AAPPS-DPP2023

Plasma position measurements of tokamak plasmas
by dual-polarization reflectometry system
Y.Q. Shen1, Z.C. Yang1, W.L. Zhong1, M. Jiang1, Z.B. Shi1, J.
Santos2, P.W. Shi1, R.H. Tong1, J. Wen1, X. Yu1, G.Q. Xue1, Y.
Zhou1, W.C. Deng1
1Southwestern Institute of Physics, Chengdu 610041, China

2

first cut-off layer and 𝑙2 is due to profile
modification. We can use the first cut-off layer
displacement detected by X-mode reflectometry tomodify the results calculated based on O-modereflectometry in order to increase the precision ofcut-off layer displacement.
Fig. 2. (a) Plasma current 𝐼𝑝

Associação EURATOM/IST, Instituto de Plasmas e FusãoandNuclear—Laboratório Associado, Instituto Superior Técnico,𝑛̅𝑒 , (b) relative displacement1049-001 Lisboa, Portugalof the first cut-off layer, (c)Email address of the main author: shenyuqi@swip.ac.cn

relative displacement of theIn tokamaks, real-time feedback control of theplasma position, which plays a crucial role indisruption avoidance and machine protection, isusually performed by magnetic diagnostics [1].
During the long pulse operation of future fusion
reactors such as ITER and DEMO, the position
deduced from magnetic diagnostics may be subject
to substantial errors due to the drift of integrators [2].
The harsh radiation environment may also induce
potential changes in the mechanical and electricalproperties of magnetic coils. Plasma Position
Reflectometry (PPR) was originally proposed to
backup or supplement the standard magnetic-based
control in ITER during the long steady-state flat-top
periods as a result of its unique capability to measure
the location of a specified density layer in thevicinity of separatrix, with sub-centimeter spatial
resolution and microsecond temporal resolution, andof its robustness to withstand the harsh ITER
environment [3].
Fig. 1. Boundary density
profiles at two moments,
used to interpret the cutoff layer displacement
consisting of the first
cut-off layer displacement and the profile
modification

Generally, the cutoff density layer movement is generated by thecombination of plasma displacement (more precisely,
the displacement of the first cut-off layer where theelectron density is zero) and profile modification,
which can be explained by the Fig. 1. When density
profile changes from moment 𝑡1 to moment 𝑡2 , thecut-off layer moves by 𝑙1 + 𝑙2. The displacement of
any cut-off layer is equivalent to the displacement of
the first cut-off layer 𝑙1 if density profile maintains
its stiffness without density modification.
Accordingly, 𝑙1 is due to the displacement of theline-averaged

density layer for1019 𝑚−3

density

1.16 ×based on phaseshift, (d) comparison of thehorizontal

relative

dis-placement (black line) frommagnetic measurement andthe displacement of the cut-off
19−3

We have tried to measure layer for 1.16 × 10 𝑚
thecut-offlayer (blue line) which has been
displacement by dual- modified by the first cut-off
polarization reflectometry layer displacementon the HL-2M tokamak, as depicted in Fig. 2. Herea new physical model based on the phase shift is
proposed to deduce the relative movement of theplasma column without density inversion. A
comparison of the cut-off layer displacement (blueline) and the horizontal relative displacement (blackline) is displayed in Fig. 2(e) after the O-mode
calculation is adjusted by the displacement of thefirst cut-off layer. In overall, there is a good
agreement between the two curves, especially during
the significant horizontal displacement that takes
place between 400 and 600 ms. Additionally, when
the active gas puffing pulses are stronger, the cut-off
layer shifts significantly outward, which laterally
demonstrates the accuracy of the cut-off layer
displacement [4].
This is preliminary evidence that the specified cutoff layer from reflectometric measurement is analternative position estimation for monitoring theplasma-wall gaps. These results could provide animportant technical basis for the further developmentof a real-time control system based on PPR.
References[1] D. Gates et al 2005 Nucl. Fusion 46 17.
[2] A. Donné et al 2012 Nucl. Fusion 52 074015.
[3] J. Santos et al 2012 Nucl. Fusion 52 032003.
[7] Y. Shen et al submitted to Rev. Sci. Instrum..