Ioannis Pagonakis

Last Name

Pagonakis

First Name

Ioannis

ORCID

0000-0003-3707-5391

Organisational unit

09681 - Barnes, Alexander / Barnes, Alexander

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Publications 1 - 10 of 13

Study of Ionized Particles in a Gyrotron Using a Full Gyrotron Simulation Model
Marti, Lea; Pagonakis, Ioannis; Sieben, Leif; et al. (2024)
2024 Joint International Vacuum Electronics Conference and International Vacuum Electron Sources Conference (IVEC + IVESC)
The ionization of the residual gas in gyrotrons causes the generation of positively charged ions and negatively charged electrons. Using the electron optics code Ariadne, it was possible to simulate the behavior of these electrons in the overall gyrotron geometry in the presence of static electric and magnetic fields as well as the electromagnetic field of the nominal transverse electric mode in the cavity. The main observation of this study is that the negatively charged electrons gain enough energy from their passage through the cavity to overcome the decelerating voltage and escape towards the collector. On the other hand, the positively charged ions remain trapped in the cavity region contributing to the neutralization effect.
198 GHz Microwaves: From the Gyrotron Output Window to the Unpaired Electron Spins in the DNP NMR Sample
Millen, Marthe; Alaniva, Nicholas Howard; Däpp, Alexander; et al. (2022)
2022 47th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz)
Versatile instrumentation including a frequencyagile gyrotron as the microwave source, a Martin-Puplett interferometer to modify the microwave polarization, a Nuclear Magnetic Resonance (NMR) spectrometer with cryogenic magic angle spinning, as well as an electron spin detection circuit is presented. The combination of these state-of-the-art technologies allows the recording of high-performance dynamic nuclear polarization spectra, a method used to enhance the sensitivity of NMR experiments via microwave irradiation.
Fabrication Procedures and Mechanical Supports of No-Insulation All-GdBCO Double Pancake Magnets in Liquid Helium
Gao, Chukun; Chen, Pin-Hui; Alaniva, Nicholas; et al. (2024)
IEEE Transactions on Applied Superconductivity
The no-insulation (NI) high-temperature superconductor (HTS) winding technique enables the fabrication of highly compact magnets with self-quench protection. NI pancake coils are implemented to develop a 28-T HTS magnet for the operation of a 792-GHz gyrotron, which is a microwave source for dynamic nuclear polarization nuclear magnetic resonance. To this end, three NI all-GdBCO double pancake (DP) magnets were fabricated using different winding diameters and procedures. The objective was to explore different mechanical coil protection mechanisms at high magnetic fields, including clamping, overband, and solder impregnation. Experiments in liquid helium using magnets with winding diameters of 18, 25, and 66 mm yielded a center field of 14.4, 11.2, and 8.1 T, respectively. The maximum currents applied to the DP magnets ranged from 780 to 1000 A. Both the 18 and 66 mm DP coils contained 400 m (2 × 200 m) HTS tape, while the 25 mm seamless DP coil consisted of only one continuous 200 m HTS tape. The 25 mm magnet with solder impregnation showed the best repeatability, although the current density was reduced owing to the solder thickness between the coil windings. Critical to the implementation of such coils in liquid helium is to effectively transfer high currents to the magnet, while not compromising the helium boil-off. Furthermore, the design of hybrid copper-HTS current leads capable of carrying current larger than 1000 A from room temperature to the HTS magnet at 4.2 K is presented. The implementation of liquid nitrogen-cooled hybrid copper-HTS current leads reduced the helium boil-off and permitted independent temperature control of the current leads.
Thorough Simulation of High-Power Gyrotron Cavity Interaction in the Hard Excitation Regime
Pagonakis, Ioannis; Genoud, Jeremy; Hogge, Jean-Philippe; et al. (2024)
2024 Joint International Vacuum Electronics Conference and International Vacuum Electron Sources Conference (IVEC + IVESC)
Using the new interaction code Phaedra, a thorough and systematic investigation of the operation of a high-power gyrotron cavity in the hard excitation regime took place. The preliminary result of this analysis shows that in the hard excitation regime, the nominal mode can potentially be excited at more than one frequency with a difference in the range of few tens of MHz and with a significant power difference at the same accelerating voltage. Furthermore, improper choice of parameters for the startup simulations can guide to an important overestimation of the stability of the mode at higher voltages.
Gyrotron-Alignment Platform with Five Degrees of Freedom
Alaniva, Nicholas Howard; Millen, Marthe; Osborn Popp, Thomas M.; et al. (2022)
2022 47th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz)
The proper placement of gyrotron relative to the magnetic field is necessary for optimal gyrotron performance. A gyrotron-alignment platform capable of independent, precise adjustment in five degrees of freedom has been developed. The effectiveness of this platform is experimentally demonstrated by alignment of a 198 GHz gyrotron used for Dynamic Nuclear Polarization
