CERN Accelerating science

  World-record current in the MgB2 superconductor
  by Thomas Hind (CERN) and Amalia Ballarino (CERN)

          
Fig 1, left. The 20-metre long electrical transmission line containing the two 20 kA MgB2 cables.
Image credit: CERN
Fig 2, right. Members of the CERN Superconductors and Superconducting Devices team in front of the test station. Image credit: CERN

Members of the CERN Superconductors team have achieved a world-record current in a Magnesium Diboride (MgB2) superconductor.

A current of 20kA was transferred at 24K in an MgB2 electrical transfer line developed at CERN. The line consists of two cables, each 20m long, made from MgB2 round wires and connected in series, with the cooling provided by a forced flow of helium gas. After operation at nominal current, a series of quench tests at 20 kA and a complete thermal cycle from room temperature to nominal operation temperature were successfully performed.

MgB2’s superconducting properties were discovered in 2001, but conductor technology only existed in the form of tape at the time. Round wire was not available when the project started, but had to be developed. The cables and associated technologies were designed and tested and CERN, with the superconducting wire the result of a joint R&D effort between CERN and Columbus Superconductors in Genova, Italy.

The project is part of the FP7 Hi-Lumi LHC Design Study, which aims to move the power converters supplying current to the superconducting magnets either to the surface or to radiation free underground areas and to use high temperature superconducting transmission lines to connect them.

These cables have further uses outside of particle physics, and their use has been proposed for innovative transmission lines used for long-distance transport of green power. The test results show that the MgB2 cables can be operated at and above the temperature of liquid hydrogen and that the basic related technology is now proven.

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  Novel Materials for Multi TeV Beam Collimation
  by Alessandro Bertarelli (CERN) and Stefano Redaelli (CERN)

 
Fig 1. Close up of Molybdenum Graphite sample at nanometre scale. Image credit: CERN

Collimators must be capable of handling impacts of intense and HEP pulses and operating safely over an extended range of temperatures and pressures, in harsh radiation environments, while minimizing the perturbations to the circulating beam through impedance effects. These challenges are even more demanding with the increase of stored beam energy in future machines. No existing material can meet all the beam collimation challenges at the same time. 

A far-reaching R&D program was launched at CERN by the LHC collimiation project with support from EuCARD2 and HiLumi LHC. In EuCARD2, the goal is to develop and test without and with beam, novel materials with high shock resistance and excellent thermal conductivity, replacing or complementing presently used collimator materials. In HiLumi, the impact of new materials on the collimation system performance is studied. Simulations of beam loss and energy deposition are used to define collimation requirements for the HL LHC. So far, the novel materials showing the most promising features are Copper-Diamond and Molybdenum Carbide – Graphite (MoGR) composites. The latter has recently achieved outstanding properties, combining an extremely low coefficient of thermal expansion in the temperature range from Room Temperature (RT) to more than 2000°C and a world record-breaking thermal conductivity of about 800 W/mK at RT. MoGR has a density of 2.6 g/cm3.


Fig 2. Molybdenum Graphite sample. Image credit: CERN

These novel materials must undergo a qualification with beam before being deployed in particle accelerators. The collimation material R&D activity has strong synergy with other domains of science where thermal shock resistance and high temperature operability are needed: braking systems for high-end automotives and aerospace, thermal management in high power electronics, hot parts for gas turbines and space components requiring extreme geometrical stability are but a few examples of potential applications.

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  Crab Cavity RF System
  by Thomas Hind (CERN) and Rama Calaga (CERN)

 
Fig 1. 400 MHz Tetrode Amplifier. Image credit: Eric Montesinos (CERN)

As part of the HL-LHC upgrade, a conceptual RF system layout for a local crab crossing scheme has been presented.

Four cavities, grouped into pairs, on each side of the collision point (IP) per beam are required to produce the transverse kick to correct the geometric effects at the collision point. A two-cavity configuration also allows for good sectorization of the cavities, both for spare policy and maintenance.

