Main research experiences:
·
MagnetoHydroDynamics (MHD) Instabilities in magnetized plasmas
1) Edge Localized Mode control
with external perturbation fields
Type-I edge localized modes (ELMs) have been mitigated at the JET tokamak
using a static external n = 1 perturbation field generated by four error
field correction coils (EFCCs) located far from the plasma. During the
application of the n = 1 field the ELM frequency increased by a factor of
4 and the amplitude of the Da
signal decreased. The normalised energy loss per ELM
DW/W
dropped to values below 2%. Furthermore, the temperature
at the outer limiters was also reduced. Transport analyses shows no or only a
moderate (up to 20%) degradation of energy confinement time during the ELM
mitigation phase. [ref. “Active control of
type-I edge localized modes with n = 1 perturbation fields on JET”, Y.
Liang, et al., Phys. Rev. Lett. 98, 265004 (2007)]
2) Error field and tearing mode
physics
The dynamic ergodic divertor (DED) on the TEXTOR tokamak allows for the
creation of static and rotating helical magnetic perturbation fields. In the 3/1 configuration the strong
m/n
=
2/1 sideband excites a locked 2/1 tearing mode above a critical perturbation field strength. The mode onset
threshold depends strongly on the plasma fluid rotation with respect to the
mode. Rotation in plasma current direction destabilizes the mode in a certain
range of rotation frequencies, whereas counter-rotation has a stabilizing
influence. The threshold shows a minimum when the frequency of the external
perturbation equals the MHD frequency of the mode. [ref. “Dependence
of the threshold for perturbation field generated m/n = 2/1 tearing modes on the
plasma fluid rotation”, H.R. Koslowski, Y. Liang, et al., Nucl. Fusion 46 No
8 (August 2006) L1-L5]
Experimental studies of the locking and unlocking of the m/n = 2/1
tearing modes using beta and plasma rotation scans have been performed on
TEXTOR. A large 2=1 island (width ~ 8 cm; ~ 17% of plasma
minor radius) near half plasma radius is seeded by application of a static (dc)
or rotating (ac) perturbation field with the Dynamic Ergodic Divertor (DED) in
3/1 configuration on TEXTOR. The 2/1 island is phase locked to the
external perturbation field, i.e. it has zero frequency in dc case. The sawteeth
are found to become stabilized after the 2/1 island has been excited.
Stabilizing of the 2/1 tearing mode by increasing of beta has been
observed in low beta plasmas. A significant influence of anisotropic pressure
induced by tangential neutral beam injection (NBI) on the locking of the 2/1
tearing modes has been observed. In the dc case, the island stays locked for a
few hundred ms after switch off of the DED, starting to spin up after co-NBI (PNBI
> 300kW) is switched off. In the 1 kHz case, the frequency of the 2/1
islands exponential decay towards zero after switch off of the DED, and stay
locked up to switch off of the co-NBI. The persistent locking of the 2/1 island
even without the external perturbation field can be attributed to coupling with
an m/n = 1/1 internal kink mode enhanced by a large anisotropic pressure
in the NBI heated plasmas. [ref. “Influence of anisotropic pressure on the locking of 2/1 tearing modes in
TEXTOR”, Y. Liang, et al., 32nd EPS Conference on Plasma Phys. Tarragona, 27
June - 1 July 2005 ECA Vol.29C, P-4.060 (2005)]
Collapse events at the q = 2 surface occurring during the mode
locking process of an m/n = 2/1 tearing mode have been observed on
TEXTOR. The plasma confinement within the q = 2 surface collapses without
much influence on the width of the island (O-point). With an external rotating
Resonant Magnetic Perturbation (RMP) field induced by the Dynamic Ergodic
Divertor (DED), secondary islands moving near the separatix of the primary large
2/1 island with the same frequency as the RMP have been observed after the
collapse events. The plasma confinement recovers when the secondary islands
vanish. [ref. “Observations of secondary
islands after collapse events occurring at the q=2 magnetic surface in the
TEXTOR tokamak”, Y. Liang, et al., Nuclear Fusion 47, (2007) L21-L25]
5) Density limit and Marfe
A significant influence of the dynamic ergodic divertor (DED) on the
density limit in TEXTOR has been found. In Ohmic discharges, where without DED
detachment normally arises at the density limit, a MARFE (multifaceted
asymmetric radiation from the edge) develops when the DED is operated in a
static regime. The threshold of the MARFE onset in the neutral beam heated
plasmas is increased by applying 1 kHz ac DED at the high-field side. The
theoretical predictions based on the parallel energy balance taking poloidal
asymmetries into account agree well with the experimental observation. [ref. “Influence
of the Dynamic Ergodic Divertor on the Density Limit in TEXTOR”, Y. Liang,
et al., Physical Review Letters
94,
105003 (2005)]
6) Measurement of Shafranov shift in the high beta plasma in Helical
Devices.
