High Field Diamond Magnetometry Towards Tokamak Diagnostics

TL;DR

This work addresses the need for radiation-hard, high-field magnetometry in tokamaks by adapting fibre-coupled ensemble nitrogen-vacancy (NV) centers in diamond to operate in fields up to $1.2 T$ using a cw ODMR scheme with microstrip microwave delivery. The approach yields high-field sensitivities in the sub-microtesla range, with $240-600\,nT/\sqrt{Hz}$ for non-degenerate NV alignments and $110\,nT/\sqrt{Hz}$ for near-$\langle111\rangle$ alignments within the $(10-150)\,Hz$ band, while demonstrating eight resonances for non-degenerate and four for near-$\langle111\rangle$ configurations. The study highlights both the promise for mm-scale, radiation-hard tokamak diagnostics and the practical challenges of vector magnetometry at high field due to the ill-conditioning of the sensitivity matrix (A-matrix), suggesting paths for optimization via alignment, isotope engineering, and advanced tracking. Overall, the results indicate that high-field diamond NV magnetometers can contribute to robust magnetometry in future DEMO/industrial tokamaks, provided engineering considerations such as dynamic range, heat management, and radiation-hard interconnects are addressed.

Abstract

Nitrogen vacancy centres (NVC) in diamond have been widely used for near-dc magnetometry. The intrinsic properties of diamonds make them potential candidates for tokamak fusion power diagnostics, where radiation-hard magnetometers will be essential for efficient control. An NVC magnetometer placed in a tokamak will need to operate within a $\geq$ 1 T magnetic field. In this work, we demonstrate fibre-coupled ensemble NVC optically detected magnetic resonance (ODMR) and magnetometry measurements at magnetic fields up to 1.2 T. Sensitivities of approximately 240 to 600 nT/$\sqrt{\textrm{Hz}}$ and 110 nT/$\sqrt{\textrm{Hz}}$ are achieved in a (10-150) Hz frequency range, for non-degenerate and near-$\langle$111$\rangle$ field alignments respectively.

Figures (31)

• Figure 1: A schematic of the experimental setup showing the sensor head within the electron paramagnetic resonance (EPR) Magnets. The inset shows a photograph of the holder with its goniometer and the EPR magnets.
• Figure 2: (a) Demodulated ODMR spectrum at a magnetic field strength of approximately 4 mT for a non-degenerate bias field alignment. A modulation depth of 400 kHz was used, such that the hyperfine resonances were visible. (b) Demodulated ODMR spectrum at a magnetic field strength of approximately 0.95 T for a non-degenerate bias field alignment. A modulation depth of 4 MHz was used. The resonances are labelled 1 to 8 for both spectra.
• Figure 3: For a non-degenerate alignment, (a) sensitivity spectra taken on the resonances 5 to 8, labelled in Fig. \ref{['fig:NondegenerateODMR']}, at a magnetic field strength of approximately 0.95 T. The inset shows the 80 Hz test field. (b) Sensitivity spectra taken at low and high-field (approximately 4 mT and 0.95 T respectively) for resonance 8. A LIA LPF of 150 Hz was used.
• Figure 4: (a) Demodulated ODMR spectrum at a magnetic field strength of approximately 1 mT for a near-$\langle$111$\rangle$ bias field alignment. (b) Demodulated ODMR spectrum at a magnetic field strength of approximately 0.95 T for a near-$\langle$111$\rangle$ bias field alignment. The resonances are labelled 1 to 8. The splitting between resonances 1 and 8 was approximately 5.7 GHz. A modulation depth of 4 MHz was used for both measurements.
• Figure 5: For a near-$\langle$111$\rangle$ alignment, (a) sensitivity spectra taken when magnetically sensitive (on-resonance) and magnetically insensitive (off-resonance) at a magnetic field strength of approximately 1 mT. (b) Sensitivity spectra taken at low and high-field (approximately 1 mT and 0.95 T respectively). A LIA LPF of 150 Hz was used. An 80 Hz test field was applied parallel to the bias field, the strength differed between low and high field.