Vertical Cryostat Research Articles

Challenges and Lessons Learned From Fabrication, Testing, and Analysis of Eight MQXFA Low Beta Quadrupole Magnets for HL-LHC

By the end of October 2022, the US HL-LHC Accelerator Upgrade Project (AUP) had completed fabrication of ten MQXFA magnets and tested eight of them. The MQXFA magnets are the low-beta quadrupole magnets to be used in the Q1 and Q3 Inner Triplet elements of the High Luminosity LHC. This AUP effort is shared by BNL, Fermilab, and LBNL, with strand verification tests at NHMFL. An important step of the AUP QA plan is the testing of MQXFA magnets in a vertical cryostat at BNL. The acceptance criteria that could be tested at BNL were all met by the first four production magnets (MQXFA03-MQXFA06). Subsequently, two magnets (MQXFA07 and MQXFA08) did not meet some of the criteria and were disassembled. Lessons learned during the disassembly of MQXFA07 caused a revision to the assembly specifications that were used for MQXFA10 and subsequent magnets. In this paper, we present a summary of: 1) the fabrication and test data for all the MQXFA magnets; 2) the analysis of MQXFA07/A08 test results with characterization of the limiting mechanism; 3) the outcome of the investigation, including the lessons learned during MQXFA07 disassembly; and 4) the finite element analysis correlating observations with test performance.

The MBRD Dipoles for the Luminosity Upgrade at the LHC: From Prototype Tests to the Series Production

The recombination dipoles MBRD for the High Luminosity upgrade of the Large Hadron Collider (HL-LHC) at CERN are double-aperture superconducting magnets generating a central magnetic field of 4.5 T in a 105 mm diameter bore, directed in the same direction in both apertures. The integrated magnetic field is 35 T-m in a magnetic length of 7.78 m: with respect to the corresponding magnet presently installed in LHC, the aperture is larger, the length is smaller and the central field is higher. The project, currently underway, includes the fabrication by ASG Superconductors and testing at CERN of a short model (1.6 m long), a prototype, and a series of 4 + 2 spare dipoles. The short model was delivered to CERN and successfully tested in a vertical cryostat in summer 2020, reaching nominal current after three quenches in the second thermal cycle, validating most of the mechanical, thermal and electrical design and giving indications for improvements that have been implemented in the prototype, which was completed and delivered to CERN in October 2021. It was tested in October 2022 in its final cold mass, 14 m long, which also includes the two orbit correctors. This contribution reports on key results from the prototype tests and on the further activities related to the construction of the first magnets of the series.

Magnetic Measurements of a Full-Scale Prototype of the HL-LHC Beam Separation Dipole

High Energy Research Organization (KEK) has engaged in development of the beam separation dipole, MBXF, toward the HL-LHC project. The magnet has a full length of 7 m and generates a magnetic field of 5.6 T in the magnet center at nominal operating condition of 12 kA and 1.9 K. Optimization of the magnetic model is performed through two and three dimensional studies and iterated until resultant field quality is within required tolerance. Field quality of the first full-scale prototype of MBXF (a.k.a. MBXFP1) is measured first at room temperature on the premises of the manufacturer, Hitachi Ltd. This result is used for comparison with the numerical models and the first validation of our design methodology. After then a further model tuning is considered for the magnetic design of the first series. The final validation of our design methodology is performed after the cold test of MBXFP1 in the vertical cryostat in KEK facilities. Finally we determine a magnetic design of the first series production with an expected sextupole integral and decapole integral, which are major systematic contributions to the field quality of MBXF, estimated to be <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$0.0 \pm 1.6$</tex-math></inline-formula> units and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1.1 \pm 0.2$</tex-math></inline-formula> units, respectively.

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.

Accelerator development at the FREIA Laboratory

The FREIA Laboratory at Uppsala University focuses on superconducting technology and accelerator development. It actively supports the development of the European Spallation Source, CERN, and MAX IV, among others. FREIA has developed test facilities for superconducting accelerator technology such as a double-cavity horizontal test cryostat, a vertical cryostat with a novel magnetic field compensation scheme, and a test stand for short cryomodules. Accelerating cavities have been tested in the horizontal cryostat, crab-cavities cavities in the vertical cryostat, and cryomodules for ESS on the cryomodule test stand. High power radio-frequency amplifier prototypes based on vacuum tube technology were developed for driving spoke cavities. Solid-state amplifiers and power combiners are under development for future projects. We present the status of the FREIA Laboratory complemented with results of recent projects and future prospects.

