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.

Dresden II - Wasteland Tourism

Ok, this is it.

The previous post covered my attempts to find any interesting dilapidated buildings near the city center of Dresden. On my way to Pirna, I stumbled across this building:

Elbtalzentrale

Let's get inside to see what "gems" it may hide...

Plenty of green colours from the moss, and algea on the left wall.

Leftover of a circuit braker / fuse box.

Wonder if the artist suffers from nightmares.

 

Window was broken, the curtains were slightly moving in the wind.

Wonderful textures!

Newspaper as tapestry. Full of GDR propaganda.

Textures, textures, textures!

Strelok was here.

Dresden I - A Wave of Capitalism

Yes, I finally bought myself a new camera, a Nikon D3100 with a 18-55 VR lens, which was recommended by several people. I got it just in time before I continued my tour across Europe. After finishing my work at CERN, Geneva and visiting Brussels, I went to Dresden and Bratislava, and wanted to do Berlin. Tour ended rather chaotic though:

• checking into a Hotel in Bratislava, I realized I left my passport in Aarhus. I've been crossing 4 countries with no passport. (I will not refer to the concurrent Danish-German border issues here.)
• going back from Bratislava to Dresden: Slovakian train conductor checks my ticket, says it's no good. I need a new one. I protest, but have to pay. A bit later new Czech train conductor arrives, says again ticket is no good. Ticket from Slovak colleague is also no good, even after heavy protests, I have to pay again.
• In Dresden I also lost my fairly new mobile phone. It was a HTC Desire, and what really bugs me is all the data which is gone with it. Of course, I did not encrypt it. Wish it was my laptop which got lost, which is fully encrypted. (Tracing it from phone company not allowed. Weird: after all those apps which main purpose is to trace where you are and what you do. But if I want to access my own data...!? Wtf!)
• Last day in Bratislava I got a bad flu, which still persists. This made me cancel the Berlin part and a course I wanted to follow in Mainz.
• One credit card was recently lost in Brussels, another forgotten in Aarhus, my last one hit the VISA 30 day limit the last day of my travels. So, suddenly I was in Dresden with no credit cards and almost no cash! Never felt so free...

Anyway, what I wanted to tell about was how the face of Dresden radically changed since 1989. Former chancellor Helmut Kohl was speaking of "Blühende Landschaften", which characterize this transformation process, but note the subtle double meaning of it, as it also can refer to nature taking over deindustrialized and depopulated areas. :-D

Dresden was more or less obliterated at the end of the second world war, and much of the voidness was filled with socialistic type architecture. Just north of the main railway station there is the Prager Straße, which more or less looked like this in 1986:

Prager Straße, 1986, with the "Inter Hotel" and the "Restaurant International".
Taken from http://flic.kr/p/4A6gVS

More or less the same buildings are there, however massively renovated and now full of "standard shops" you find in any city in Germany.

Prager Straße, 2011.

A few more impressions:

Prager Straße 1991. From: http://www.flickr.com/photos/eastgermanpics/2197012589/in/photostream/

Same street 2011, guess the fountain was turned and moved a bit further to the north.

Prager Strasse 1991: Aluminum padded building which served as a central store.

Panels were partially reused for a new building.

Over the socialistic soviet-style reminiscences, a wave of capitalism crushed, cladding the dilapidated buildings with its usual representatives: Karstadt, Douglas, Jack Wolfskin, McDonalds, Deutsche Bank, Esprit... they are all there, effectively removing any uniqueness, rendering the shopping street identical with that in Frankfurt, Stuttgart or any other medium sized town.

I searched for a few remanences which still prevail from socialistic times. Some are easy to find, but they get less:

A crystal tree at Pirnaisches Tor.

Robotron was the chip and computer manufacturing company of the former GDR.

Blühende Landschaften, in its other meaning. (Note the GDR typic lanterns.)

I was somewhat surprised how difficult it actually is to find some industrial wastelands. Way over 90 % of the entire city is renovated in some way, I spent an entire day on bike going through clean streets without finding what I was looking for. Even asking the locals did not really help much.

Almost all of Dresden is fully renovated, and streets look like this.

But then, after roaming the city for an entire day, I finally found somenthing:

Dresden 2011, somewhere near Zwickauer Str.

Garages with characteristic "droplet" shaped lamp.

Those lanterns are mostly mounted on poles made of concrete.

Black graffiti says: "Here a MURDER happened! A piece of art was destroyed."

Dresdner Frauenkirche, 1991.

... and again, as in 2011. That part seen in the former picture is the darker left of the church.

Another thing I really enjoyed about Dresden was how easy it was to get in contact with people. Nomatter where I were, people came to chat, and they openly discussed anything about their lives, the former GDR etc.

I had a very long chat with this guy at a flea market. Note the cylinder of mono-crystalline silicon at the lower edge in the middle of the picture. (He wanted 10 € for that.)

Reminiscences from second world war can of course also be found, this one I found interesting:

"Museum, do not bomb"

I could not understand what it said, my very limited knowledge of the Russian language only identified "Museum, do not...?", so I decided to wait next to the inscriptions until I found some Russian tourists to help me out. I only had to wait one minute, a group of young Russians gave me the answer  (I even asked in Russian, and they understood what I wanted, feel very proud :-).

Anyway, this concludes more or less the first part of my Dresden visit. Thanks to Alina for hosting, being my tour guide and holding my cup of coffee while taking the picture below.