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.. :-)

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