Overview

The Electrostatic Storage Ring (ESR) at TUM is being developed as a compact, table-top storage ring for charged particles at low kinetic energies. It will provide a uniquely controlled environment for precision measurements of spin dynamics and searches for new physics beyond the Standard Model.
Operating purely with electrostatic fields, the ESR enables long-term storage and manipulation of ions and molecules without magnetic fields, offering an ideal platform for future electric dipole moment (EDM) and dark sector searches.

Scientific Motivation

Electrostatic storage rings offer several advantages for fundamental physics experiments:

• No magnetic fields → enabling “frozen spin” conditions,
• energy-independent focusing for slow, non-relativistic ions,
• and extended spin coherence times due to minimal perturbations.

The TUM ESR aims to confine polarized ions or molecules (such as H₂⁺ or molecular ions) with kinetic energies around 30 keV, circulating for up to several hours.
Such conditions allow testing for ultralight dark matter interactions, axion-like couplings, and tiny EDM-induced spin rotations with an effective magnetic sensitivity below 10⁻²¹ T/√Hz in the sub-mHz range.

Experimental Setup

The current ESR design of a closed electrostatic beamline of approximately 6 m circumference, built from:

• sextupoles and double quadrupoles for focussing and beam stability
• steering electrodes for orbit correction,
• and a beam injection section connecting to the ion source and diagnostics.

Ions are injected with kinetic energies of 10–30 keV and guided entirely by static electric fields at potentials up to several kilovolts.
The electrode geometries have been optimized using finite-element field simulations, ensuring high homogeneity and minimal aberrations. 

The system operates under ultra-high vacuum (UHV) conditions (≈10⁻⁸ mbar) to minimize scattering and charge exchange, with all deflection and focusing elements mounted on precision-machined, electrically isolated supports.
A fast-switching injection system enables pulsed beam injection and storage, while diagnostics allow for detailed orbit characterization.

Current Status and Outlook

Initial beam transport through individual ring sections has been demonstrated, and commissioning of the closed-orbit configuration is ongoing. The next milestones include achieving first stable storage, lifetime measurements, and the characterization of electric field homogeneity.

In the long term, the TUM ESR will serve as a prototype platform for investigating:

• the long-term stability of electrostatic beam storage,
• spin coherence and systematic effects relevant for EDM measurements,
• and novel detection schemes for slow, stored ions and molecules.

Through its compact size and highly controlled environment, the TUM Electrostatic Storage Ring provides a flexible foundation for precision measurement development and for training new generations of researchers in experimental beam physics.

Publications and finished Theses

• Brandenstein, Chiara, et al. "Towards an electrostatic storage ring for fundamental physics measurements." EPJ Web of Conferences. Vol. 282. EDP Sciences, 2023.

Involved Persons and Former Members

Currently involved persons: Dr. Hans Steiger,  Adil Muggo

Former Members: Chiara Brandenstein