion energy

The high-current experiment (HCX) at LBNL is a driver scale single beam injector that provides a 1MeV K+ ion beam current of 0.18A for 5μs. It transports high-current beams with large fill factor (ratio of the maximum beam envelope radius to the beam pipe radius) and low emittance growth that are required to keep the cost of the power plant competitive and to satisfy the target requirements of focusing ion beams to high-power density. Beam interaction with the background gas and walls desorbs electrons that can multiply and accumulate, creating an electron cloud. This ubiquitous effect grows at higher fill factors and degrades the quality of the beam. We review simulations and diagnostics tools used to measure electron production, accumulation and its properties.

High-current SRF cavity design

For high current applications, it is desirable for the cavity shape to have a low longitudinal loss factor and to have a high beam-breakup threshold current. This paper briefly describes three different cavities designed for this purpose: a six-cell elliptical cavity for particles traveling at the speed of light, a two-cell elliptical cavity for subluminal particle speeds, and a single cell cavity which uses the TM012 mode for acceleration. SUPERFISH simulations predict the peak fields in both of the elliptical cavities will not exceed the TeSLA values by more than 10% but both will have 28.7% larger apertures. The elliptical designs assume the bunch frequency equals the accelerating mode frequency. The beam pipe radius is chosen so that the cutoff frequency is less than twice that of the accelerating mode. Hence all of the monopole and dipole higher-order modes (HOMs) that can be driven by the beam have low loaded Q values. This simplifies the problem of HOM damping. The TM012 cavity is predicted to have much higher peak fields than a π-mode elliptical cavity, but offers potential advantages from its simplified shape; it is essentially a circular waveguide with curved end plates. This basic shape results in easier fabrication and simplified tuning.

Evaluation of external Q of the superconducting cavity using Kroll-Yu method

Design and development of superconducting (SC) cavity having βg&amp;gt;0.42 has been taken up as a part of the accelerator driven subcritical system project. An input coupler is designed for the SC, elliptical cavity operating at 1.056GHz, using the Kroll-Yu method. The evaluation procedure is optimized and the method has been successfully implemented for the evaluation of high Q external. The effect of beam pipe radius, coupler dimensions, penetration depth, and the position of the coupler on the external Q has also been studied.

The new beampipe for the Belle experiment

We have upgraded the beampipe and the silicon vertex tracker of the Belle detector for increased luminosity and to achieve better vertex resolution. The radius of the beampipe was reduced from 2 to 1.5 cm to achieve better vertex resolution. In order to reduce the beam background that could accompany the reduction of the beampipe radius, a massive mask made of tantalum and tungsten was placed on each side of the central section. The total weight of them is 40 kg. The maximum radiation dose of the silicon tracker is measured about 80 krad/yr. This is lower than the dose of 100 krad per year with the previous beampipe even though the beampipe radius is smaller. The beampipe is designed to minimize heating, and enough cooling power is provided by the liquid cooling.

Longitudinal space charge effect in slowly converging or diverging relativistic beams

Beginning with the Green function for a rod beam in a round beam pipe we derive the space charge induced average energy change and rms spread for relativistic beams that are slowly converging or diverging in round beam pipes, a result that tends to be much larger than the 1/{gamma}{sup 2} dependence for parallel beams. Our results allow for beams with longitudinal-transverse correlation, and for slow variations in beam pipe radius. We calculate, in addition, the space charge component of energy change and spread in a chicane compressor. This component indicates source regions of coherent synchrotron radiation (CSR) energy change in systems with compression. We find that this component, at the end of example compressors, approximates the total induced voltage obtained by more detailed CSR calculations. Our results depend on beam pipe radius (although only weakly) whereas CSR calculations do not normally include this parameter, suggesting that results of such calculations, for systems with beam pipes, are not complete.

