Accelerator Tunnel Research Articles

Surface construction vibration impacts on underground tunnels: Approaches for using measurements and finite-element modeling to predict vibration in the frequency and time domain

Quantitative analyses of vibration impacts on underground tunnel structures were carried out on two projects. The first project was a new facility situated above an existing physics laboratory accelerator tunnel embedded in soil where the concern was vibration interference with sensitive research. Published and in-house data for construction equipment source levels were combined with measured transfer functions, between the ground surface and the tunnel, to estimate vibration transmitted to the tunnel in the frequency domain. This case study presents an approach for site-specific evaluations of construction vibration where spectral data are needed for evaluating vibration-sensitive equipment/receivers. The second project was a new facility situated above a storm relief tunnel embedded in rock where the concern was for potential vibration damage to the tunnel. A 3-D finite-element model of the rock and tunnel structure was developed to calculate the peak particle velocity and strain response of the tunnel walls in the time domain. This case study presents an approach for site-specific evaluations of construction vibration where peak velocity amplitudes and strain data are needed for evaluating structural damage potential.

Investigation of the neutron radiation fields inside the accelerator tunnel during operation of the Swiss Light Source (SLS)

The Swiss Light Source (SLS) is a third-generation synchrotron light source with an electron energy of 2.4 GeV. It consists of a linear accelerator, a booster and a storage ring providing light for various applications. As SLS is a high-energy accelerator, its operation is accompanied with production of ionizing radiation fields, including neutrons. This leads to additional challenges from a radiation protection point of view. The long-term investigation of the radiation fields at this accelerator facility can provide useful information for the residual activation studies and optimization of shielding. This work describes measurements of the neutron radiation fields inside the tunnel and results of this study.

Research on methods to suppress the error accumulation in the elevation direction for accelerator tunnel control network

In recent years, the 4th generation synchrotron radiation light sources have gradually emerged. However, the accumulated errors in Laser Tracker multi-station measurements make it challenging to meet the accuracy requirements for the control network measurement, especially in the direction of elevation. To address this issue, this paper proposes to utilize the Hydrostatic Leveling System (HLS) to provide high-precision external constraints for the control network measurement. Firstly, based on the measurement principle of capacitive HLS sensor, a system is designed and constructed which was used to optimize the measurement accuracy of the HLS for level difference between difference points and to obtain the external reference. Comparison of the HLS measured values with the nominal values measured by the CMM shows that the optimized HLS can achieve a level difference measurement accuracy of 5 μm. On this basis, some constraint methods are proposed, which have been verified by simulations and measurements. The results show that the error accumulation in the elevation direction is suppressed compared with the classical adjustment. For the 70 m linear tunnel control network of Hefei Light Source, the maximum error of the measurement is reduced by 90 μm (23% improvement in accuracy) and the standard deviation of the error is reduced by 26 μm (35% improvement in accuracy) after adding constraint.

FCC circumference studies based on RF synchronization

As the study of the Future Circular Collider (FCC) project progresses, the choice of the exact circumference of the accelerator tunnel becomes essential. It is defined by RF-related constraints, mainly the RF frequency of the FCC-hh, as well as the revolution frequency ratios with the hadron injector. It is shown firstly how favourable circumference options assuming the present LHC RF frequency have been identified and ranked from the RF point of view. Secondly, the entire frequency range of interest has been systematically scanned, assuming the possibility of operating SPS or LHC as the potential injector of FCC-hh. Considerations for the RF synchronization between injector and collider have been taken into account. Several options are studied in detail, amongst them the particularly attractive circumferences of 90478.6 m and 90837.7 m, and compared to the present working hypothesis of 90658.2 m. The results are analysed and rated from the point of view of flexibility and longitudinal beam dynamics.