Improvement of vacuum characteristics of cryostats for HPGe gamma-radiation detectors

Using mass spectrometry methods, the composition of residual gases and its temporal dynamics in experimental cryostats for HPGe detectors of gamma radiation, have been investigated. The studies were carried out for a vertical dipstick cryostat with vacuum chamber of small volume (1200 cm3) and an U-type cryostat having its vacuum chamber combined with vacuum cavity of a Dewar vessel (total volume 36500 cm3). Results of residual gas analysis are presented, showing the effect of Combogetter on a composition of residual gases of warm cryostats. Its use as an auxiliary getter for cryostats with HPGe detectors, maintaining a vacuum during storage or transportation, is considered.

Cryostat for cooling samples in the study of low-temperature structural and magnetic phase transitions by neutron diffraction

A vertical cryostat was developed built around a Sumitomo RDK-101D cold head. The cryostat is intended for investigating the structure and phase transformations of different materials via neutron diffraction experiments on samples under high vacuum of up to 10−5 mbar. A working temperature of the sample close to 2 K was reached. The drift diameter of the head is 54 mm, making it possible to cool samples with a height of 55 mm and 15 mm in diameter. Test measurements were conducted and illustrative spectra were recorded on the example of a sample of HoFeO3. The cryostat was designed and manufactured at the Frank Laboratory of Neutron Physics of the Joint Institute for Nuclear Research.

Retro-Fitting Earth-Field Compensation Coils to the Vertical Cryostat GERSEMI in FREIA

We describe the design and construction of coils to compensate the Earth’s magnetic field in the vertical cryostat GERSEMI in the FREIA laboratory at Uppsala University.

First acceleration of heavy ion beams with a superconducting continuous wave HIM/GSI CW-linac

In the future a new superconducting (sc) continuous wave (cw) high intensity heavy ion Linac should provide ion beams with a max. beam energy above the coulomb barrier for the Super Heavy Element (SHE) program at GSI Helmholtzzentrum für Schwerionenforschung. Key features of this new Linac are the acceleration of different ions from protons to uranium as well as a smooth energy variation from 3.5 to 7.3 MeV/u for design ions with a mass-to-charge ratio A/q = 6 and even above for lighter ions. As a first step a newly developed superconducting 15-gap Crossbar H-cavity (CH-cavity) operated at 217 MHz has been successfully tested with heavy ion beam up to the design beam energy of 1.85 MeV/u for the first time. The design energy gain of 3.5 MV within a length of less than 70 cm has been validated with heavy ion beams of up to 1.5 pμA. The measured beam parameters showed excellent beam quality, while a dedicated beam dynamics layout provides beam energy variation between 1.2 and 2.2 MeV/u. The beam commissioning is a milestone of the R&D work of Helmholtz Institute Mainz (HIM) and GSI in collaboration with Goethe University Frankfurt (GUF) and the first step towards a sc heavy ion cw-Linac with variable beam energy. The first tests under cryogenic conditions of the next two CH-cavities have already been started at GUF in a vertical cryostat. The results of the first successful heavy ion beam acceleration with a superconducting CH-cavity will be presented.

Vertical electropolishing for 1.3 GHz single- and nine-cell superconducting niobium cavities: A parametric optimization and rf performance

Electropolishing (EP) is a standard process for the treatment of the interior surface of niobium (Nb) superconducting radio-frequency (SRF) cavities used in particle accelerators. We previously conducted a study on vertical electropolishing (VEP), in which the cavity was set vertically during EP of a 1.3 GHz Nb single-cell cavity. In that study, we showed that the major cause of asymmetric removal in VEP is the accumulation of hydrogen gas bubbles, generated during the EP process, on the upper half cell of the cavity. Three types of unique rotating cathodes were tested with the aim to resolve the issues of asymmetric removal and rough surface. Appropriate cathode design and parameters reduced the asymmetric removal in the cell and yielded a smooth surface of the cavity. In this study, we have performed an extensive investigation on VEP of both single- and nine-cell Nb 1.3 GHz cavities with a modified cathode. The VEP parameters with single- and nine-cell coupon cavities were investigated, which facilitate an in situ study of the coupon currents at various positions of the cavities and observation from the viewports located near the iris and equator positions of the cavities. The modified cathode and optimized VEP process for the single-cell cavity yielded uniform removal in the cavity cell and a smooth surface of the interior of the cavity. A novel acid flow method that allows the separation of the acid flow in the cavity and cathode housing was employed in the nine-cell cavity to efficiently remove the gas bubbles from the cavity. The optimized acid flow rates in the cavity and cathode housing, along with other adequate parameters including temperature, cathode rotation speed, and applied voltage, made the surface smooth and significantly reduced the removal nonuniformity in the nine-cell cavity. The single- and nine-cell cavities after an average removal of 46 and $130\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$, respectively, were tested in a vertical cryostat. The single- and nine-cell cavities achieved $31\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at a ${Q}_{0}$ value of $9\ifmmode\times\else\texttimes\fi{}{10}^{9}$ and $28.3\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at a ${Q}_{0}$ value of $6.7\ifmmode\times\else\texttimes\fi{}{10}^{9}$, respectively, in the rf tests at the temperature of 2 K. The achieved SRF performances were as good as the baseline performances achieved after the horizontal electropolishing process of the same cavities.

