2026 Volume 21 Article ID: 1202034
Plasma experiments on electron cyclotron resonance heating (ECRH) using a high-power optical vortex beam (OVB) have been demonstrated for the first time in the Heliotron J stellarator/heliotron device. The OVB is generated by a spiral phase plate in a 70 GHz ECRH system. Plasmas are successfully sustained up to the second harmonic extraordinary (X) mode cut-off density. Virtually identical performance in terms of stored energy and density limits confirms the injection of the OVB as a heating tool comparable to Gaussian beams. These first results showed that the OVB is a possible method for the research on wave and plasma physics in magnetically confined fusion systems.
Electron Cyclotron Resonance Heating (ECRH) is primarily used for plasma heating in magnetically confined fusion devices [1, 2]. In stellarator/heliotron (S/H) devices and tokamaks, it can be utilized for plasma initiation, sustainment, control, and physics study, as well as for current drive. Although ECRH is recognized as an effective heating scheme, the issue is that the accessible density is limited by a cut-off density for electromagnetic EC waves. In recent years, the Optical Vortex Beam (OVB), previously explored in other research fields, has been introduced into the microwave frequency regime in fusion research; it is a beam form possessing a rotating wavefront and carrying orbital angular momentum (OAM) [3]. Theoretical calculations and numerical simulations reveal that the OVB is promising for overcoming the cutoff via mode conversion [3, 4]. The generation of the OVB was experimentally demonstrated in a free-space propagation [5]. Recently, we have introduced an OVB generation system into a 70 GHz ECRH system [6] on the Heliotron J S/H device. This paper reports on the first plasma experiments using the OVB on Heliotron J.
To generate high-power OVB, a 70 GHz ECRH system on Heliotron J has been modified with a switching system that includes an off-axis spiral phase plate (SPP) [5, 7], miter bends, and corrugated waveguides to convert the standard Gaussian beam.
Prior to the plasma experiments, we used the same waveguide system to conduct a low-power (mW) measurement using a Gunn oscillator and a high-power (kW) measurement using a gyrotron. The beams were converted into an OVB with a topological charge of l = 1 via the SPP. The low-power measurement yielded an OVB beam radius of 58 mm at 50 cm from the waveguide outlet, which aligns well with results (57 mm) calculated by OVB formula. In the high-power measurement, the existing gyrotron was employed to generate 250 kW ECW. The beam intensity distributions were measured using a thermal target paper and an infrared (IR) camera. These results were compared with those of a Gaussian beam, as presented in Fig. 1. To ensure experimental consistency, both OVBs and Gaussian Beams were transmitted through an identical setup replicating the actual injection configuration. Comparative dummy load measurements reveal an approximate 10% power loss for the OVB relative to the Gaussian beam, attributed to the conversion of the SPP. While the measured radius of the generated l = 1 OVB shows agreement with mathematical computation, some discrepancies in distribution are observed. These deviations are likely attributable to appearance of spurious modes during generation at the SPP or the transmission. Consequently, this OVB configuration will be employed for the subsequent plasma injection experiments. The details of the OVB system and measurement results will be reported in the forthcoming paper.
To analyze the plasma behavior under these injection scenarios, plasma parameters including line-averaged electron density (ne), stored energy (Wp), and Hα signal are measured. Additionally, a stray millimeter-wave radiation diagnostics system (SMWR) is employed to estimate the plasma absorption of the injected EC power, which reflects the total absorption rate. The previous study showed that estimation of EC power absorption rate is close to simulation results from the TRAVIS ray-tracing code [8].
The standard (STD) magnetic configuration is chosen with the magnetic field of 1.25 T. Plasmas are initially produced using 2.45 GHz, 7.2 kW non-resonant microwaves generated by a magnetron [9]. Subsequently, the ECRH power of 250 kW with perpendicular launch (N|| = 0) is injected in four injection cases: extraordinary (X)-mode and ordinary (O)-mode, each with the SPP branch engaged (for OVB generation) or disengaged (for conventional Gaussian beam injection).
Two discharges of the X-mode injection with OVB injection or Gaussian Beam are shown in Fig. 2 for comparison. The timing diagrams for the magnetron and ECRH are included to confirm the consistency of the preset discharge conditions. The electron density is ramped up by a gas puff to study the accessible density. The results show that the maximum electron density reached nearly the cut-off density, ne = 3 × 1019 m−3 in both cases. The plasma collapses at the end of discharge due to radiation loss. This demonstrates that the OVB can sustain plasmas comparable to the Gaussian beam. During the ramp-up phase of the plasma parameters, Wp is comparable, and the EC absorption rate for the OVB appears to be slightly lower than that for the Gaussian Beam. Both parameters are nearly identical when the maximum ne is reached. The Hα data, which can reflect the plasma boundary conditions, indicate that the plasma possessed similar boundary conditions under both injection conditions [10]. Although the accessible density did not exceed the cut-off density in the experiment reported here, these results suggest that the OVB can achieve plasma sustainment comparable to those of a conventional Gaussian Beam.
The relationship between the EC power absorption rate and electron density has been analyzed for the two X-mode injection scenarios, as presented in Fig. 3. In all cases, the absorption efficiency peaks around ne = 1 × 1019 m−3. The power absorption rate for the X-mode OVB is 80% comparable to that for the X-mode Gaussian beam. The single-pass absorption rate for the X-mode Gaussian beam is 99 % according to the TRAVIS code simulation. We will compare them more quantitatively by measuring the electron temperature with a Thomson scattering diagnostic, which was not available in this experiment. The relationships between plasma stored energy and electron density were analyzed for three scenarios: O-mode OVBs and X-mode Gaussian beams and OVBs. It was found that at an electron density of ne = 1 × 1019 m−3, the plasma stored energy reached its peak for three injection methods. Specifically, the stored energy for X-mode Gaussian beam and OVB injections was nearly identical at approximately 1.4 kJ, even despite a 10% loss in beam power. At the same density under O-mode OVB injection, the peak stored energy was approximately 1.2 kJ. These findings support the potential for further comparative studies on Gaussian beam and OVB heating in the future. Further experiments are necessary to clarify the heating performance of the OVB under high-density conditions by adjusting the injection mode and angle.
In conclusion, the world first plasma heating experiments using OVB injections have been done, the plasma experiments have demonstrated that OVB can be used to heat plasma like Gaussian beams, and the OVB injection achieves plasma heating and sustainment comparable to a conventional Gaussian beam at densities close to the cut-off density. Crucially, even with a notable 10% power loss, the OVB achieved stored energy and electron density levels equivalent to those of the Gaussian beam, implying its potential for EC wave and plasma physics research in future. While these initial results are promising, the dataset is limited in this period of experiments for heating optimization. Future work will focus on experiments at higher densities to validate theoretical predictions and on the investigation of plasma heat transport using OVB heating.
The authors are grateful to the Heliotron J group for their support in conducting the experiment. This work was conducted with support from the NIFS Collaborative Research Program (NIFS10KUHL030, KFFT003), Grant-in-Aid for Scientific Research, MEXT (Kiban (B) 26K00685), and JSPS Core-to-Core Program, A. Advanced Research Networks, PLADyS.