High-precision and high-efficiency measurement method of accelerator tunnel control network based on total station angle observation

Given the importance of accelerator devices, major developed and emerging countries around the world have invested substantial human and material resources into the development, construction, and operation of more advanced large-scale accelerator devices. High-precision and high-efficiency alignment of the control network is crucial for the stable operation of accelerators. This study proposes a method for measuring tunnel control networks using total station angle observation. By taking advantage of the high precision of angle observation with a total station, we use the measured results of the designed and deployed control points measured by the laser tracker to simulate the total station angle observations, combined with some length scale values in the control network measured by the laser range finder. With the angle observations and several known distances, the point coordinates can be calculated by angle intersection methods. Only angle observations are used from total stations, while different length scale reference conditions are added to constrain the control network. This is done by calculating the point accuracy of the China Spallation Neutron Source circular accelerator. The overall plane point accuracy can reach 66 μm. By using this method, personnel will spend less time on the site and are at lower risk of radiation exposure, for example by automating measurements at night. This method can be used at nighttime for measurements, avoiding the need for daytime work and shielding employees from severe radiation exposure. By employing this method, efficiency can be increased twice. For on-site measurement, it enables monitoring and automated measurement during operation, as well as providing reference for the installation and measurement of tunnel control networks for large scale accelerators such as the Circular Electron Positron Collider.

Commissioning experience with two in-vacuum undulators at PETRA III

In 2018 the high energy X-ray beamlines P07[1] and P21b[2] at DESY’s storage ring PETRA III received new undulators. The two identical devices were built by an industry collaboration of Research Instruments GmbH and Kyma S.p.A. according to DESY’s specifications. They have a magnetic length of 4 m, a period length of 21.2 mm and a deflection parameter K of 1.41 at a gap of 7.0 mm. We report about experience during magnet tuning, the final acceptance measurements and the commissioning in the accelerator tunnel.

Research on the layout of primary control network and new measurement method for the HIAF survey and alignment

HIAF (High Intensity heavy ion Accelerator Facility) is one of the major national science and technology infrastructure projects in the “12th Five-Year Plan” undertaken by the Institute of Modern Physics, CAS. The primary control network, as the basic framework of the local coordinate system control of the particle accelerator, is the basis for calculating the coordinates in the process of accelerator construction and implementation, and plays a very important role in the survey and alignment during the construction period and the maintenance and monitoring during the operation period. Based on the multi-dissimilar design structure of HIAF and the unique geological conditions of the construction area, a new high-density layout method of the primary control network and a high-precision 3D(three-dimensional) measurement method were studied, and were successfully applied to the survey and alignment of HIAF. Inside and outside the accelerator tunnel, a high-density 3D primary control network that can form a full envelope for the accelerator beam is respectively laid out. All primary control networks are embedded in the bedrock layer with stainless steel reference inner cylinder and outer protective cylinder, and special waterproof filling layer is set up. All primary control network points in the accelerator tunnel are equipped with measuring intervisible holes and measurement platforms that can directly perform 3D measurements on the ground. By setting up the laser tracker on the horizontal attitude of the measurement platform, the 3D coordinates of the primary control network points inside and outside the accelerator tunnel can be measured directly and with high precision, and the simultaneous adjustment of the measurement data can be completed. The HIAF primary control network layout and measurement method proposed in this paper overcomes the problems of heavy pile foundation, poor crack resistance and poor water seepage resistance of the primary control network laid by the traditional method and solves the problem that the traditional primary control network must separate the plane network and the elevation network for independent measurement. The traditional method of tunnel point and forced centering measurement is avoided. By using a laser tracker to directly measure the 3D coordinates of the primary control network points inside and outside the tunnel, the measurement efficiency and measurement accuracy of the primary control network are improved, and the reliability of primary control network measurements is increased. The feasibility and correctness of the HIAF primary control network layout and measurement method are verified by comparing the actual multi-system measurement results in the field. This method has a certain reference value for the layout of the primary control network of the same type of accelerator.