The Shafranov shift is derived from the two-dimensional profile of x-ray
intensity measured with a soft x-ray CCD camera in LHD. The accuracy of the
measurement of the magnetic axis is 3% of the Shafranov shift at high beta. The
measured Shafranov shift increases linearly up to 280 ± 3 mm, which is 47% of
the minor radius as the volume averaged beta <bdia> measured with a diamagnetic loop is increased up to 2.6%. The Shafranov
shift measured with a soft x-ray CCD camera agrees with that calculated with the
three dimensional equilibrium code VMEC.
[ref. “Measurement of Shafranov shift
with soft x-ray CCD camera on LHD” Y. Liang, et al., Plasma Phys. Control. Fusion, Vol. 44, 1383
(2002)]
The radius of the plasma magnetic axis has been measured with soft X-ray
CCD camera in its imaging mode. The Shafranov shift of plasma magnetic axis
measured from the x-ray images was found to be larger than that estimated from
diamagnetic loop in the low-density NBI heated plasmas in CHS. This discrepancy
is considered to be due to the beam pressure driven by a tangentially-injected
neutral beam.
[ref. “Imaging of soft x-ray by
using soft x-ray CCD camera”, Y. Liang,
et al.,
Proc. of the 2nd IAEA TCM on
Steady State Operation of Magnetic Fusion Device – Plasma Facing Component,
Fukuoka, Japan, 25-29th Oct. 1999, FURKU Report 99-05(67), Vol. III,
pp. 736-751”]
·
Energy and particle transport of magnetized plasmas
8) Internal transport barrier in
electron heat transport in Helical Devices
The internal transport barriers triggered by neoclassical bifurcation are
observed with electron cyclotron heating (ECH) in low-density plasmas. The
two-dimensional profiles electron temperature measured with the soft x-ray CCD
camera in CHS ITB plasma show a sharp temperature gradient appeared at the layer
in between the electron root and ion root of neoclassical transport where a
large radial electric field shear (Er shear) exists.
[ref.
“Photon
counting CCD detector as a tool of x-ray imaging”, Y. Liang, et al., Review of Scientific
Instruments, Vol. 72, 717
(2001)]
9) Internal transport barrier in
particle transport in Helical Devices
The radial profiles of titanium Ka
spectra are measured with photon counting x-ray CCD cameras for plasmas with a
neoclassical ITB in CHS. The impurity transport analysis based on the radial
profiles of emission and averaged energy of titanium Ka lines indicates that the diffusion coefficient
inside the neoclassical ITB is
one order of magnitude lower than that of the plasma without neoclassical
ITB. [ref. “Observation
of Low Impurity Diffusivity inside the Neoclassical Internal Transport Barrier
(ITB) in CHS”, Y. Liang,
et al.,
1) X-ray Pin-diodes Arrays
“Observation
of mode structure and mode locking using the Dynamic Ergodic Divertor on TEXTOR”,
Y. Liang, et al., 31st EPS Conference on Plasma Phys. London, 28 June -
“Radiation
power profiles in the plasma with the Dynamic Ergodic Divertor on TEXTOR”,
Y. Liang et al., 31st EPS Conference on Plasma Phys. London, 28 June -
3). X-ray CCD camera system
“Energy and spatial resolved measurement of soft x-ray
emission with photon counting x-ray CCD camera in CHS”, Y. Liang,
et al.,
Review of Scientific Instruments, Vol.
71, 3711 (2000).
“Photon
counting CCD detector as a tool of x-ray imaging”, Y. Liang,
et al., Review of Scientific
Instruments, Vol. 72, 717 (2001)
“Measurement
of soft x-ray image by using CCD camera for long pulse discharge“, Y. Liang, et al.,
Journal of Plasma and Fusion Research SERIES, Vol. 3, 427
(2000).
”Measurement
of the Shape of Magnetic Flux Surfaces in a High Temperature Plasma Using a Soft
X-ray CCD Imaging Camera”, Y. Liang,
et al., IEEE Transactions on Plasma
Science, Vol. 30, 84 (2002)].
4). Charge Exchange Spectroscopy (CXS)
“Measurements of
poloidal rotation velocity using charge exchange spectroscopy in a large helical
device”, K. Ida, S. Kado, and Y. Liang, Rev. Sci. Instrum. 71, 2360 (2000)
5). Soft X-ray Pulse Height Analysis (PHA)
“Diagnostics
of high temperature plasma with soft x-ray spectrometer under high count rates”,
Y. Liang, Nuclear Fusion and Plasma Physics (in Chinese), Vol. 17, 57,
(1997).
“Soft
x-ray spectrometer on HT-7 Tokamak”,
Y. Liang, et al., Fusion
Engineering and Design, Vol. 34 and 35, 201, (1997).