T2K Horn Testing

I've been in Japan for the the past week, working on horn tests for T2K's third neutrino horn at KEK (pictured at right). The horns are devices that create strong magnetic fields which are used to focus charged pions into a long decay region where they, not surprisingly, decay into neutrinos. T2K will employ three horns in series to create the world's most powerful neutrino beam. Each horn itself is essentially two concentric aluminium [sic] cylinders which carry
a high electric current. Between the two cylinders, a powerful magnetic field is created and that bends the paths of any charged particles passing through.

The horns must generate strong magnetic fields but they run in a high radiation environment so they must be very robust. The T2K horns will operate with an electric current of 320 kA. For this much current, the power is distributed along large aluminium striplines (pictured below) which feed the horns.



The work this week was mainly to test the structural stability of the newly constructed third horn. The first horn was tested extensively earlier this year. The currents and fields created by the horns are so strong that they exert tremendous mechanical stresses on the devices and must be tested extensively before being deployed in their final locations in the beamline. The mechanical forces are so strong that the horns emit an incredibly loud bang each time they are pulsed; it sounds as if someone were banging them with a sledgehammer. Given the amount of energy being pulsed through the horns when they operate, that's not a bad analogy.

The tests themselves are simple enough: we just run the horns in the same way that we will in the experiment. The horn experts spent the past few weeks preparing for the tests by setting up the DAQ and monitoring tools so that non-experts (like me) could come in and watch to make sure nothing went wrong. The plan was to test the horns for 10.5 hours per day in three shifts of 3.5 hours each. We didn't test the horn overnight because the noise is so loud you can hear it off-site, and it's important to keep good relations with the community surrounding the lab. The only hitch has been the extreme heat that Japan has been experiencing this summer. The building where the horns were tested does not have air conditioning, and if the temperature inside went over 37 C we halted operation until it went back down below that threshold. We lost three afternoons to the heat last week! Nevertheless it was a successful week: the horns were pulsed over 44,000 times last week while I was there.

Ask me... I'm an expert

Remember the posts about the Big Bang exhibition at the Science Museum? Well, if you haven't seen it yet hurry up. It's on until the 7th of October, after which it will be replaced by [not allowed to tell ;-) ].

To add a little more action to the displays the Science Museum set up a little interactive event last week where they would show some kit and have a couple of scientists explaining it. The plan was to have a cosmic ray detector and some Geiger counters. We provided the latter but they were quite useless in the end as nobody was able to bring anything even remotely radioactive to make them tick. So we concentrated on cosmic rays, which was not a bad idea anyway. Our colleagues from Bristol provided a scintillator and a small spark chamber, definitively the star of the show. It's much smaller than the huge one Imperial and RAL built long ago and we could not get to work again. At least the Bristol chamber worked nicely. Don't know if there's a lesson here...

It was a lot of fun explaining cosmic muons to the general public (although the general public found at the Science Museum is a quite biased sample) and answering the most unexpected questions. How do we know they are muons? Where do the cosmic rays come from? How does it work? Could they have triggered life? What's this black thing? (the trigger). Wouldn't the muons have all decayed before reaching the ground? (This guy knew more than expected). How do you get a satellite on orbit? Hmmm, somewhat unrelated, but I had a tag saying "asking me... I'm an expert" so I just had to know.

Many thanks to the Bristol guys David, Clare, Ben and John for providing the nice kit and to Will and Tom for representing Imperial.

Finally I agree with Gavin: we should try to build a similar chamber and display it on level 5.