Each cavity will have an independent powering system for precise control and reliable operation of the cavities. The input power coupler will use a single coaxial disk window to separate the cavity vacuum and the air side. These powering systems are assumed to use two 40kW LEP type Tetrodes modified to 400MHz to deliver the specified 80kW to cope with beam and cavity transients with some additional margin.


Fig 2. Schematic of the crab cavity layout on one side (not to scale) Image credit: Rama Calaga (CERN)

The cavity controls consists of a fast loop to ensure a rapid response time (around 1 microsecond) and a central (slow) control loop, which performs the task of field and phase control in the cavity.

A proof of principle test will be carried out in the SPS as a pre-requisite before an installation in the LHC.

Due to the limited space available in the interaction region, a detailed study is underway to determine the best option to fulfil the RF requirements in a cost effective manner.

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  Pile-up management at HL-LHC and Crab-Kissing
  by Stephane Fartoukh (CERN) with Agnes Szeberenyi (CERN)

 
Crab-kissing collision to level the luminosity and strongly mitigate the pile-up density at HL-LHC.
Image credit: S. Fartoukh (CERN)

The HiLumi LHC Design Study investigates novel technologies and concepts to extend the discovery potential of the LHC. The overall scheme targets an integrated luminosity higher by a factor of 10 with respect to the LHC, but without compromising on the quality of the data delivered, both in terms of total number of interactions per bunch crossing (pile-up), but also their line density along the luminous region. A novel collision scheme might lead the way to succeed.

This so-called crab-kissing scheme is complementary to the baseline crab-crossing scheme already in use. It assumes, however, that additional crab-cavities are installed in order to achieve a bunch rotation in the plane perpendicular to the crossing plane (the so-called parallel separation plane). In this way the two beams collide with a side-slip angle in one of the two transverse planes, for levelling the collision time and therefore the luminosity, while mitigating the interaction rate at the centre of the luminous region, and therefore spreading out the events more efficiently over the luminous region.

Assuming a double harmonic radio-frequency system for the HL-LHC, as already planned for other purposes, the longitudinal bunch distribution can be made more uniform. Then combined with the crab-kissing scheme, a pile up density of only 0.5-0.6 event /mm is within reach for the HL-LHC, which would definitely maximise the efficiency of the new detector and might even be partly used to further increase peak and integrated luminosity of the HL-LHC

The connection between pile-up density and crab-cavity via the crab-kissing scheme, gives an additional boost to this very challenging RF device which remains a keystone of the HL-LHC project. First beam dynamics analysis were reported at the 6th crab-cavity workshop CC13, validating the soundness of the proposal, in particular in terms of beam-beam effect.

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  From the FP7 HiLumi Design Study towards the High Luminosity LHC
  by Agnes Szeberenyi (CERN) and Livia Lapadatescu (CERN)


Group picture from the 3rd Joint HiLumi LHC-LARP Annual Meeting 2013.
Image credit: HiLumi LHC.

To mark the approval of the High Luminosity LHC Programme by the CERN Council in June 2013, the 3rd Joint HiLumi LHC-LARP Annual Meeting was held in conjunction with the HL-LHC kick-off.

The event took place at the Daresbury Laboratory in the UK between 11-15 November 2013, with an outstanding attendance of more than 160 scientists from all over the world, including Japan, Russia and the US. Directors of major accelerator laboratories were present as invited prominent speakers.

The first part of the event covered the kick-off of the HL-LHC project, advanced as a first priority for CERN in the years to come. Following the format of the previous annual meetings, the second part of the event focused on the progress in design and R&D of the FP7 HiLumi Design Study. Plenary and work package parallel sessions were organized and focused on the work of the Parameter and Lay-Out Committee, the collaboration with LARP, the achievements and reports from work packages. The outcomes and recommendations of the CERN Review of LHC & Injector Upgrade Plans Workshop (RLIUP) were also reported.

The 4th Joint HiLumi Annual Meeting is planned to take place in November 2014 at KEK, the High Energy Accelerator Research Organization in Japan.

