The Felsenkeller underground ion accelerator lab
The Felsenkeller underground ion accelerator lab has been built jointly by HZDR (group of Daniel Bemmerer) and TU Dresden (group of Kai Zuber).
The laboratory includes two tunnels, respectively called tunnel VIII and IX, of the Felsenkeller underground site in Dresden.
The Felsenkeller is divided into three areas: Area A includes the scientific installations and is a radiation safety controlled area. Area B is used to access area A, and it includes the plants serving the laboratory. Area B' is used as emergency exit and for part of the climatization unit.
The tunnels were blasted in 1856-1859 into the hornblende monzonite rock, in order to create cool storage for the adjacent Felsenkeller brewery. From 2016-2018, TU Dresden and HZDR jointly refurbished tunnels VIII and IX for laboratory usage, with support from the Excellence Initiative of the German federal and state governments.
In the photo to the right, accelerator engineer Bernd Rimarzig is shown inspecting the sulphur hexafluoride storage tank in tunnel IX. This gas is needed for high-voltage insulation. When the accelerator tank is filled with 5-6 atmospheres of this gas, a potential of 5 million volts can be maintained on the high voltage terminal without spark discharges.
When the accelerator is undergoing maintenance, the gas is stored in the SF6 tank. The pumping station needed to transfer the gas from this storage tank to the accelerator tank and back is visible in the background of the photo.
At the far back, the entrance to the controlled area A is visible.
Bernd is standing on the original granite plaster that was maintained from old times in order to limit construction costs - and it makes for a stylish atmosphere!
PhD student Felix Ludwig and Bachelor's student Julia Steckling are testing the first part of the ion acceleration system: the external ion source. A multi-cathode SNICS (MC-SNICS) cesium sputter ion source by National Electrostatics (NEC, Middleton, WI, USA) is used. The ion source produces intensive beams of 12C- ions of about 70 keV energy that are then transmitted via an electrostatic analyzer and a large electromagnet (in the center back of the figure) to the main accelerator.
Felix and Julia have recently managed to extract 140 µA of analyzed 12C- beam for two hours, better than the quoted performance of the source.
Different from the main accelerator, the sputter ion source is operated in air. Hence the blue Faraday cage to protect people from accidentally touching the part of the source that is at potential. When a door is opened, the high voltage is automatically switched off.
The ion accelerator has been acquired secondhand by HZDR. In its previous service, it was located in York/UK and used for 14C analyses by accelerator mass spectrometry. The machine is a 15SDH-2 pelletron-type accelerator produced by NEC, with double charging chains for a total rated upcharge current of 300 µA. Already in July 2012, the entire system including all beam lines has been expertly dismounted in York by David Chivers of Ion Beam Systems and transported to Dresden by tractor-trailer and ferry.
Postdoc Tamás Szücs is checking the electrical power cabinet that serves the ion accelerator system.
The ion accelerator can work both in so-called tandem mode and as single-ended accelerator.
In tandem mode, its original mode of operation, the negatively charged carbon ions are accelerated to the positively charged high voltage terminal, inside the accelerator tank. There, an electron-stripping system (in our case, a nitrogen gas cell) rids the ions of two or more electrons, thus converting them to positively charged ions. These positive ions are then once more accelerated to ground potential, to the high-energy side of the accelerator.
In single-ended mode, an additional mode installed between 2012-2017 on the accelerator destined for Felsenkeller, positive ions are directly generated on the high voltage terminal and accelerated just once. This mode is very useful when noble gas ions are to be accelerated.
During its storage in Dresden-Rossendorf, the high voltage terminal of the accelerator was completely reworked.
A second ion source, a radio-frequency ion source (made by NEC) for positive 1H+ and 4He+ ions, has been mounted on the high voltage terminal. This source allows for the generation of high-intensity 4He beams that would not be feasible in tandem mode due to the fact that helium is a noble gas.
The source, and a connected electrostatic deflector, have been tested outside the accelerator tank and gave up to 90 µA 4He+ beam. For permission reasons, it was not possible to test source and accelerator together at the surface of the earth.
PhD student Marcel Grieger and Master's student Simon Rümmler are seen inside the tank, making sure that the terminal ion source is properly mounted and connected. They have already shown that the plasma ignites also inside the tank and cannot wait to extract and accelerate 4He beam. In order to do this, they have to climb out of the tank!
The newly installed radio-frequency ion source and many other components are controlled by Labview programs running on Beckhoff hardware.
Technician Maik Görler is seen fine-tuning his Labview virtual instrument, for now controlling it from a desk right next to the accelerator.
During final operations, the accelerator will be remote controlled from the outside of the tunnel. There, in addition to control and preparation rooms, a large experiment user room is available just across the street from the tunnels.
Once accelerated, the ion beam is transported via evacuated beam lines to the high-energy electromagnet, and hence to the target chamber for the actual experiment.
Technician (and deputy radiation safety officer) Toralf Döring works on the vacuum generation and measurement in the drift beamline directly after the high energy end of the magnet. Due to the gas load provided by the terminal ion source, Toralf had to replace the original cryopumps with large turbomolecular pumps backed by multistage roots pumps.
A further challenge was the proper alignment of the beam lines under the cramped conditions of the tunnel with few reference points, which was achieved using optical theodolites and levellers brought in from Rossendorf.
The scientific experiments are carried out in two concrete bunkers, which are surrounded on all sides by a 40 cm layer of reinforced concrete. The cement, sand, and gravel had been analyzed before each load of concrete was mixed, in order to ensure a specific radioactivity of less than 20 Bq/kq both for the uranium and for the thorium chain of naturally occuring radioactive materials.
The activation bunker, shown in the picture on the right, contains an ultra-low background setup, acquired by TU Dresden based on support by the DFG Großgeräte der Länder program. A very large, 150% relative efficiency, high-purity germanium detector is surrounded by a graded copper and lead shield of very low intrinsic radioactivity. This detector, together with its recently acquired smaller sibling, will make this laboratory the most sensitive underground radioactivity-counting facility in Germany.
The in-beam experiment bunker will contain the scattering chamber for the ion-beam based experiments. Several different chambers can be mounted, depending on the need for the precise experiment to be performed. The room has sufficient space to install any kind of γ-ray detection apparatus for low-energy, low countrate studies of astrophysically important nuclear reactions.
The tour of the Felsenkeller lab concludes with a photo of 2015 Physics Nobel Laureate Professor Arthur McDonald in tunnel VIII, in front of the entrance to the two concrete bunkers. Professor McDonald was kind enough to speak at the topping-out ceremony of the new laboratory in 2017, i.e. after the concrete was poured but before any of the services had been installed.