PENTATRAP Project

Motivation

The new Penning trap project PENTATRAP for high-precision mass measurements on highly charged stable ions related to tests of fundamental symmetries and constants is presently under construction at the MPIK. PENTATRAP aims for a relative precision of the mass measurement of δm/m <= 10^-11. For example, it will open a way to an experimental determination of the upper limit of the neutrino mass in the electron capture sector with an accuracy of ~1eV [1]. Another example is the test of quantum electrodynamics in the little explored regime of extreme fields.

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Experimental Setup

The PENTATRAP experiment will be attached to the Heidelberg EBIT [2] (see Fig.1) or later to the HITRAP project at GSI [3], where the ions of interest will be produced.

Figure 1: Sketch of the planned experimental setup: Highly charged ions are either delivered from the Heidelberg EBIT or later from the HITRAP experiment and enter the PENTATRAP setup from the top. Ion optical elements will guide the ions to the trap-tower which is situated inside the 7-T superconducting magnet. (b) Setup built around the Penning trap. The trap being cooled by the liquid helium cryostat of the magnet is placed in a vacuum chamber and can be aligned towards the magnetic field lines. The associated electronics is placed below in an extra vacuum chamber. - click to enlarge

Then the highly charged ions will be guided by ion optical elements into the Penning trap setup, which is placed in a superconducting magnet of 7 T (see Fig. 2).

Figure 2: Superconducting 7 T magnet situated in the temperature controlled PENTATRAP lab. - click to enlarge

The trap-tower as well as the detection electronics will be surrounded by a 4 K environment. A translation and tilt stage enables the online alignment of the Penning traps towards the magnetic field lines. Furthermore, to ensure stable and well-controlled environmental conditions, the setup will be placed in a room, in which the temperature fluctuations will be reduced below 0.1 K. To minimize disturbing effects due to vibration, the magnet is supported by anti-vibration pneumatic pads. Moreover, a stabilization of the helium level and the gas pressure in the liquid helium dewar of the magnet will be implemented. Advanced feedback control of the magnetic field via flux-gate magnetometry will be used to correct fluctuations in the vertical component of the earth magnetic field.

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Penning trap system

The mass determination of an ion in a Penning trap is done by measuring its free cyclotron frequency

ν_c = 1/(2π) ⋅ qB/m ,

where q/m is the charge-to-mass ratio of the ion and B the magnetic field. Only relative mass measurements, where the magnetic field is calibrated via a mass comparison are carried out to achieve highest precision. Thus, the frequency ratio between an ion of interest (IOI) and a reference ion (RI) has to be alternately measured. Therefore, undetected B-field fluctuations are one of the main uncertainties limiting the precision of high-precision mass measurements.

At PENTATRAP a novel trap system (see Fig. 3 and 4) consisting of five independent trapping regions will guarantee high-precision frequency measurements. All traps are cylindrical five-pole Penning traps [4].

Figure 3: Prototype of the trap tower. The electrodes are made of oxygen free copper and are isolated by sapphire rings. - click to enlarge

Figure 4: Sketch of the Penning trap stack at PENTATRAP. - click to enlarge

The inner traps will be used for high-precision frequency measurements. Applying the measurement scheme shown in Fig. 5, a fast (but adiabatic) exchange during the measurement cycle of the two ion species (IOI and RI) is possible, without any loss of time due to ion preparation processes. At the bottom as well as at the top of the tower two traps serve as so called "monitor traps" for the observation of the magnetic field fluctuations via the detection of the cyclotron frequency of a permanently stored ion. Thus, a magnetic field correction between the two measurements of the IOI and the RI can be carried out, to reduce the uncertainty in the mass determination additionally. With a set of five Penning traps a variety of other measurement schemes is possible.

Figure 5: Example of a measurement cycle at PENTATRAP. The frequency of the reference ion (RI) is measured at t₁ in the central precision trap and the ion of interest (IOI) is meanwhile prepared in one of the preparation traps. At t₂ the ions are transferred one trap further and the frequency of the IOI is measured in the central trap. In a third step the ions are moved in the initial position and the RI is measured again. During the whole cycle the magnetic field is observed via the measurement of the cyclotron frequency of two ions stored in the monitor traps. Hence, a magnetic field correction between the two measurements of the IOI and the RI can be carried out - click to enlarge

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Ion detection

The frequency determination will be carried out with the Fourier-transform ion-cyclotron resonance detection technique. Here, the very tiny image current (~fA) induced by the ion motion in the trap electrodes is non-destructively detected. To detect these very tiny current, it is necessary to ensure a sufficient signal-to-noise ratio. To this end, the electrodes are connected to tuned circuits, which consist of a high-Q inductor (example see Fig. 6) followed by a high-impedance amplifier. These circuits are placed close to the trap in the 4 K environment, since thermal noise is reduced and low-loss superconducting circuits can be applied. Typical values are a Q-factor in the range of 10⁴ and a voltage noise of the amplifiers below 1 nV/(Hz)^1/2.

Figure 6: Picture of a helical copper coil used for the frequency detection.

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References

Comments

The research within the framework of the PENTATRAP project has close affiliation with the goals of the Extreme Matter Institute (EMMI)  at GSI/Darmstadt . The institute was created by the Helmholtz Alliance "Extremes of Density and Temperature: Cosmic Matter in the Laboratory". This Alliance connects GSI with 7 national partners (among them the Max Planck Institute for Nuclear Physics at Heidelberg) as well as 4 international partners. With EMMI Europe will get a unique infrastructure for interdisciplinary investigations of matter under extreme conditions.