Опасная зона

Опасная зона

Sunday, October 16, 2011

A Tour of the Tandem EN Accelerator in Aarhus

As I mentioned earlier we have several particle accelerators at the Department of Physics and Astronomy in Aarhus. After showing our little KN Van de Graaff from HVEC, this post is dedicated to our 5 MV EN Tandem, which recently suffered irreparable damage after a series of indoor explosions.

The EN Tandem is the smaller brother of the FN Tandem by HVEC. Our EN was installed in Aarhus during the early 70ies. Our tour today starts in the control room of the accelerator:

Main control room.
The panel shows an outline of the facility. Beam starts in one of the two ion sources to the right, the accelerator in the center, energy selection magnet to the left, and finally the beam selection magnet to the lower left.

Ion source controls.
When starting the accelerator, you first turn on the power for the control system with the key in the upper right. Fire up the ion source (filament, sputtering and oven current) or if you are running a gas source, increase the gas pressure. Then make sure you get a beam out of the ion source.

Injector controls.
The ion sources are lifted to some negative high voltage, such as -70 kV. You can control the voltage with the upper right dial. The blue panels are new controls which were designed in-house by the accelerator engineer Klaus Bahner, who gradually upgraded the old technology.

The inflection magnetic field directs the beam from the active ion source to the Tandem accelerator. Addiational a chopper was there for attenuating the beam, along with electrostatic beam steerers. Two quadrupole pairs assures the focusing of the beam.

Beam current monitors.
Of course, you must somehow get an idea if you have a beam or not. Several destructive monitors can be inserted into the beam. The measured current could be coupled with a speaker, where the pitch of the tone was proportional with the current. This way you could concentrate on tuning the beam on the most optimal setting without constantly staring at the amperemeter. This feature was used a lot, as you can see in the picture above the text for setting the speaker volume is worn off.

Accelerator controls.
The Tandem accelerator itself is controlled with the panels shown above. Here you find the switches for the belt motor and the charge supply, which sprays charge on the belt. The terminal voltage could be adjusted with the "charge current" button and further stabilized with the "corona distance".
Inside the terminal you can install several kinds of strippers, either a gas stripper, or various foil strippers. The negative ions are here stripped by their electrons, then becoming positive and accelerated again from the positively charged terminal.

Analyzer magnet controls.
The accelerated beam leaving the Tandem may contain several ion types. With the analyzer magnet you can select the beam you want.

Beamline selection and focussing.

And finally, after some additional tuning and focussing, you can select which beam line you want to use.
Control logic
Safety system bypass switches...

This concludes our tour in the control room, next lets take a look at the accelerator vault. Depending on what kind of ion and what currents you operate with, the accelerator may create radiation. Mostly gammas, but occasionally also neutrons if you run e.g. high current protons. A massive concrete door slides aside...

Once inside, you can see the Tandem accelerator (to the left) and the two ion sources to the right.

The ion source to the back next to the blue 19' rack, is a new ion source for C-14 dating. It contains a large wheel with dozens of carbon samples to be measured for C-14 contents. Also an airlock makes it possible to change the samples without breaking the vacuum.

Here is the old C-14  ion source which was used for many years. Both the new and the old ion source is a sputter ion source where Cesium ions are bombarding the carbon target, thereby releasing negatively charged ions.

The accelerator itself is a big pressurized tank containing a vaccum tube and the high voltage charging apparatus. The tank is filled with mix of dry Nitrogen and SF6 (stored in the tanks under the ceiling). Due the damage the accelerator suffered, it was depressurized for further inspections. So let's have a look inside...

Before you enter, you must make sure that proper venting is assured. There has been a casualty at another accelerator of this type, where people fainted and suffocated.

There are four manholes, where you can enter. You can see the ring structures which ensure a homogenous field gradient from the high voltage terminal.

Looking from the low energy section to the high energy section, you can just glimpse the terminal in the middle.

A part of the acceleration tube at the high energy section.

Once the beam exits the accelerator, it passes a quadrupole pair... 

... and the orange analyzing magnet shown to the left.

Finally it passes another set of quadrupoles and the switching magnet, which directs the beam to one of the 8 beam lines.

Magnets are water cooled.

A water cooled beam dump.

Radiation detectors monitor the gamma and neutron field in the accelerator vault.

Signs tell if the accelerator is running, and if radiation is present.

Hand held radiation monitors.
The sad story is that due to some renovation of nearby labs, it was decided to destroy two other radiation protection doors (of similar type like the ones shown before) with dynamite. Unfortunately the five explosions set off a blast wave which destroyed the Tandem beyond repair. The pressure tank took damage, and cannot be repressurized, the accelerating column is bent downwards and there are many other issues.

A movie of the explosion of one of the two radiation protection doors can be seen here:

A damage report (in Danish only, but with fancy pictures) is here:

This is the end of our tour at the Tandem accelerator.
The complete picture gallery with additional fotos was uploaded to picasa:
If you are still reading this, you are welcome to drop by for a cup of coffee in my office upstairs.. :-)

Saturday, October 15, 2011

I am a Wave, a new Wave

So, this is the recipe for a successful conference..
I went to a conference in Kemer: the NUFRA2011 on nuclear fragmentation.  It was arranged at an all-inculsive five star resort, which I generally believe was a terrific idea. (As an arranger, you save all the catering fuzz, and its not all that expensive!)

