2026 年 21 巻 論文ID: 21.1201059
Additional electromagnets enable adjustment of the external DC magnetic field condition by varying the DC current applied to each electromagnet in a developed, large-diameter (LD), low-aspect-ratio (LAR) radiofrequency (RF) plasma source using a flat spiral antenna. The magnetic field topology downstream changes in three cases: diverging, uniform, and converging fields, while the field strength near the flat spiral RF antenna remains nearly unchanged. Electron density increases stepwise with increasing RF power, and the transition thresholds systematically shift toward lower RF power as the downstream magnetic field becomes more convergent. The lowered threshold power may be related to reduced axial electron loss under the converging-field condition. These results indicate that control of the downstream magnetic-field topology can modify the RF power threshold for accessing higher-density plasma branches in the LD-LAR RF plasma source.
High-density radiofrequency (RF) plasma sources provide high etching rates on semiconductor wafer surfaces [1] and increase ionized particle flux, which is required to optimize performance in plasma thrusters using a magnetic nozzle [2]. Helicon wave propagation in a magnetic field effectively increases RF plasma density nonlinearly. High density up to 1013 cm−3 is obtained with extensive operational parameters [3]. Large-diameter RF plasma sources can be applied as negative-ion sources for neutral beam injection (NBI) in the fusion community [4]. Several studies have developed large-diameter (LD) RF plasma sources and investigated plasma generation efficiency [5–7].
An LD RF plasma source with a low-aspect ratio (LAR) was developed [8], and the feasibility of high-density plasma generation in such a plasma source is being evaluated. LAR sources have the advantage of efficient processing in a large area and high thrust efficiency for RF plasma thrusters. Dependence of electron density, ne, on RF input power, PRF, was found in the previous experiment [8]. However, a nonlinear density increase, so-called “density jump,” was not achieved. Instead, ne increased monotonically with PRF.
Previous studies have reported abrupt density jumps in RF sources, and multiple density jumps were also investigated [9–11]. Another discrete density increase has also been observed in W-mode plasma even under an open magnetic field topology [9]. Shamrai et al. experimentally and analytically demonstrated that nonlinear RF power absorption and plasma-loading responses can produce multiple density branches and hysteretic transitions in a magnetized inductively coupled plasma (ICP) source [10]. In a numerical fluidic study considering wave propagation, Isayama et al. clarified that successive density increases in a high-density helicon plasma can be associated with different axial wave modes [11]. In contrast, Shinohara et al. reported a continuous density increase with increasing RF power in an ICP source using a spiral antenna without an abrupt density jump [12].
In the present experiment, multiple stepwise density increases were observed when an external DC magnetic field was applied downstream of the source. Since no direct RF-wave measurement was performed, these transitions are not identified as a W-mode transition. Instead, this study focuses on the possible origins of the stepwise density response and on the systematic shift of the transition thresholds with the downstream magnetic-field topology.
To adjust the magnetic-field topology, two additional solenoid coils, composed of 60-turn enamel wire wound around an aluminum alloy bobbin, are newly mounted inside a vacuum vessel. Converging magnetic fields can increase plasma density by reducing axial plasma wall losses, which can dominate in an LAR source [6, 7]. Previous studies clarified that plasma generation at a density of order 1013 cm−3 was accomplished using an axially converging magnetic field configuration in the Large-Diameter Device at Kyushu University [6]. Given helicon wave propagation from an RF antenna, the downstream magnetic field configuration is a critical factor even in LAR plasma sources such as Ref. [7]. This communication briefly compares plasma-density enhancement with additional coils that vary the field topology and discusses the potential for further high-densification in the LD-LAR-RF source.
