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Stored and Cooled Ions Division
 
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THe-TRAP Project

Experimental setup

Sketch of the experimental setup at THe trap.
Fig. 4.1: Sketch of the experimental setup at THe trap. Further explanation in the text.

An overview of our experimental setup is shown in Figure 4.1.
The system consists of two main parts. The external ion source (top) and the two hyperbolic Penning traps (bottom). In the ion source, the 3He- or 3H-gas is ionized by collisions with electrons emitted from a cathode filament. The ions then travel through the ion optics (see Figure 4.2) to the Penning trap section.

Schematic diagram of the ion source of THe trap (left) and a photo of the real ion source (right).
Fig. 4.2: Schematic diagram of the ion source of THe trap (left) and a photo of the real ion source (right). The ion source consists of a unit (the cathode plate, the cathode ring and cathode mesh) which is used to create an electron beam. Within the anode the ions are created and then accelerated towards the ion optics, which focus the ion beam and forward it to the hyperbolic Penning trap. To all these elements, voltages can be applied from the outside through a DB9 connection (see right picture).

The ion cloud is stored above the hyperbolic Penning traps (see Figure 4.1) in the ion capture segment. There, it is trapped by the magnetic field and an additional electric potential. When the electric potential is reduced for a short time, parts of the ion cloud reach the first hyperbolic trap, called capture trap (see Figure 4.4). (Further information can be found in Marius Tremer's diploma thesis externer Link.)
The hyperbolic Penning traps are located in a superconducting 6 T magnet and are cooled by liquid helium to a temperature of 4 K. Due to manufacturing tolerances, holes in the endcaps for ion injection and truncated size electrodes, they slightly deviate from the ideal shape (see Figures 3.1 and 4.3).

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Cross-section image of the hyperbolic Penning trap used in THe-Trap.
Fig. 4.3: Cross-section image of the hyperbolic Penning trap used in THe-Trap. In addition to the endcaps and the ring, there are two correction electrodes. The field emission point (FEP) serves as an electron source. With the electron beam generated by the FEP, the residual gas can be ionized. The lower and upper skimmers are used to reflect the electron beam.

In addition the ring- and endcap-electrodes, there are two correction electrodes, which are intended to tune anharmonicities and are used for exciting the ion with radiofrequency electric fields.

Schematic of the storage tube and the two Penning traps.
Fig. 4.4: Schematic of the storage tube and the two Penning traps. In reality, the structure is rotated by 90° clockwise. Ions from the ion source are first stored in the drift tube in the ion capture segment and then shot in short pulses to the first hyperbolic Penning trap (capture trap). There, they can be stored longer. The actual frequency measurement takes place in the precision trap.

Actually, even one hyperbolic Penning trap would suffice for the measurement of a mass ratio of two ions, because two ions can be stored easily in a Penning trap. But in this case, the two charged particles interact with each other and thus interfere with the measurement. Therefore, it is advantageous to store one ion in the upper trap, while the frequency of the other ion is measured in the precision trap (see figure 4.4). In addition, the ion in the upper trap can be used to monitor fluctuations of the magnetic field or the ring voltage. A fast exchange of the ions reduces the effect of long-term fluctuations of the electric and magnetic field.