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

Improved plasma performance via low-Z powder injection in theLarge Helical Device
F. Nespoli1, K. Tanaka2,4, S. Masuzaki2,3, N. Ashikawa2,3, M. Shoji2, E.P. Gilson1, R. Lunsford1,
A. Bortolon1, D.A. Gates5, K. Ida2,3, G. Kawamura2,3, T. Kawate2,3, N. Kenmochi2,3, T. Kinoshita4,
A. Mollen1, T. Morisaki2,3, G. Motojima2,3, A. Nagy1, S. Ohdachi2,3, T. Oishi6, M. Osakabe2,3,
N.A. Pablant1, C. Suzuki2, H. Takahashi2,3, Y. Takemura2,3, N. Tamura2, M. Yoshinuma2,3
1Princeton Plasma Physics Laboratory, Princeton, NJ, USA 2 National Institute for Fusion Science,Toki, Japan 3The Graduate University for Advanced Studies, SOKENDAI, Japan 4Interdisciplinary
Graduate School of Engineering Sciences, Kyushu University 5Thea Energy, Princeton, NJ, USA
6
Department of Quantum Science and Energy Engineering, Tohoku University, Japane-mail (speaker): fnespoli@pppl.gov
We report the results from recent impurity powder
injection experiments in the Large Helical Device inJapan, conducted over the last three years (20192022). With the main goal of improving wallconditions and plasma performances, different low-Zmaterials (B, BN, B4C, C, Li) sub-millimeter powders
have been injected into the plasma under the action ofgravity using the Impurity Powder Dropper (IPD) [1].
The installation of the IPD was guided by EMC3EIRENE and DUSTT simulations [2] to optimize thepowder penetration in the LHD unique magnetic
geometry.
One of the main results of this series of experiments
is the improvement of wall conditions: reduction ofintrinsic impurity content C, O, Fe and reduction ofwall recycling, both on a shot-to-shot basis and in realtime [3].
Leveraging these results by injecting boron powder inminute-long discharges, an otherwise occurring
radiative collapse was successfully avoided [4].
Furthermore, the access to a reduced-turbulence
improved confinement regime was observed uponboron powder injection [5] at constant power and line
averaged density, resulting in an increase of theplasma temperature on the order of 25%, but with anincrease of ion temperature that can be as high as 50%.
At the same time, the turbulent fluctuations amplitude
has been measured to decrease by up to a factor of 2.
Access to this regime has been investigated inextensive dedicated experiments [6], and has been
observed for different powder materials, heating
schemes, plasma main ion species (H and D), and forboth directions of the magnetic field.
The main mechanism behind this improvement ofconfinement is not fully understood yet, but is mostprobably due to the suppression of ITG turbulence
driven by plasma profile modification and increase ofeffective charge resulting from the impurity powderinjection.
Leveraging the increase of confinement during
powder injection, B powder has been injected into thehigh ion temperature scenarios (low density and highpower plus C pellet injection), resulting in a further
increase of Ti of about 50% [7].
Simulation efforts to better understand dynamics of
powder injection into the LHD plasma and help

optimize real time wall conditioning and the access tothe improved confinement regime are ongoing [8].

Figure 1: Radial profiles of (a) electron density (b)electron (solid lines) and ion temperature (dashed lines)(c) turbulent fluctuation amplitude measured by PCI, fora boron injection discharge (#167234, red) and its
reference (#167233, blue), at t=5.25 s (during injection).
Reprinted from ref. [6].
References[1] A. Nagy et al., Rev. Sci. Instrum. 89 10K121 (2018)[2] M. Shoji et al., Contrib. Plasma Phys. 60 e201900101(2019)[3] R. Lunsford et al., Nucl. Fusion 62 086021 (2022)[4] S. Masuzaki et al., this conference[5] F. Nespoli et al., Nat.Phys. 18 350–6 (2022)[6] F. Nespoli et al., Nucl. Fusion 63 076001 (2023)[7] H. Takahashi et al., in preparation[8] M. Shoji et al., this conference