Analysis of the longitudinal collective behavior in a50GeV×50GeVmuon collider ring

Simulations of the longitudinal instability in the $50\mathrm{GeV}\ifmmode\times\else\texttimes\fi{}50\mathrm{GeV}$ muon collider ring have been performed. Operation of the ring close to the slippage factor ${\ensuremath{\eta}}_{1}\ensuremath{\simeq}{10}^{\ensuremath{-}6}$, such that synchrotron motion is frozen, minimizes the need for rf to maintain the bunch length. However, there is still an energy spread due to the bunch wake. For design parameters of the ring, this induced energy is too large and must be controlled. This paper demonstrates that the bunch wake may be compensated for by two rf cavities with low rf voltages. These studies were made at the nominal design point, and sensitivities to errors were explored. It is seen that the small energy spread of the beam ($\ensuremath{\delta}E/E=3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$) in the $50\mathrm{GeV}\ifmmode\times\else\texttimes\fi{}50\mathrm{GeV}$ muon collider ring can be maintained during the 1000 turn lifetime of the muons. Controlled beam dynamics requires proper choice of rf parameters (rf voltage, rf frequency, and phase offset) for two cavities; these parameters depend on the ring design through the impedance, beam pipe radius, and momentum compaction. The simulation also shows that the computation of wake field using bins of variable width (each with a constant number of macroparticles in each bin) gives an accurate wake and also yields reduced computing time compared to an evaluation of the wake as the direct sum over the wakes of all preceding macroparticles.

Longitudinal space charge in final-focus systems for linear colliders

In the final-focus system of a linear collider, the effect of longitudinal space charge can be significant. A diverging or converging ultra-relativistic beam experiences a longitudinal space-charge force which is independent of the beam energy. This force, though weak and often neglected, causes an energy variation across the bunch which depends on the beam size, beam-pipe radius and bunch population and this may affect the chromatic correction of a beam line. Because of the inherent large chromaticity of a typical final-focus system, a very small energy variation, induced in the final focus, can lead to an intolerable increase of the spot size at the interaction point. The space-charge force, thus, gives rise to a limit on bunch intensity beyond which the resulting spot-size increase will degrade the collider performance. In this paper, we evaluate the effect of the longitudinal space charge and derive intensity limits for the three existing or proposed final foci.

Test installation of a 2.3 cm radius beam pipe and vertex drift chamber in CLEO at CESR

We installed a 2.3 cm radius beam pipe with a three-layer drift chamber around it at the CLEO interaction region of the Cornell electron storage ring to study feasibility of various future vertex detectors for CLEO II. We ran the chamber inside the CLEO detector in order to evaluate synchrotron radiation and beam gas backgrounds produced in the CLEO detector using such a small radius pipe. We also evaluated temperature stability and wire currents in this chamber to see if the environment around the pipe would be suitable for a high-resolution vertex detector.

High-frequency behavior of the longitudinal impedance for a cavity of general shape.

An ultrarelativistic beam bunch traveling along the axis of an azimuthally symmetric cavity of general shape connected to a beam pipe generates wake fields. An integral equation is derived for the longitudinal electric component of these wake fields at the beam-pipe radius. The kernel for this equation is approximated at high frequency for several different cavity shapes and the resulting integral equation is solved for the electric field at the beam-pipe radius. The longitudinal coupling impedance is obtained and shown to vary inversely as the square root of the frequency, in agreement with recent results obtained by others using different approximation techniques. The results are also shown to depend only on the axial extent of the cavity at the beam-pipe radius, but otherwise are independent of cavity shape for four different cavity geometries. In addition, the results are shown to be in agreement with the analytic properties of the coupling impedance as a function of complex frequency required by causality for an ultrarelativistic beam.

Longitudinal impedance of a periodic structure at high frequency.

An ultrarelativistic beam bunch traveling along the axis of an azimuthally symmetric cavity of general shape connected to a beam pipe generates wake fields. We extend an earlier derivation of the integral equation for the axial electric field at the beam-pipe radius to the case of a periodic structure. This equation is then solved in the high-frequency limit, and we show that the reciprocal of the impedance per cell has a particularly simple form, closely related to the admittance for a single cavity. We also show that the imaginary part of the impedance per cell varies as ${\ensuremath{\omega}}^{\mathrm{\ensuremath{-}}1}$ and the real part of the impedance per cell varies as ${\ensuremath{\omega}}^{\mathrm{\ensuremath{-}}3/2}$. Finally, the results are shown to satisfy the requirements of causality for an ultrarelativistic beam.