Comparison of Semi-autonomous Mobile Robot Control Strategies in Presence of Large Delay Fluctuation

We propose semi-autonomous control strategies to assist in the teleoperation of mobile robots under unstable communication conditions. A short-term autonomous control system is the assistance in the semi-autonomous control strategies, when the teleoperation is compromised. The short-term autonomous control comprises of lateral and longitudinal functions. The lateral control is based on an artificial potential field method where obstacles are repulsive, and a route is attractive. LiDAR-based artificial potential field methods are well studied. We present a novel artificial potential field method based on color and depth images. Benefit of a camera system compared to a LiDAR is that a camera detects color, is cheaper, and does not have moving parts. Moreover, utilization of active sensors is not desired in the particle accelerator environment. A set of experiments with a robot prototype are carried out to validate this system. The experiments are carried out in an environment which mimics the accelerator tunnel environment. The difficulty of the teleoperation is altered with obstacles. Fully manual and autonomous control are compared with the proposed semi-autonomous control strategies. The results show that the teleoperation is improved with autonomous, delay-dependent, and control-dependent assist compared to the fully manual control. Based on the operation time, control-dependent assist performed the best, reducing the time by 12% on the tunnel section with most obstacles. The presented system can be easily applied to common industrial robots operating e.g. in warehouses or factories due to hardware simplicity and light computational demand.

Application of recoil-imaging time projection chambers to directional neutron background measurements in the SuperKEKB accelerator tunnel

Gaseous time projection chambers (TPCs) with high readout segmentation are capable of reconstructing detailed 3D ionization distributions of nuclear recoils resulting from elastic neutron scattering. Using a system of six compact TPCs with pixel ASIC readout, filled with a 70:30 mixture of He:CO2 gas, we analyze the first directional measurements of beam-induced neutron backgrounds in the tunnel regions surrounding the Belle II detector at the SuperKEKB e+e− collider. With the use of 3D recoil tracking, we show that these TPCs are capable of maintaining nearly 100% nuclear recoil purity to reconstructed ionization energies (Ereco) as low as 5keV˙ee. Using a large sample of Monte-Carlo (MC)-simulated 4He, 12C, and 16O recoil tracks, we find consistency between predicted and measured recoil energy spectra in five of the six TPCs, providing useful validation of the neutron production mechanisms modeled in simulation. Restricting this sample to 4He recoil tracks with Ereco>40keV˙ee, we further demonstrate axial angular resolutions within 8° and we introduce a procedure that under suitable conditions, correctly assigns the vector direction to 91% of these simulated 4He recoils. Applying this procedure to assign vector directions to measured 4He recoil tracks, we observe consistency between the angular distributions of observed and simulated recoils, providing first experimental evidence of localized neutron “hotspots” in the accelerator tunnel. Observed rates of nuclear recoils in these TPCs suggest that simulation overestimates the neutron flux from these hotspots. Despite this, we estimate these hotspots to produce the majority of neutron backgrounds in the accelerator tunnel at SuperKEKB’s target luminosity of 6.3×1035cm−2s−1, making them important regions to continue to monitor.

LCLS-II helium cryoplant and cryo distribution system installation

The helium cryoplant and cryo distribution system (CDS) are key elements of the new superconducting Linac Coherent Light Source (LCLS-II) and will provide superfluid helium to the accelerator. The cryoplant consists of two helium refrigerators with an equivalent 4.5 K refrigeration capacity of 18 kW each. The 37 cryomodules of the LINAC will be operated at a temperature of 2.0 K to accelerate a 4 GeV electron beam that will generate extremely bright X-ray laser light. Two five-stage cold compressor cold boxes will be utilized to provide superfluid helium II for the superconducting cavity structures with a total cooling capacity of 8 kW at 2.0 K. This paper describes the installation of the LCLS-II cryoplant and CDS. The LCLS-II cryoplant was designed and contributed by Jefferson Lab. To expedite the project completion the reuse of proven design and technology from the Jefferson Lab CHL-2 cryoplant was the preferred strategy. The CDS consists of ∼260 m thermally shielded and vacuum super insulated transfer lines, two distribution boxes and eight feed and end caps, and was designed and contributed by Fermilab. SLAC installed the cryoplant components into a newly erected building and the CDS components into the existing accelerator tunnel and klystron gallery with strong engineering support from both partner labs.