23 Tesla high temperature superconducting pocket magnet
Gao, Chukun; Chen, Pin-Hui; Alaniva, Nicholas; et al. (2024)
Superconductor Science and Technology
We present a compact 23 T no-insulation (NI) magnet that was wound with 60 m of 10 mm wide high temperature superconducting (HTS) tape. The magnet consists of only one pocket-sized double pancake (DP) coil with an inner diameter of ∼6 mm, a height of 20 mm, and an outer diameter of 41.6 mm. Another NI coil of similar size but with a larger inner diameter of 8 mm reached a slightly lower magnetic field of 21 T. We also present a smaller coil which was wound with only 20 m of HTS tape and still achieved a magnetic field of 16 T. During the experiments in liquid helium, each coil was charged to a current between 690 A and 850 A, corresponding to a high current density of 1500–1900 A mm−2. The small bore size and high current density contributed to the high fields generated by these coils. We present the fabrication details, helium tests and repeatability analysis of these 'pocket' magnets.
Experimental characterization of the TCV dual-frequency gyrotron and validation of numerical codes including the effect of After Cavity Interaction
Genoud, Jérémy; Alberti, Stefano; Hogge, Jean-Philippe; et al. (2024)
EPJ Web of Conferences ~ 22nd Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating (EC22)
A dual-frequency gyrotron has been developed within the context of the recent Tokamak à Configuration Variable (TCV) upgrade. The gyrotron is designed to generate a 1 MW, 2 seconds RF wave at 84 or 126 GHz. Before integrating the gyrotrons in the TCV tokamak ECRH system, an extensive characterization of their behaviour has been performed. This paper focuses on presenting the results of these experiments at the two operating frequencies. The power measurements are systematically compared with numerical simulations. This comparison highlights the validation of numerical codes and the effect of After Cavity Interaction (ACI), a crucial factor that must be considered for achieving a good agreement between theoretical predictions and experimental results.
Development of Kilowatt THz Gyrotrons for Pulsed Dynamic Nuclear Polarization - Cavity Design Code Phaedra -
Pagonakis, Ioannis; Genoud, Jérémy; Gao, Chukun; et al. (2022)
2022 47th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz)
A significant effort has recently been initiated at ETHZ for the development of high frequency, high power, frequencyagile gyrotrons for DNP-NMR spectroscopy. In this context an in house cavity interaction code is under development. In this paper, some details on the development of the new cavity code and the preliminary results will be presented.
Cryogenic magic-angle spinning continuous wave EPR and DNP spectroscopy at 7 T with a gyrotron
Millen, Marthe; Alaniva, Nicholas; Björgvinsdóttir, Snædís; et al. (2025)
Journal of Magnetic Resonance
ynamic nuclear polarization (DNP) relies on the transfer of electron polarization to nuclei through microwave irradiation and is typically performed under cryogenic magic-angle spinning (MAS) at high magnetic fields. Gyrotrons are commonly used microwave sources in DNP because of their ability to produce high-power microwaves over a broad frequency range. An important step towards a more in-depth understanding of DNP mechanisms and rational optimization of DNP performance is the access to instrumentation, which can provide information about the DNP process. Continuous wave (CW) electron paramagnetic resonance (EPR) can reveal important information on the electron spin system during DNP experiments. Here, we present a dual CW EPR/DNP spectrometer operated under MAS at 100 K and 7 T using a frequency-agile 198 GHz gyrotron. The measured sensitivity for CW EPR at 198 GHz using an MAS stator is 4 × 1013 spins/(G√𝐻𝑧). To illustrate the electron and nuclear detection capabilities of our setup, DNP profiles and CW EPR spectra of a diamond powder and a trityl sample were recorded under the same conditions, specifically at 100 K and under MAS.
A gyrotron cavity interaction simulation approach
Pagonakis, Ioannis; Barnes, Alexander B. (2025)
Applied Mathematical Modelling
Microwave generation in a gyrotron occurs within the interaction cavity, where a high-energy electron beam resonates with a transverse electric mode of the cylindrical cavity. Numerical tools are essential for designing and analyzing cavity operations, with the gyro-average approach traditionally fulfilling this role. This paper introduces an alternative model for fast and accurate simulation of the interaction cavity. Two innovative methods are proposed for calculating electron beam dynamics: the first employs a perturbation technique, while the second utilizes a particleless approach by expanding beam properties in a Fourier series. A specialized code has been developed based on this novel framework. This paper provides a detailed overview of the simulation model, its implementation, and initial results obtained using the new tool.

Publications 1 - 10 of 13

Ioannis Pagonakis

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