All-depth dispersion cancellation in spectral domain optical coherence tomography using numerical intensity correlations

In ultra-high resolution (UHR-) optical coherence tomography (OCT) group velocity dispersion (GVD) must be corrected for in order to approach the theoretical resolution limit. One approach promises not only compensation, but complete annihilation of even order dispersion effects, and that at all sample depths. This approach has hitherto been demonstrated with an experimentally demanding ‘balanced detection’ configuration based on using two detectors. We demonstrate intensity correlation (IC) OCT using a conventional spectral domain (SD) UHR-OCT system with a single detector. IC-SD-OCT configurations exhibit cross term ghost images and a reduced axial range, half of that of conventional SD-OCT. We demonstrate that both shortcomings can be removed by applying a generic artefact reduction algorithm and using analytic interferograms. We show the superiority of IC-SD-OCT compared to conventional SD-OCT by showing how IC-SD-OCT is able to image spatial structures behind a strongly dispersive silicon wafer. Finally, we question the resolution enhancement of 2–? that IC-SD-OCT is often believed to have compared to SD-OCT. We show that this is simply the effect of squaring the reflectivity profile as a natural result of processing the product of two intensity spectra instead of a single spectrum

Fast spectrally encoded Mueller optical scanning microscopy

Mueller microscopes enable imaging of the optical anisotropic properties of biological or non-biological samples, in phase and amplitude, at sub-micrometre scale. However, the development of Mueller microscopes poses an instrumental challenge: the production of polarimetric parameters must be sufficiently quick to ensure fast imaging, so that the evolution of these parameters can be visualised in real-time, allowing the operator to adjust the microscope while constantly monitoring them. In this report, a full Mueller scanning microscope based on spectral encoding of polarization is presented. The spectrum, collected every 10 μs for each position of the optical beam on the specimen, incorporates all the information needed to produce the full Mueller matrix, which allows simultaneous display of all the polarimetric parameters, at the unequalled rate of 1.5 Hz (for an image of 256 × 256 pixels). The design of the optical blocks allows for the real-time display of linear birefringent images which serve as guidance for the operator. In addition, the instrument has the capability to easily switch its functionality from a Mueller to a Second Harmonic Generation (SHG) microscope, providing a pixel-to-pixel matching of the images produced by the two modalities. The device performance is illustrated by imaging various unstained biological specimens

Cryogenic operational experience from the LHC physics run2 (2015 – 2018 inclusive)

With end of year 2018 the LHC has completed its second physics run and started its second two-years long shut down period dedicated to planned consolidation, maintenance and upgrade activities. The run2 – four-year physics operation period started in spring 2015 – was used mainly for luminosity production but also to allow the optimization and adaptability of the cryogenic system capacity to compensate for generated operational heat loads. Several tests and qualifications were studied and applied to the configuration of the available equipment in order to reach a deep understanding of the real operation limits. Dedicated global improvements were implemented in the control system, especially in regards of handling the beam induced dynamic heat load during transitory operational states. Adequate modifications were also applied for the Inner Triplet magnets control system to compensate for dynamic heat load related to secondaries, close to the interaction points of the ATLAS and CMS detectors. This paper will give a general overview of the LHC cryogenics operation with specific information on encountered operational difficulties and applied solutions on the system. Helium inventory management, including process use and leaks, as well as the system overall availability indicators will be presented

Cryogenics Experience with High Luminosity Running & Feedforward Controls

The first part of the presentation will summarize the overall cryogenic performance and availability for 2017 and the expected performance for 2018 taking into account the beam induced heat loads in the inner triplets magnets and the beam screen circuits. The second part will focus on the update of the expected cryogenic limitations as well as on the available cryogenic power studies and associated tuning in order to deliver the required cooling power with respect to the 2018 beams operational conditions. The feedforward controls and refrigeration capacity improvements on the inner triplets and beam screen circuits will be discussed

Heat load profile estimates on LHC beam screens by thermal transient analysis

The LHC beams are producing significant dynamic heat loads on the LHC cryogenic system. These heat loads are deposited on beam screens, where they must be properly extracted with dedicated cooling loops between 4.6 K and 20 K. Since 2015, unexpected beam-induced heat loads are observed in specific locations of the machine and their origin is still not completely understood. In order to improve our understanding on the heat load origin and on the spread between them on different areas of the accelerator, the thermal transients occurring after the beam dumps have been analyzed to reconstruct the heat load profiles of the beam screens. Following a description of this specific issue, the paper presents the methodology used for the measurements and estimates of the heat load profiles and its validation against some experimental data and dynamic simulations

Cryogenics Experience during Run 2 and impact of LS2 on next run

The first part of the presentation will summarize the overall cryogenic performance and availability for Run 2. Main evolutions and effects on the cryogenic system during Run 2 will be presented. The second part will focus on the LS2, in particular the main maintenance and evolutions engaged. We will present their expected effects on the global availability on one hand and their expected influence on the cryogenic limitations with respect to the required cooling power during Run 3 on the other hand. Finally, we re-formulate the expected cooling power limitations taking into account the balance between 1.9 K cooling power used for Inner Triplet and 4.5-20 K non-isothermal cooling power used for Beam Screens

Electron cloud observations and mitigation for the LHC Run 3

When operated with the nominal bunch spacing of 25 ns, the Large Hadron Collider (LHC) suffers from significant electron cloud effects. During the second operational run (Run 2) of the LHC, beam-induced conditioning allowed a satisfactory exploitation of 25 ns beams for luminosity production but could not fully suppress electron cloud formation. It has since been understood that this limitation was due to a degradation of some of the beam screen surfaces that occurred with beam operation after air exposure during the first long shutdown period. In the LHC Run 3, several electron cloud effects are expected to become even more important due to the increase in bunch intensity foreseen during the run. In addition, the beam screens have again been exposed to air during the preceding shutdown period, leading to a reset of most of the conditioning acquired in Run 2 and opening the possibility for further degradation. In this contribution, we describe the experimental observations of electron cloud effects during operation with beam after the start of Run 3 in 2022 and discuss their implications for future operation and mitigation strategies for the remainder of the run

Process control system evolution for the LHC Cold Compressors at CERN

The Large Hadron Collider (LHC) operates using superfluid helium provided by eight large refrigeration units (2.4 kW @ 1.8 K each). These units supplied by specialized cryogenic industrial suppliers are composed of serial hydrodynamic cold compressors based on an axial-centrifugal impeller coupled with volumetric warm screw compressors. The process control systems delivered by the suppliers have been installed, commissioned and operated reliably for more than 13 years. However, the implemented process control closed configuration approach limits of the operational diagnostic, and required operational flexibility and adaptability of the cold compressors systems. In the frame of the CERN evolution of process control standards, the LHC cryogenic operational requirements together with the end of electronic components life cycle has motivated an upgrade of the whole process control system. Through a step-by-step analysis process including the initial operational risk analysis, CERN has conceived and engineered two prototypes with their dedicated functional analysis and process control logic to cover the complete range of system operation. These prototypes have been initially fully tested in an off-line configuration and after that validated in real system operation. This paper presents the whole process, the successful results obtained and the perspectives for the future deployment during the LHC Long Shut-Down 2 (2019-2020) period