Evidence of increased radio-frequency losses in cavities from the fundamental power coupler cold window

High radio-frequency (rf) losses measured for cavities in original Continuous Electron Beam Accelerator Facility (CEBAF) cryomodules, compared to the losses measured in single-cavity tests, have been a long-standing issue related to their performance. We summarize experimental evidence of increased rf losses in CEBAF cavities arising from the fundamental power coupler cold window and waveguide, respectively. Cryogenic rf tests were done on cavities tested in vertical cryostats as well as inside cryomodules in the accelerator tunnel. The cold window metallization losses were assessed by combining numerical results with measured data obtained with an existing cryogenic waveguide resonator setup. The results showed that the cold window metallization losses can increase the cavity rf heat load at 2.07 K by up to 86%, depending on the standing-wave pattern in the fundamental power coupler waveguide, and that such losses are reduced if the distance between the waveguide and the cavity cells is increased.

Development of Disaster Prevention System for Accelerator Tunnel

Development of Disaster Prevention System for Accelerator Tunnel

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.

Development of a compact Local Power Distribution System for the ILC

The Local Power Distribution System (LPDS) of the International Linear Collider (ILC) transmits RF power from a 10 MW klystron to 39 cavities. A variable hybrid and variable phase shifter are adopted in the LPDS to have been adopted in the LPDS to achieve the efficient beam acceleration in the ILC. As it is difficult to construct the LPDS in an accelerator tunnel, the LPDS can be assembled on a cryomodule on the ground. Then, the cryomodule integrated with the LPDS can be installed in the tunnel. Furthermore, production cost can be decreased by utilizing identical variable hybrids and variable phase shifters. In this study, a compact LPDS was developed to satisfy these two requirements. A compact variable hybrid and variable phase shifter were designed for a compact LPDS. The power transmitted to a cavity was adjusted by the variable hybrid to achieve the maximal accelerating field, which exhibited a variation of ±20% depending on cavity performance. The variable phase shifter compensated the phase shift of the variable hybrid. Fixed phase shifters were designed to adjust the phase difference between adjacent cavities for beam acceleration. These RF components were manufactured, and the S parameters and power handling capability of the components were tested. An L-band resonant ring was constructed at the Super-conducting RF Test Facility (STF) at KEK. The power handling capability of these RF components were tested using a power of 5.2 MW with a pulse width of 200 μs and a power of 2.0 MW with a pulse width of 2 ms. The S parameters and power handling capability satisfied the requirements of the compact LPDS. The compact LPDS will be constructed using these RF components and demonstrated at the STF.

Evaluation of Skyshine from an Accelerator Facility: Dependence on Distance and Angle.

Neutron skyshine from Linac Coherent Light Source II 4 GeV electron beam operation at SLAC National Accelerator Laboratory can contribute to prompt radiation exposure to the public at distances far beyond the accelerator tunnel housing. One of the shielding design requirements at SLAC is that the annual dose to a member of the public is no more than 0.05 mSv y. This study uses Monte Carlo code FLUKA to simulate the generation of neutrons from 4 GeV electron beam losses on a thick copper target inside a generalized geometry of the Linac Coherent Light Source II Beam Transport Hall accelerator tunnel section. The effective dose from neutron skyshine was characterized as a function of both distance from the tunnel wall (up to 1 km away) and angle relative to the beam direction (between 0° and 180°). This new methodology for evaluating neutron skyshine dose is applicable to high-energy GeV-range electron accelerator facilities both at SLAC and elsewhere.

High-energy proton irradiation effects on GaN hybrid-drain-embedded gate injection transistors

The characterization of commercial-grade power transistors upon high levels of particle irradiation is required to enable radiation tolerant LED power supplies for the new luminaires of CERN accelerator tunnels, which represent a harsh environment for semiconductor devices. This work describes the effects of 24 GeV/c proton irradiation on commercial GaN hybrid-drain-embedded gate injection transistors (HD-GITs) after a fluence of 5.9 × 1014 p/cm2. Measurements of drain leakage current, threshold voltage and Ids − Vds curves show that only a minor variation occurs in the electrical properties of GaN HD-GITs after the considered fluence; for example, an average increase of ≈11–13 mV is found in the threshold voltage upon irradiation. We also put forward a physical explanation of the observed degradation caused by proton irradiation; in particular, the electron drift velocity in the 2DEG channel at high electric fields appears to decrease due to a radiation-induced increase in phonon relaxation rate. Finally, an AC/DC LED power supply with current control using GaN HD-GITs is proposed for the new luminaires of CERN tunnels, meeting the requirements in terms of radiation hardness and light quality.