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  Successful test of HQ02: Nb3Sn is getting closer to proton beams
  by Ezio Todesco (CERN)

 
Fig.1, left: Ramp rate dependence of b3 on HQ01 without cored cable. Image credit: LARP collaboration.
Fig.2, right: Ramp rate dependence of b3 in HQ02 with cored cable. Image credit: LARP collaboration.

US LHC Accelerator Research Program (LARP) scientists in collaboration with CERN under the framework of HiLumi LHC are working on the development of the Nb3Sn technology to produce accelerator magnets that are able to operate at 12 T peak field in the coil. After the first successful test of HQ02 further tests are on-going to test other critical aspects of the magnet.

HQ02 is the first magnet that makes use of a “cored” cable, i.e. a 25-mm-thick strip between the superconducting strands to increase the resistance between strands. In case of Nb-Ti LHC magnets, the interstrand resistance was controlled through an oxidation of the strands. This method cannot be used for the Nb3Sn which becomes superconductive after a heat treatment at 650 ºC lasting more than one day. First results show that the field quality versus current is extremely smooth compared to the behaviour of a magnet without cored cable. The core also allows reducing the heating induced by a fast ramping, so that the magnet is superconductive even at ramp rate higher than nominal.

In its first test, HQ02 reached the power supply limit of 15 kA, corresponding to operational current at 1.9 K, with a few quenches. The second powering is being done at 2.2 K, after a warm up and cool down, showing a perfect memory (i.e. the magnet was able to reach the same current as before warm up without any quench), and capability of operating at 16 kA, i.e. a margin of 1 kA. The powering is still in progress and a special program is now on-going to test other critical aspects as quench protection. The new inner triplet to be installed at the beginning of next decade will be based on this design, taking all the features that have been successfully proved by LARP.

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  New developments in unconvetional RF structures
 
by Graeme Burt (ULANC) with Mathilde Chaudron (CERN)

 
Fig 1: 4-rod crab cavity for HL-LHC. Image credit: ULANC.
Fig 2: 56 MHz quarter wave cavity. Image credit: BNL.

A lot of attention is paid to getting very high accelerating gradient RF structures, either for superconducting or normal conducting machines. Most of these structures use the TM010 accelerating mode in elliptical or pillbox type cavities, similar to those used in the LHC, however there are several other types of structures under development and not all of them are for acceleration.

Crab cavities are required to rotate bunches prior to collision and are being developed for LHC, ILC, CLIC and SPX. Quarter-wave cavities and capacitive loading of cavities have been tested by Lancaster University/CERN and ODU/Jlab. Another type of dipole RF structure is the RF undulator which uses RF fields to deliver short period undulators that can vary polarisation very quickly.

There are also developments in SRF monopole cavities. Niowave and ODU are developing spoke resonators for compact light sources, while BNL has developed a low frequency SRF gun based on a quarter-wave structure. Also for acceleration and for deflecting, several photonic bandgap (PBG) cavities are being developed, high gradient tested by SLAC and MIT, while LANL has been developing an SRF photonic cavity to extract HOMs. Additionally, a novel idea from Yale and Manchester University is to have a cavity that operates at two harmonics.

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  Collimator upgrades toward HL-LHC 
  by Mathilde Chaudron (CERN) with Stefano Redaelli (CERN)

 
3D drawing of the by-pass cryostat/collimator assembly designed to install a warm collimator in the cold dispersion suppressor between two 11 T dipoles that would replace a standard dipole. Image credit: A. Bertarelli (CERN).

In the frame of the LHC upgrades towards the High Luminosity LHC (HL-LHC), the improvement of the LHC collimation system is a critical aspect. The Achromatic Telescopic Squeezing (ATS) optics, foreseen to be used in the HL-LHC, introduces major changes to the optics in the experimental Interaction Regions (IR).