Figure 1 shows the magnetic field topology (a) without and (b) with the additional electromagnets in the LD-LAR RF source. The source consists of a stainless-steel vessel with an inner diameter of 538 mm and a length of 300 mm, and the source aspect ratio is 0.56. A four-turn, oxygen-free copper, flat spiral antenna with an outer diameter of 380 mm is mounted on the surface of a quartz window with a thickness of 40 mm. The background pressure inside the vacuum vessel is 2.2 × 10−4 Pa, and the pressure is maintained at 0.9 Pa while continuously introducing Argon neutral gas. An RF power, PRF, is applied to the antenna with a 13.56 MHz RF power supply. Here, the PRF corresponds to the power setting of the RF power supply. The impedance matching network was manually adjusted to minimize the reflected power, which was typically below 5% of the forward power at each power setting. Pulsed discharge with a pulse duration of 40 ms was performed to avoid heat damage in the RF power supply system. Figure 1(c) compares the field strength on the z-axis, |Bext|, between the diverging (div.), uniform (uni.), and converging (conv.) cases downstream of the source. DC coil currents applied to the outer, inner upper, and lower coils, Iout, Iupper, and Ilower, are set to (Iout, Iupper, Ilower) = (2.2 A, 10 A, 0 A), (2.3 A, 6 A, 6 A), and (2.3 A, 5 A, 9 A) for the div., uni., and conv. cases, respectively. A straight-type Langmuir probe is inserted from the peripheral wall of the vessel at z = 140 mm. This probe tip is made of tungsten wire with a length of 3 mm and a diameter of 1 mm. This study does not use RF compensation. Effective electron temperature, Te, is derived from Te = 2⟨ε⟩/3, where ⟨ε⟩ is calculated from the first energy moment of the electron energy distribution function (EEDF), and electron density, ne, is given by integrating the EEDF. The detailed equations are explained elsewhere [13]. Probe current was measured during the quasi-steady phase of the discharge after the initial transient.
Figure 2 shows the dependence of measured Te and ne at (z, r) = (140 mm, 0 mm) on PRF as well as electron pressure, pe = eneTe, where e is the elementary charge. Te gradually decreased with higher PRF regardless of the magnetic field cases and reached ~7 eV at PRF = 1.8 kW, as shown in Fig. 2(a). Relatively high Te values were found when ne was low at PRF less than 0.8 kW for the uni. and conv. cases. The higher Te may be related to reduced axial electron loss and an increased effective confinement time due to the strengthened downstream magnetic field.
Figure 2(b) shows that the transition threshold PRF, at which ne increases nonlinearly, depends on the magnetic field conditions. Based on the previous characterization of this source [8], the present discharge is regarded as a magnetized ICP in the density range of 1011–1012 cm−3. The vertical lines indicate the RF powers below the observed density transitions. In addition to the first stepwise increase, a second nonlinear enhancement was observed at higher PRF, particularly in the converging configuration, resulting in three distinguishable density branches. These stepwise density increases are not taken as evidence of a transition to the helicon W-mode because no direct RF-wave measurement was performed. Further measurements of the RF magnetic-field structure are required for an unambiguous mode identification. The physical origin of the second transition remains unresolved, but we simultaneously observed a change in the impedance-matching condition. Under the converging magnetic field condition, reflected RF power temporarily increased near 0.9 and 1.2 kW before adjusting the matching network. After rematching, the reflected powers were decreased to below 5% of the forward power. A previous study discussed a similar nonlinear density transition related to changes in plasma loading resistance and circuit loss [14]. The increase in pe with PRF follows the corresponding increase in ne, as shown in Fig. 2(c). The converging magnetic-field configuration therefore enables access to a higher-electron-pressure state at lower RF power.
We estimated the axial electron loss using a simple magnetic-mirror model. The electron motion is expected to be approximately adiabatic, since the electron Larmor radius is much smaller than the magnetic field scale length. For the converging magnetic field condition, the mirror ratio Rm = Bext(z = 140 mm)/Bext(0) on the z-axis gives an estimated transmitted electron-flux fraction of approximately 1/Rm = 0.89. This suggests that the converging magnetic field can reduce axial electron loss. Although the ions may not satisfy the same magnetization condition, suppressing electron loss can modify the ambipolar potential and thereby influence the overall axial particle loss [15]. This mechanism is considered a possible explanation for the observed density increase rather than a confirmed confinement mechanism.
In summary, two nonlinear density increases were observed in an inductively coupled discharge in the LAR-LD RF source. The transitions were accompanied by changes in the impedance-matching condition, indicating nonlinear variations in the plasma loading. The threshold powers shifted systematically toward lower values under the convergent downstream magnetic-field configuration, suggesting that the convergent field promotes access to higher-density discharge branches. This study demonstrates multiple stepwise density increases in the LD-LAR-RF plasma source while systematically varying the downstream magnetic-field topology from diverging to uniform and converging configurations with only a small change in the magnetic field near the spiral antenna. The transition thresholds systematically shifted toward lower RF power as the downstream field became more convergent.
Although direct RF magnetic-field measurements are required to determine whether the highest-density branch corresponds to a W-mode, further increases in RF power may enable plasma densities comparable to those reported in other LAR helicon sources.
We acknowledge partial support of NIFS Collaboration Research Programs (Grant Nos. NIFS25KSPM006, NIFS22KIER002, and NIFS22KUGM194) and the young research funding program at Kobe University in 2025.