Vertical Test of a 500-MHz SRF Cavity With a Closed-Loop Cryogenic Helium System

A vertical test for a 500-MHz superconducting radio frequency (SRF) cavity succeeded in the RF laboratory of the National Synchrotron Radiation Research Center. The test system, consisting of a vertical test cryostat, an RF power amplifier, a tunable coupler, and a low-level RF control system is integrated with the existing long-distance cryogenic transfer system and radiation shield. Relying neither on a gasbag nor a purifier, the vertical test cryostat is directly connected to the 400-W cryogenic plant that simultaneously provides liquid helium to the SRF module in the Taiwan Light Source. Applying special cleaning, drying, and purging processes, the vaporized helium gas suffers no pollution during the vertical tests. The valve on the return line for the gaseous helium regulates the helium pressure of the vertical test cryostat at an acceptable fluctuation of +/– 100 Pa. The measurement results for a KEKB-type 500-MHz SRF cavity are illustrated herein to demonstrate the integrated performance of this measurement system.

Cryogenics Performance of the Vertical Cryostat for Qualifying ESS-SRF High Beta Cavities

Cryogenics Performance of the Vertical Cryostat for Qualifying ESS-SRF High Beta Cavities

Qualification of a vertical cryostat for MLI performances tests between 20 K and 60 K to 4.2 K

Boil-off calorimetric measurements of Multilayer Insulation (MLI) performance from 20 K-60 K to 4.2 K are being performed at CERN, extending the range of measurements of MLI to cover cryostat applications where thermal shields are operated at lower temperatures than that of LN2. Possible applications include future large accelerators like the Future Circular Collider (FCC). The tests are carried out in an existing vertical test cryostat shielded with a liquid nitrogen cooled vessel and with an inner cylindrical liquid helium tank carrying the wrapped MLI sample. The cryostat has been modified to insert an additional intermediate thermal shield maintained at controlled temperatures in the 20 K-60 K range, by means of regulated power input from an electrical heater. This paper describes the test cryostat and the MLI performance measurement methodology as well as experimental results of the commissioning and calibration of the instrument.

Thermal performance of the new superfluid helium vertical test cryostats for magnet tests at CERN

In the frame of the R&D program of the HL-LHC, the upgrade of the Large Hadron Collider (LHC), CERN augmented its test facility with two new large-scale vertical test stations for superconducting magnet testing. The cryostats composing the core of these test stations share design features but are of different dimensions: one allows testing magnets up to 1.5 m diameter, 2.5 m long for a maximum weight of 15 t, while the other can accommodate magnets up to 0.9 m diameter, and 5.5 m long for a maximum weight of 18 t. These test stations are designed to operate at 1.3 bar and at controlled temperatures in the range 1.9 - 4.5 K. They can provide for safe dissipation of stored energy in the magnet coils of up to 10 MJ. After a brief description of the cryostats, this paper describes the qualification measurements of heat loads at 4.5 K and 1.9 K of both cryostats, some design enhancements that were made and the performance improvements obtained.

First operational experience of the upgraded cryogenic infrastructure of the vertical magnet test facility in the SM18 hall, including HFM and cluster D test stations.

As part of the R&D program for HL-LHC, the future upgrade of the Large Hadron Collider (LHC), the vertical magnet test facility in the SM18 cryogenic test hall at CERN has seen significant development. Since the installation of HFM and Cluster D cryogenic test stations in 2017, the facility is comprised of five vertical test cryostats of various sizes operating at either 1.9 K or 4.2 K. It is now possible to test a wide range of superconducting high field magnets with weigh up to 18 tons, maximum stored energy of 10 MJ and powering requirements up to 30 kA. This article describes the first cryogenic operational experiences with these test stations, their cool-down and warm-up characteristics and the heat in-leak measurements of their components. Major upgrades made to the cryogenic infrastructure of the vertical magnet test facility as a whole are also detailed. This includes the adoption of an improved safety strategy to ensure the continued safe operation of the test facility during all operational modes. As well as the incorporation of quench recovery systems on four of the five test stations to reduce the amount of helium lost during testing and the installation helium guards on infrastructure operating at 1.9 K to avoid the ingress of air to cryogenic circuits.