The development carried out within the HiLumi LHC Work Package 5 aims at verifying that the cleaning of beam halo (the safe and controlled removal of the unavoidable beam losses by collimators during standard operation) and losses in the high-luminosity experimental regions remain appropriate for all the HL-LHC challenges, including higher beam stored energies and optics layout changes. Proposed collimation upgrades in Long Shutdown 1 (LS1) and LS2 are designed to be compatible already with the ultimate HL-LHC requirements.

Large losses are created by the collisions inside the LHC experiments. These so-called "Physics debris" losses have to be studied separately from beam halo matters in order to make sure that the local protection of the cold magnets downstream of the experiments is appropriate, i.e. that they can operate below the quench limit and that radiation damage remains under control. The studies performed so far indicated that the proton beam operation in IR1 and IR5 until LS3 could be compatible with the expected LHC parameters with an upgrade layout proposed for implementation in LS1. On the other hand, collision losses during ion beam operation will induce losses well above the quench limit of superconducting magnets in the dispersion suppressor. This calls for an action that might be already taking place in LS2, with priority given to IR2 where the ALICE detector is installed.

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  A first layout for the High Luminosity Upgrade
  by Ezio Todesco, Stephane Fartoukh (CERN)

 

The tentative layout for the insertion: sequence of magnets versus distance to the interaction point in ATLAS or CMS (0 m is the centre of the experiment). The flags indicate a possible origin of the in-kind contribution. 
Image credit: HiLumi LHC

A first baseline for the layout of the High Luminosity inner triplet and associated magnets has recently been defined. This is the second major milestone, after the choice of a 150 mm aperture for the quadrupoles in July 2012.

A layout is the selection of the sequence of magnets, their strength, their length, and the associated technology: therefore, it requires inputs both from the optics and from the magnet teams. Its definition is a delicate equilibrium between the search for maximum performance and the need of minimizing complexity and associated risks. Nb3Sn technology had been already selected for the triplet quadrupoles – these magnets are planned to be built with a CERN-US collaboration, heavily relying on the work carried out by LARP in the past 10 years.

After the triplet, a separation dipole of 5.5 T, in Nb-Ti, is being studied by the Japanese team in KEK. The layout is complemented by the challenging orbit correctors, also based on Nb-Ti technology, with nested coils both providing horizontal and vertical field of up to 2 T. A package of higher order correctors relying on superferric technology is also available, in order to correct for the inevitable field imperfections of the inner triplet at the 10-4 level. The triplet corrector package is based on the design developed in Spain by CIEMAT.

As a result, the energy deposition team can start simulations to have a precise estimate of the radiation damage and heat loads coming from the collision debris. It is a heavy shower and magnets will need thick shielding to avoid falling in pieces before reaching the project ambitious goal of 3000 fb-1. The layout has been presented at CERN and will be extensively discussed at theHiLumi/LARP collaboration meeting, 8-10 April 2013, Napa Valley.

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Superconducting Technologies for the Next Generation of Accelerators
  by Marina Giampietro (CERN), Lucio Rossi (CERN), Agnes Szeberenyi (CERN)


Fig1: High Luminosity LHC will rely on cutting-edge 13 Tesla superconducting magnets, very compact and ultra-precise superconducting cavities for beam rotation, and 300-metre-long high-power superconducting links with zero energy dissipation. Image credit: CERN

CERN, in collaboration with TIARA and HiLumi LHC European FP7 projects,  organized the Workshop on Superconducting Technologies for Next Generation of Accelerators at the Globe of Science and Innovation on 4th and 5th December, 2012.

The 2-day event focused on key areas such as the development of high- and low-field superconducting magnets, superconducting cavities, cryostats and superconducting links.

The workshop was the one of a series of initiatives at CERN aimed at connecting research infrastructures, facing specific technical challenges, with potential commercial partners, for R&D collaborations and knowledge exchange. More than 100 specialists, about half from industry and half from research laboratories and institutes, met to exchange information on: technologies, processes, materials, facilities, work organization and training of next generation engineers and technicians. In the workshop talks form HiLumi LHC, TIARA and ESS were complemented by presentations and booth stands from companies in forefront research of superconductivity.

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