I was invited to give a talk here, and surely it was one of the most enjoyable conferences I have ever been to. I gave a talk about the impact of nuclear models on particle therapy. (I will provide a link to my talk here, once it becomes available, or possibly make a post about it on my work-blog.)
Update: here it is.

A very brilliant idea was to keep the talks in the morning and evening, leaving the afternoons for hanging out on the beach.

Pictures here: https://picasaweb.google.com/niels.bassler/NUFRA2011?authuser=0&feat=directlink

We had terrific weather the entire week, though the trip home was more or less a disaster due to heavy rain, see video:

My flight was delayed a day, and on monday even a plane had a kind of mini-crash (fortunately no casualties!) which blocked one runway, causing further delays, see:

Nonetheless... what I really found fun and enjoyable was that the resort "Hotel Fantasia" was primarily for Russians. All employees were very friendly and fluent in russian language, better than English or German (very surrealistic). At the evenings somehow some of us ended up in the local "disco", called "Monkey Club" (see the irony?) ("Whooo whoo whoo") which had a propensity to play awful Russian dance music.


1st evening: "WTF! This is really bad."

2nd evening: "OMG, they play the same music as the day before. Wtf!"

3rd evening : putting your hands into the air and sing along:

Я волна, новая волна,
Подо мной будет вся страна
Подожди, скоро навсегда,
Затоплю ваши города.

Я волна, новая волна,
Подожди, скоро навсегда,

Video here (just for the records, of course):


I even smuggled this piece into the physics Friday bar play list at our university the following Friday and everyone freaked out dancing when it was played. (How bad can it get?!!?)

Meh... big thanks to Анна Пшеничнова, for making this tolerable.
Take-san and Vakkas in the background to the right.

Conclusion: Russian dance music is not worse than anything else, you just have to get used to it.
Thanks also to Vladimir, Konstantin, Take-san, Vakkas and our "princess..." !

Sunday, August 28, 2011

A 5 MV Van de Graaff Accelerator

At the Department of Physics and Astronomy in Aarhus, we have a number of particle accelerators in our basement. This post is about our 5 MV Van de Graaff accelerator, manufactured in the 60ies by High Voltage Engineering Corporation (HVEC). (Checkout this old ad.)

The machine was taken apart for service, so I had a chance to take a closer look at the interior of it.

The machine is of the KN type. On the picture above you can see the pressure tank which is filled with a mixture of nitrogen and SF6 for spark suppression.
 The terminal cap is removed, exposing the interiors.

This is the ion source. The copper tube at the top leads the gas to be ionized into the glass tube. RF connectors for ionizing the gas are detached here. The red cylinder to the right is a solenoid which provides some focusing of the beam.

RF power generator.

The red gas container is filled with deuterium gas. It can be used for neutron production, e.g. by bombarding beryllium targets with a beam of deuterium ions.

But mostly, proton and helium ion beams are required. The two blue gas containers shown here hold hydrogen (left) and helium gas (right).

In order to achieve a good beam, it is important to maintain a regular voltage gradient from the terminal to the base which is on ground potential. The dark green columns inside the metal rings are high voltage resistors.

The accelerator is not in too good condition. The dust which can be seen here (and everywhere) is from the rubber belt which delivers the charge to the terminal.
The last time I tried to operate the accelerator, the maximum voltage achievable was 2.3 MV. This is sufficient for most experiments done here which deal with radiation induced defects in silicon.

The accelerator seen from the other side, the terminal cap in front.

The terminal cap of the accelerator. If you zoom in, you can see plenty of markings from high voltage sparks which happen from time to time.

This is the high energy end of the accelerator. The beam tube is that vitreous thing in the middle going from the left to the right. Notice how clear the glass is on this end...

... but as we get closer to the terminal, the glass darkens...

... and at the ion source, the glass is dark brown. I presume the explanation is that the accelerator mostly runs with positive charge for accelerating protons and other ions. (It can also run negative voltage, if you want to have a beam of electrons.) The ion beam itself does not do much radiation damage to the acceleration tube, as the beam is contained in the tube. However, on the surface electrons are going the opposite direction, gaining more energy and intensity as they get close to the terminal. Therefore, the radiation damage in the glass is most significant close to the ion source.

After a mayor reconstruction in the basement of our institute, it will not be possible to run electrons with this accelerator anymore. The needed radioprotection was removed to make room for new laboratories.

Spark gaps between each element.

Here the rubber belt is seen and the charge pickup / deposit (depending on polarity). The belt also drives a current generator for feeding all electrical equipment in the terminal with power. (One cannot have a cable feeding this part, since this would break down the HV... )

For the same reason, any communication with the terminal (focus setting, gas setting, gas selection, pressure reporting, etc..) happen via rods made of non-conducting material (probably PMMA).

Here is the beam exit, still not connected. The two red things before the exit are electrostatic beam steerers.

(I'm not thinking what you might be thinking. And I don't wanna hear about it.)

I have also done a ton of pictures of our High Voltage Engineering EN Tandem accelerator, which I will do in a later post. And later (as it seems) I will post the sad story of decommissioning it.
Also waiting in the queue is a post on the recent course I took in how to run a nuclear research reactor. Just returned this week from Mainz, where I had the fantastic chance of running a TRIGA Mark II reactor. Hope I will get the clearance for posting those pics.
Then my Bratislava visit, two more unusual computer games I always wanted to blog about, and more thoughts about art and music.