Quench localisation in SRF cavity tests with transition edge sensors

Transition edge sensors (TES) have recently been developed at CERN as second sound detectors for quench localisation on superconducting radio frequency (SRF) cavities. After validation of the concept of heat source localisation by TES in the laboratory, TES have been implemented as a diagnostic tool in SRF vertical testing cryostats at CERN SM18 facility. Two cavities were forced to quench by applying sufficiently high values of RF power. Simultaneously, RF and TES second sound signals were recorded. In this paper we present the particularities of each of the relevant tests, the nature of the read signals, as well as the analysis performed on them to locate the origin of the quenching spot.

Heat leakage of a vertical test cryostat for superconducting cavity based on variable arranged radiation shield

The vertical test cryostat, which is used to measure the cryogenic performance of a superconducting cavity is a liquid helium open vessel, and has high requirements for thermal insulation performance. The inner vessel of the cryostat is high-vacuum multilayer insulation, and the open side is multi-shield insulation. In order to further reduce the heat leakage through the cover plate of the cryostat at the open side, a variable arranged radiation shield spacing is proposed. The coupling heat transfer model was established for the cover plate, radiation shields, liquid surface and helium gas, and was verified by experiment. The influence of the radiation shields distribution on the heat leakage and the optimal radiation shield arrangement were obtained by numerical calculation. As the number of radiation shields is 11, the method of the variable arranged radiation shield, which is to arrange 7-layer radiation shields in the high temperature region and 4-layer radiation shields in the low temperature region, has a minimal heat leakage. The method can reduce the heat leakage by 4% compared with the equal spacing method.

Design and Fabrication of the 1.9 K Magnet Test Facility at BNL, and Test of the First 4-m-Long MQXF Coil

The future high luminosity upgrade of the Large Hadron Collider (LHC) at CERN will include 20 4.2-m-long Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn high gradient quadrupole magnets, which will be components of the triplets for two LHC insertion regions. In order to test these and four preproduction models, the vertical superconducting magnet test facility of the Superconducting Magnet Division (SMD) at Brookhaven National Laboratory (BNL) has been upgraded to perform testing in superfluid He at 1.9 K and 1 bar, the operational condition at the LHC. This has involved extensive modification of the 4.5 K cryogenics plant, including piping, compressors, and other upgraded components; a new vertical test cryostat, which can accept larger diameter magnets; a modernized power supply system upgraded with IGBT switches and fast shutoff capability, and that can supply 24 kA to test high field Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn magnets; and completely new data acquisition, signal analysis, and control software and hardware, allowing for fast, high precision, large volume data collection. This paper reports on the design, assembly, and commissioning of this upgraded test facility, and presents results of the first magnet test performed.

Cryogenic test facility instrumentation with fiber optic and fiber optic sensors for testing superconducting accelerator magnets

The magnets for the next steps in accelerator physics, such as the High Luminosity upgrade of the LHC (HL- LHC) and the Future Circular Collider (FCC), require the development of new technologies for manufacturing and monitoring. To meet the HL-LHC new requirements, a large upgrade of the CERN SM18 cryogenic test facilities is ongoing with the implementation of new cryostats and cryogenic instrumentation. The paper deals with the advances in the development and the calibration of fiber optic sensors in the range 300 - 4 K using a dedicated closed-cycle refrigerator system composed of a pulse tube and a cryogen-free cryostat. The calibrated fiber optic sensors (FOS) have been installed in three vertical cryostats used for testing superconducting magnets down to 1.9 K or 4.2 K and in the variable temperature test bench (100 - 4.2 K). Some examples of FOS measurements of cryostat temperature evolution are presented as well as measurements of strain performed on a subscale of High Temperature Superconducting magnet during its powering tests.

Impact of Remanent Magnetic Field on the Heat Load of Original CEBAF Cryomodule

The heat load of the original cryomodules for the continuous electron beam accelerator facility is ~50% higher than the target value of 100 W at 2.07 K for refurbished cavities operating at an accelerating gradient of 12.5 MV/m. This issue is due to the quality factor of the cavities being ~50% lower in the cryomodule than when tested in a vertical cryostat, even at low RF field. Previous studies were not conclusive about the origin of the additional losses. We present the results of a systematic study of the additional losses in a five-cell cavity from a decommissioned cryomodule after attaching components, which are part of the cryomodule, such as the cold tuner, the He tank, and the cold magnetic shield, prior to cryogenic testing in a vertical cryostat. Flux-gate magnetometers and temperature sensors are used as diagnostic elements. Different cool-down procedures and tests in different residual magnetic fields were investigated during the study. Three flux-gate magnetometers attached to one of the cavities installed in the refurbished cryomodule C50-12 confirmed the hypothesis of high residual magnetic field as a major cause for the increased RF losses.