Compact single-shot electro-optic detection system for THz pulses with femtosecond time resolution at MHz repetition rates.

Electro-optical detection has proven to be a valuable technique to study temporal profiles of THz pulses with pulse durations down to femtoseconds. As the Coulomb field around a relativistic electron bunch resembles the current profile, electro-optical detection can be exploited for non-invasive bunch length measurements at accelerators. We have developed a very compact and robust electro-optical detection system based on spectral decoding for single-shot longitudinal bunch profile monitoring at the European X-ray Free Electron Laser (XFEL) for electron bunch lengths down to 200 fs (rms). Apart from the GaP crystal and the corresponding laser optics at the electron beamline, all components are housed in 19 in. chassis for rack mount and remote operation inside the accelerator tunnel. An advanced laser synchronization scheme based on radio-frequency down-conversion has been developed for locking a custom-made Yb-fiber laser to the radio-frequency of the European XFEL accelerator. In order to cope with the high bunch repetition rate of the superconducting accelerator, a novel linear array detector has been employed for spectral measurements of the Yb-fiber laser pulses at frame rates of up to 2.26 MHz. In this paper, we describe all sub-systems of the electro-optical detection system as well as the measurement procedure in detail and discuss the first measurement results of longitudinal bunch profiles of around 400 fs (rms) with an arrival-time jitter of 35 fs (rms).

A remote-controlled robot-car in the TPS tunnel

A remote-controlled robot-car named “PhotonBot” was put into the TPS accelerator tunnel and is equipped with a 360 ° LiDAR for SLAM and navigation, two cameras for perception and first-person view, and a thermal imaging system. The robot can be remotely controlled and can send data to a remote PC through Wi-Fi. With SLAM, it can go more freely without being restricted to a designated path. In order to ensure it can work continuously, there is a wireless charging station in case of a low battery.

The First Replacement of the RF Window of the ACS Cavity

In 2013, the Annular-ring Coupled Structure (ACS) cavities were installed to the Japan Proton Accelerator Research Complex (J-PARC) linac. Since then, the ACS cavities have been stably running. Although any serious problem induced by the RF window had not yet observed, we decided to replace the RF window of one ACS cavity by the newly manufactured one. The major motivations of the replacement are to check the surface condition of the RF window which have been under operation for nearly five years, to confirm that the new RF window fully meets specifications, and to learn how much time is required for high-power conditioning of the new RF window. By making use of the summer maintenance period of 2018, we carried out the replacement. This was the first experience for us to replace the RF window installed to the ACS cavity in the linac accelerator tunnel. As for the removed RF window, there was no any abnormal warning found with the visual examination. At the resuming of the cavity operation after the maintenance period, we conducted the high-power conditioning in a measured manner. It took around fifty hours so that the targeted peak power was stably input to the cavity through the new RF window. The ACS cavity with the new RF window is now stably operating.

Dosimetry Mapping of Mixed-Field Radiation Environment Through Combined Distributed Optical Fiber Sensing and FLUKA Simulation

Distributed optical fiber (OF) radiation sensing is a technique recently adopted at CERN for the monitoring of radiation dose levels in the proton synchrotron (PS) booster and in the PS. In this paper, we present the progress made on our previous work by benchmarking the rich information contained in the spatially distributed OF dose measurement with the radiation levels simulated with the FLUKA Monte Carlo code. We begin by discussing the experimental results for the qualification/calibration of a new radiation sensitive single-mode OF, currently being deployed in the accelerator tunnels. We performed this calibration tests online under 60Co $\gamma $ -rays source. We then move on to report the results of the online irradiation tests that we carried out in the CERN high energy accelerator mixed-field facility. Finally, we compare the OF radiation maps with the dose levels calculated along the OF cable with FLUKA for two very different configurations of the facility. This paper is a first promising example of combining 3-D Monte Carlo simulations of complex mixed-field environment with the 1-D spatially distributed dose measurements provided by the OF-based dosimetry.