Triplet magnets program progressing on both sides of the Atlantic

Panagiotis Charitos — Tue, 06 Dec 2016 11:19:36 +0000

Triplet magnets program progressing on both sides of the Atlantic
by G. Ambrosio, P. Ferracin, E. Todesco (CERN)

The Nb3Sn 150 mm aperture quadrupoles MQXF, to be installed in the inner triplets around ATLAS and CMS in 2024-5, are entering a critical phase; the first two 1.5-m-long models have been manufactured and tested since the beginning of this year.

This magnet development program, carried out as a joint effort between CERN and US LARP foresees the construction and testing of five 1.5-m-long models to validate the design and fine tune the assembly features during 2014-17.

These magnets rely on the Al shell and bladder&key structure, allowing easy and fast disassembly, and a precise tuning of the coil prestress. Mechanics is a critical part in the design of these large aperture quadrupoles, featuring an 11.4 T peak field in the coils (50% larger than the peak field in the LHC dipoles operating at 6.5 TeV).

The first model, MQXFS1, was assembled in the U.S. with two CERN coils and two LARP coils, and was confirmed to fulfil performance requirements in April 2016 (see Figure 1). The performance requirements included a) reaching the ultimate current (8% higher than the nominal current of 16.4 kA), and b) reaching nominal current after a thermal cycle with at most one quench.

Figure 1: Training of MQXFS1: quenches (markers), nominal and ultime current (solid lines) and short sample limit (dotted line). (Credit: HL-LHC WP3 collaboration)

The memory after thermal cycle has outperformed expectations by exceeding ultimate current in the first quench after the thermal cycle. However, training has been slower than expected, reaching nominal current after nine quenches. After this first cycle of testing, the transverse pre-stress in the magnet was increased by 30%, to ensure a better support to the coils.

In October 2016, the second assembly was tested at FNAL, reaching 18.8 kA; which is 15% more than the nominal current, and close to 90% of the maximum theoretical performance of the magnet. Some detraining has been observed sporadically, reducing the magnet performance but keeping it always well above the nominal current.

At 4.2 K the magnets shows the ability to reach the same current, thus demonstrating the existence of a considerable margin in temperature, meaning the magnet should tolerate local heating).

The second model, MQXFS3, (MQXFS2 has been postponed to 2017) has been tested at CERN in October 2016, using a novel test station (HFM) planned to be used for the Fresca II dipole.

The magnet reached nominal current with nine quenches, as MQXFS1, but reached a current only 4% above nominal after 20 quenches. A significantly larger detraining than in MQXFS1 was observed, pushing the magnet performance well below nominal (15.0 kA).

Nonetheless, the maximal performance of 17.2 kA has been recovered after ramp rate tests. In addition, 4.2 K test, shows the same performance reached at 2.1 K and also demonstrates the existence of a considerable temperature margin.

Training of MQXFS3: quenches (markers), nominal and ultimate current (solid lines) and short sample limit (dotted line). (Credit: HL-LHC WP3 collaboration)

Work is now focussed on understanding the relationship between the quenches and the mechanical structure. As quenches are mainly located in the coil heads the longitudinal preload will be increased. Further testing after the thermal cycle is expected for the end of the year and . three additional models are foreseen in 2017.

The program will run in parallel with the development of the long coils (4.2 m in US and 7.15 m in CERN) required for the full size magnets.

“The short model program is a fundamental tool to master the design and construction of superconducting magnets, and it is even more important for a novel technology as Nb3Sn” says L. Bottura, leader of the CERN Magnet, Superconductors and Cryostat group. “If needed, we will prolong the short model program to improve our understanding and to reduce the risks in the construction of the prototypes and of the series.”

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HiLumi

LHC - Higgs factory

Margarita Synanidi — Thu, 27 Nov 2014 09:05:58 +0000

LHC - Higgs factory
by Mike Lamont (CERN)

Fig1: Joe Incandela, CMS Spokesperson presenting at Higgs search update on 4 July 2012. (click to enlarge) Image credit: CERN
Fig2: Improvements in LHC performance are clear - this graph shows the luminosity delivered to the Atlas experiment in 2010 (green), 2011 (red) and 2012 (blue) for proton-proton collisions. Image credit: CERN

With a wry smile, Fabiola Gianotti showed the significance of the combined gamma-gamma and 4 lepton channels. "We observe in our data clear signs of a new particle, at the level of 5 sigma, in the mass region around 126 GeV." She gave heartfelt thanks to "the whole LHC exploitation team, including the operation, technical and infrastructure groups, for the outstanding performance of the machine, and to all the people who have contributed to the conception, design, construction and operation of this superb instrument".

It hasn't been easy. First beams were injected on the 10th September 2008. 9 days later - disaster, and it was November 2009 before beams were injected again following a Herculean effort to repair the seriously damaged sector 34. The causes of the incident were examined closely and caution dictated running at less than design energy: 3.5 TeV in 2010 and 2011; 4 TeV in 2012.

2010 was devoted to commissioning with beam with a very careful eye on the critical machine protection system. Towards the end of the year luminosity production really started. The total for 2010: around 50 inverse picobarns. The total by the 4th July 2012: around 10,000 inverse picobarns.

The machine has a limited number of parameters it can use to increase the collision rates seen by the experiments: number of bunches; number of protons per bunch; and the beam size at the interaction point. Over the last couple of years the LHC and its injectors have pushed hard on all options with considerable reward. Squeezing the beam sizes down to around 70 microns at the interaction points in ATLAS and CMS, coupled with small beam sizes and high bunch intensities from the injector chain have resulted in truly impressive collision rates. This performance, together with good machine availability, has allowed the LHC to deliver something like 800 trillion collisions to each of ATLAS and CMS. A very few of these have produced something that looks like a Higgs.

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Triplet magnets program progressing on both sides of the Atlantic

LHC - Higgs factory