PANDORA: a new facility at LNS

PANDORA is a facility whose construction is supported by INFN in the frame of PANDORA_Gr3 project; it aims at building an innovative compact and flexible magnetic plasma trap, for fundamental physics studies and interdisciplinary and applied research. The main goal is the study of -decays in the plasma (never done so far), i.e. in ionization conditions similar to some stellar environments and relevant for nucleosynthesis of chemical elements in the cosmos. PANDORA mainly consists of three subsystems (see figure 1):

1. An innovative superconducting magnetic plasma trap, able to produce and confine plasmas with electron-ion density up to 10¹³ cm^-3 and electron temperature of T_e~0.1-30 keV;
2. An advanced plasma multi-diagnostic system, consisting in a set of non-invasive diagnostic tools capable of operating simultaneously for the non-intrusive monitoring of the thermodynamic plasma properties and the measurement of plasma parameters;
3. An Array of 14 HPGe (High-purity Germanium) detectors for -ray spectroscopy, surrounding the plasma trap.

The procurement and installation of the whole system is expected by mid 2023. The first experimental measurements in PANDORA will be possible by the end of 2023 or early 2024.

Figure 1: Detailed sketch of PANDORA setup with its main subsystems, including plasma diagnostics such as X spectrometers, pin-hole camera systems for X-ray imaging, optical spectrometers, microwave Interfero-Polarimeters, RF probes, mass spectrometry and HPGe detectors array for -ray spectroscopy (SXR: soft-X ray; OMT: Ortho-mode transducers).

The superconducting magnetic trap

PANDORA conceived to be equipped by an innovative (the biggest in the minimum-B configuration), superconducting magnetic trap providing a magnetic field able to confine the plasma at densities and temperatures suitable to reach ionization states comparable with the astrophysical ones.
The magnetic system (see figure 2) is made of 3 superconducting coils (NbTi) for axial confinement and a superconducting hexapole (NbTi) for radial confinement.
It is the largest magnetic trap ever designed in a "minimum-B scheme", supporting a plasma chamber of 70 cm length and 28 cm inner diameter. The aim of this so-called "minimum-B trap" is to provide the confinement of a plasma located around the central axis of the magnetic system.

Figure 2: Left - 3D conceptual model of the structure including the coils (in red), the cryostat overall geometry (transparent blue object here represented as a bulk volume) and the iron yoke (in gray), as well as the holes that shall be done in order to provide proper lines of view for plasma diagnostics and HPGe detectors. Right - Sketch of the setup including ray-tracing simulations for γ-rays emitted after the beta decay. 

The maximum fields are 3 T along the chamber axis and 1.5 T along the radius, at the chamber inner surface, and these values will allow MHD stable plasma generation through an RF field at the frequencies of 18 and 21 GHz via Electron Cyclotron Resonance.
The magnet cryostat houses the complete superconducting magnet system and ensures its safe and reliable operation. The cryostat provides safe mechanical support and efficient thermal insulation for its cold mass. It also provides magnetic shielding of the stray field produced by the superconducting magnet system, and it is thought to host the HPGe diagnostic.
Some selected characteristics of the most performing existing ECR ion sources are compared with those of PANDORA in the table below.

The multi-diagnostic system and the 14 HPGe detectors array

The developed advanced multi-diagnostic system (see figure 3) consists of:

• A Silicon Drift Detector (SDD) for volumetric spectroscopy in soft X-ray domain;
• Two CCD cameras with pin-hole systems and multi-disks collimators for imaging and space-resolved spectroscopy in the soft X-ray domain. A CCD camera will be radially installed, the other one axially;
• An optical spectrometer for the plasma-emitted visible light characterization;
• Two RF probes installed inside the plasma chamber and connected, respectively, to a scope and to a spectrum analyzer;
• Two highly directive horn antennas operating in two diagnostic configurations: i) microwave Interferometer (when connected to a waveguide reference leg) and ii) a microwave Polarimeter (when connected to the Orthomode Transducers OMTs);
• An analyzing magnet with a Faraday-cup in order to measure the charge state distribution and to analyze the extracted beam.

Moreover, the PANDORA trap will be surrounded by 14 HPGe detectors array for -rays spectroscopy and in hard X-rays domain, with conical collimators to intercept the plasma core only. All diagnostics tools will operate simultaneously.
In the figure 3 the overall sketch of the PANDORA multi-diagnostics system is shown.

Figure 3: Detailed sketch of the subsystems, including plasma diagnostics such as optical and X spectrometry, pin-hole CCD camera systems for X-ray imaging, microwave Interfero-Polarimetry, probes RF, mass spectrometry and an array of HPGe for spectroscopy (SXR: soft-X ray; OMT: Ortho-mode transducers).

The most relevant characteristics of the developed tools in terms of sensitive range and resolution and the experimental measurement that can be done, with the correspondent typical experimental errors, are summarized in the following table.

The plasma heating and injection system

The PANDORA trap microwave injection system includes three different microwaves injections lines in order to excite (even simultaneously) the plasma by means of the Electron Cyclotron Resonance (ECR).
Three separate frequency generators will be used to provide different drive signals for the three Klystron:

• two microwave injections (primary and secondary) provided by two Klystron Amplifiers operating in the frequency range from 17.3 to 18.1 GHz at 2.4 kW (output power);
• a tertiary microwave injection provided by a 1.5 kW CW (output power) Klystron amplifier operating in the frequency range 21-22 GHz.

This configuration will allow to operate in different microwave injection schemes: besides the single frequency heating, both the double frequency scheme ("far" and "close" operating with a frequency gap, respectively, of GHz or hundreds of MHz) and the triple frequency scheme will be possible.
Finally, the trap will also be equipped with vaporization oven systems for metal ions, also to produce plasmas of rare isotopes. The expected concentration ratio, with respect to the primary plasma (typically of He, O or Ar) is 1:100 (for metal isotopes) or 1:3 (for gaseous) in terms of partial densities.

Physics

PANDORA is a facility conceived for multidisciplinary studies especially in the astrophysics framework - laboratory plasmas can emulate, in terms of density, temperature and ionization states, certain conditions of astrophysical plasmas - but also for applications. PANDORA_Gr3 could "add unique research capabilities" [CVI-report 2019] in Astrophysics and Nuclear Astrophysics in laboratory, ensuring:

1. for the first time, -decay measurements in plasmas, with huge impact on nuclear physics and stellar nucleosynthesis;
2. plasma opacity measurements in conditions similar to kilonovae ejecta, allowing to study heavy elements production in n-star merging;
3. an unprecedented setup for applications: it will be the biggest B-minimum magnetic trap with potentiality as ion source; as testbench for magnetic fusion; as radiation source for Archaeometry. This could have the potentiality to design new ion and radiation sources for science and technology.

Further details are summarized in the PANDORA Technical Design Report, which describes the main subsystems of the facility. The main physical cases already identified and their nuclear astrophysics and applied physics interest, the experimental method, the available skills as well as the project management approach (project structure, collaborations, budget and risk analysis) are also summarized in the document.

References and Docs

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[PS21] Palmerini, S. et al., The Astrophysical Journal, 921, 7 (2021)