Tuesday, 17 August 2010

The Conference Excursion

I'm back from California now, and enjoyed the Nuclear Structure 2010 conference. I'll probably blog about more of the talks - I particularly enjoyed George Dracoulis' on Tantalum-180 - but not this evening. Instead, I want to talk about the conference excursion.

For those that don't have the pleasure of going to scientific conferences, let me explain the conference excursion. Not every conference has one, but often one afternoon of a week-long conference will involve taking a trip somewhere of interest near to the conference venue. It doesn't have to be somewhere of relevance to the conference topic. It could be a local place of historical interest, or something like that. The excursion is partly an excuse to socialise with the other people at the conference, which is an important part of the purpose of getting all the attendees together at a conference. Indeed physicists sometimes need help in socialising, and these events can lead to useful discussions and collaborations. Of course the excursions are also partly for fun.

The excursion at Nuclear Structure 2010 certainly counts as a physics excursion, and also a fun one. We went to NIF: the National Ignition Facility, based at Lawrence Livermore National Laboratory not too far from San Francisco. The facility is being built to make small pellets of Hydrogen-2 and Hydrogen-3 nuclei fuse together to give off energy, just as they might do in a future fusion reactor and just as they actually do in thermonuclear weapons.

The US has a stockpile of thermonuclear weapons (the phrase used to describe hydrogen bombs, which work by fusing hydrogen isotopes together as opposed to nuclear fission bombs used in anger in WW2), but it has agreed that it will no longer test them in either atmospheric or underground explosions. However, it would like to understand that they are being well-maintained and still functional - something that I suppose it not obvious when you have a rather hi-tech device which you have built and then left on the shelf for many years. The way around the test-ban is to build a kind of controlled thermonuclear bomb, and that is what the NIF is. They certainly make no secret that the driving purpose of the facility is weapon "stewardship" but given that the weapons already exist and will continue to do so, it seems that they have managed to build something that allows quite a bit of interesting basic physics research to take place, piggybacked onto the weapons programme.

To get to see the facility, alien attendees at the conference had to get security-checked months in advance, and thankfully I passed the tests (though I don't know what I was being tested for). So last Wednesday, I boarded a bus in Berkeley, showing my passport before I even got on, and we drove to the lab. We stopped at the security gate for a while, and were escorted to the badge office to get our temporary badges. The whole procedure was generally taking so long that I feared we would have an hour-long trip to the security office and a 10-minute tour of the facility. We were all enjoying joking about it, though, which I was also nervous wouldn't endear us to the facility people... but all was well.

The building in which NIF sits is a rather ugly warehouse-looking place, but inside it is impressively hi-tech. I've been to a few facilities (as a mere theoretical physicist, I don't actually see inside labs all that often) and I think it's fair to say that I've never seen one so sparkling, shiny, sophisticated and hi-tech. The fusion will take place by having a tiny capsule of H-2 and H-3 that you could hold in your fingertips, placed at the focus of a couple of hundred laser beams - the most powerful in the world, which will collapse the capsule causing compression and heating and then nuclear fusion. They haven't started fusion runs yet, but have fired the lasers at non-fusion pellets and everything looks good so far.

The current set up is such that the pellet is placed very carefully in a big spherical chamber into which the lasers are beamed. They will be able to make one explosion every four hours or so when it is all up and running. If they want to actually get fusion energy out of this, they said that they need a rate of 10 Hz - i.e. 10 explosions per second. That seems quite ambitions to me, but I expect that they will learn some important things about the hydrodynamics of fusing hydrogen plasma, which is really what is needed. Certainly the nuclear physics reactions are well enough understood.

After the tour, and I wish I could show you pictures but cameras were verboten, we were treated to cookies and a talk about the basic science that might come out - about matter at the extremes of density - and the promise that the majority of the experiments would be unclassified. Then it was time to head back to the bus, and back to Berkeley. Back from the borders of the sunny desert to the perennially cloudy Bay Area...