OT: pre zaujimavost :-)

[Daniel Valuch]

Po 3 tyzdnoch chladenia dosiahol 223 tonovy supravodivy magnet 
experimentu CMS teplotu 4K. Vnutorne pole by maloo byt pri nominalnom 
prude 20kA ca. 4 Tesla, vid clanok nizsie...

obrazok:
http://doc.cern.ch//archive/electronic/cern/others/PHO/photo-cms/magnet/magnet-2006-002.jpg


The CMS solenoid is cold - and that's an understatement. At -269° C, the 
gigantic coil is chilling out at its operating temperature and is 
superconducting, an important milestone for the AT/ECR cryogenics group, 
the CMS coil collaboration (which includes CERN, ETH Zürich, Fermilab, 
INFN Genova, ITEP Moscow and Saclay) and the CMS collaboration at large.

The coil consists of 14.5 tonnes of superconducting cables, which are 
embedded into 74 tonnes of pure aluminum and reinforced with 126 tonnes 
of high mechanical-strength aluminum alloy and 9 tonnes of insulation. 
Added together, that's a 223.5-tonne magnet. Cooling a component of such 
massive proportions is anything but easy.

'Superconductivity so far, despite it's great potential, has not yet 
made a mark in the commercial world because there is the complication of 
cooling down to liquid helium temperature, which is sophisticated, 
complicated and expensive,'said CMS Magnet and Integration Group Leader 
Domenico Campi. 'Nevertheless, for such a huge experiment like CMS, we 
cannot avoid the use of superconductors and thus of cryogenics. It is 
simply inconceivable to make a magnet of such power with a resistive 
conductor within acceptable parameters.'

After completion, the coil was suspended from the vacuum tank using 
large titanium tie-bars, under the supervision of Bruno Levesy from 
Saclay. The coil's vacuum vessel was then welded closed by the firm DWE 
under the supervision of Hubert Gerwig, Andrea Gaddi and collaborators. 
The cool-down process began in early February. Cryogenics engineers had 
to carefully cool the magnet evenly and slowly to protect it from 
deformation and damage.

The cool-down is achieved by circulating helium gas through a series of 
pipes surrounding the coil. In the first phase, the helium is steadily 
refrigerated to ever lower temperatures by the means of -196° C liquid 
nitrogen. Then, to achieve the lowest temperatures, the helium is 
injected into small turbines, cooled to liquefaction temperature and 
sent to fill up the coil circuits.

After the three-week cooling campaign, the superconducting magnet is now 
resting at its operating temperature.

The service and operation of the helium refrigeration plant is led by a 
team of two: project leader Goran Perinic and technical engineer Thierry 
Dupont. They are supported by a team of operators and their colleagues 
from the technical support sections led by Marco Pezzetti and Olivier 
Pirotte of the AT-ECR cryogenics group.

The cool-down system is fully automated, but Perinic and Dupont have 
spent the last three weeks close to the plant, in the control room or in 
front of a computer at home, closely watching the slow refrigeration 
process and ready to intervene at the smallest sign of trouble. They've 
both worked on the project for six years.

'It is nice to add a tiny milestone to a technology whose roots are in 
Geneva,' Perinic said. 'It is just more than 128 years ago that Raoul 
Pictet managed to refrigerate a gas to a cryogenic temperature and to 
liquefy it for the first time at the University of Geneva. I doubt that 
he had imagined an application like this.'

For the nominal current of 20,000 Amps, the solenoid is designed to 
reach a magnetic field of 4 Tesla, almost 100 000 times the Earth's 
magnetic field. It will have enough stored energy (2.7 GigaJoule) to 
melt 18 tonnes of gold. The superconducting coil will be kept at 
operating temperature for magnetic testing starting in May. It will be 
lowered into the CMS cavern, 90 metres underground, before the end of 
the year.