7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Volumetric recombination of high density plasma in converging fieldfollowing an ECR plasma source
A. Okamoto, K. Takeda, K. Sato, Y. Yamada, M. Koike, M. Sugimoto, and T. Fujita
Graduate School of Engineering, Nagoya University
e-mail (speaker): a-okamoto@energy.nagoya-u.ac.jp
Atomic processes and transport of high-density plasmas
in external magnetic field are significant processes forthe divertor plasma study, where comprehensive
understanding on volumetric recombination ofhigh-density plasma flowing along the field line isrequired. We have developed a high-density plasma
production method applicable to arbitrary higher
magnetic field than the ECR magnetic field,[1] in which
both trapped electrons and an overdens electromagnetic
mode play important roles. Because resonantly
accelerated electrons are trapped in a magnetic mirror
generated between the ECR plasma production region
and the higher magnetic field region, the electrons
efficiently ionize before they diffuse to a chamber wall.
In experiments using an ECR plasma device NUMBERwith a frequency 2.45 GHz microwave power supply. Aright-hand circularly polarized microwave is injectedfrom an end of the chamber, where a condition of highfield side injection along magnetic beach is satisfied asshown in Fig. 1. Helium gas is supplied to both theproduction region and the higher magnetic field region(test region). Then a high-density 𝑛e ≃ 1 × 1018 m−3
helium plasma in magnetic field 𝐵 ≃ 0.3 T has beensuccessfully obtained.
Particle fluxes are measured by the ion saturation
current of Langmuir probes for various helium gaspressure. Particle flux at the middle of the test region
monotonically increase with the helium gas pressure,while that at the end plate saturates for 𝑝 > 0.8 Pa. Thenthe ratio of particle flux at the endplate normalized tothat at middle of the test region increases with the gaspressure for 𝑝 < 0.5 Pa, while it decreases with thepressure for 𝑝 > 0.8 Pa. The phenomenon is similar toso-called the density roll over, which is observed in theplasma detachment from divertor targets. In suitableneutral gas pressure 𝑝 ≃ 1 Pa , line emission spectrafrom highly excited states of helium atom, He I (𝑛 ≤ 13),have been observed in the middle of the test region.
These spectra are good indicator of volumetricrecombination. Sufficiently low electron temperature
𝑇𝑒 ≃ 0.06 eV to induce the recombination has beenobtained from Boltzmann plot as shown in Fig. 2. While
the volumetric recombination and the density roll overare often observed in DC arc discharge plasmas, this is
the first time observation in ECR plasmas.[2] Then anopen question is relaxation process for the energeticelectrons generated by ECR.
In order to answer the question, the measurement of
anisotropic electron energy distribution using a laserThomson scattering system is ongoing as well as
electrostatic measurement. Initial results on theelectrostatic measurement of floating potential both in
the ECR plasma production region and in the test region
suggest higher electron temperature clump is transported
into the higher magnetic field region.[3]The work is supported by JSPS KAKENHI Grant Nos.
26420848, JP19H01869, JP20H01883, JP22H01202 andJP23H00148 and the NIFS collaboration researchprogram,Grant
Nos.
NIFS20KBAH031
andNIFS22KIIP008.
References[1] A. Okamoto, et al. Plasma Fusion Res. 14, 2401005(2019)[2] K. Yagasaki, et al., Plasma Fusion Res. 18, in press(2023)[3] A. Okamoto, et al., Jpn. J. Appl. Phys. 62, SL1022(2023)

Figure 1 Schematic of NUMBER device and magnetic
field profile on the chamber axis. OES denotes theposition of optical emission spectroscopy.

Figure 2 Boltzmann plot of He I line emission spectra.