Snake Perturbations during Pellet injection in the HL

Snake Perturbations during Pellet injection in the HL
Snake Perturbations during Pellet injection
in the HL-1M Tokamak
Key words
Soft X-ray
FU Bingzhong
DONG Yunbo
Pellet injection
Snake oscillation
Pellet injection experiments have been widely carried out with various toroidal plasmas,
peaked density profile and improved confinement are frequently observed, and some
interesting phenomena have also been found. One of them is the long-lived m=1/n=1
oscillation observed on soft X-ray emission and other diagnostics (notably the microwave
interferometers and the ECE systems) after pellet injection in JET [1], JT-60 [2], and Tore Supra
, which has been called the snake oscillation.
Snakes are observed in HL-1M after high-speed solid hydrogen pellets injection during
ohmic discharges. Preliminary results have been already reported [5]. A more thorough
discussion of the available data on snakes is the purpose of this article. The experimental
observations on snake after pellet injection are presented.
Experimental setup Snake Perturbations in pellet fuelled plasma
The HL-1M tokamak has circular cross section with major radius R=1.02m, minor radius
ne=0.3-3.6 × 1019 m −3 , central electron temperature Te (0) ≈ 900ev,plasma current Ip in the range
100-200kA, toroidal magnetic field in the range 2.0-2.6T at R=1.02m. High speed hydrogen
pellets with a size of d P =1.2 mm and with velocities in the range of 200 to 1100m/s have
been injected into ohmically heated plasma in the HL-1M tokamak. One set of the SX array, a
H α emission array, and a CCD camera are located at same poloidal section with pellet
injector to monitor the plasma radiation. In HL-1M, soft X-ray (SX) and H α signals are
obtained by means of PIN diodes array consisting of 20 channels each. An interference filter
with 656.3nm is used for H α detectors and a Berylium foil with 25 µm is covered on the SX
camera. The time resolution of measurement is 10 µs for both soft X-ray and H α detectors.
The CCD camera at about 13.5o inclination angle with respect to the midplane is for pellet
ablation cloud photographing.
2 Snake Perturbations in pellet fuelled plasma
Excitation of large snake-like oscillation has been observed in the core region after
injection of high-speed pellets into the HL-1M plasma. At moderated density
(ne=1.5-3.0 × 1019 m −3 ) the pellets penetrated to and beyond the q=1 surface. The crossing of
the q=1 surface was accompanied by a very considerable drop in the pellet ablation rate-an
effect which has been associated with the reduced reservoir of hot electrons available for
ablating the pellet within the resonant flux tube. Fig. 1 shows the corresponding snake after
pellet injection as seen by the soft X-ray camera, it has a minor radius (rs) of roughly 3cm and
has been found with an m=1 topology by a comparison of the phase of the snake on all the
Strong bursts of snake oscillation frequently appear when the pellets cross the q=1
surface, but this is not invariable. Sometimes, a transient m=1 sinusoidal MHD oscillation is
often seen or there is no MHD activity observed just after pellet injection. In ohmic plasmas,
under the condition of pellet having crossed the q=1 surface, snakes are produced in about
60% of the cases. The probability of snake production becomes high when pellets penetrate
deeper inside the q=1 surface. On the other hand, when snake is produced, sawteeth very
rarely appear. This is contrast to the observations in other tokamaks where the snake can
co-exist with sawtooth and survive to the sawtooth crashes.
An ohmic plasma is considered with the following parameters:
I=200kA, B=2.4T,
Te=0.9kev and ne ≈ 2.1 × 1019 m −3 before pellet injection at about 344.0ms. In Fig.2, a snake is
observed in the central soft X-ray signals, while sawtooth activity is observed after pellet
injection. The lifetime of snake is about four times than that of sawtooth at a time before
pellet injection. Hidden-line perspective plot of the snake observed by vertical multi-channel
detectors, shows a snake-like oscillation
Further experiments developing different methods to measure the current modifications
induced by the pellet injection, particularly during the transient phase, are needed for a better
understanding of the snake formation.
Sometime, the recurrence of sawteeth and/or m=1 mode was found at the later phase of
RF-pulse in which the sawteeth were suppressed. It suggests that the current profile may be
barely stable and that a slight change in the imposed RF current profile may be sufficient to
cause central MHD instability. Furthermore, a rotating m=1 mode oscillation with snake-like
features was frequently observed at the end or later phase of the suppression of sawteeth (or
m=1 mode), which will be discussed below.
3 Conclusion
Snake phenomena are observed in HL-1M after pellet injection for a large range of
density under certain conditions related to the penetration depth. Location of snake oscillation
and its relationship to q=1 surface have been analyzed from multi-channel soft X-ray and
H α measurements. The plasma q profile is deduced from photographing of a CCD camera
during pellet ablation. Quantitative analysis of the experimental results indicated that, when
the plasma initial q profile is quite flat, pellet injection may cause the rational q=1 surface
shift inside to a inner region (where a large amount of particles are deposited) because of
changes of local temperature and density, thus a magnetic island that traps particles released
from the pellet is formed leading to a snake oscillation. Further experiments are needed to
estimate the current profile modification induced by the pellet with different methods. In the
next step, we intend to increase the view angle for the camera or use two CCD cameras to
observe the ablation process from different directions, thus results with more accuracy of q (r)
profile will be obtained. Owing to the lake of means of direct measurements, the perturbations
of electron temperature and electron density in the snake region in the HL-1M plasma have no
obtained. The mechanism of snake-like perturbations will be further studied.
The authors would like to thank Prof. Ding Xuantong and Prof. Guo Gancheng for
useful discussion.
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图 2 The time-evolutions of plasma current, electron density and soft X-ray intensity, a、b、
c、d、e are soft X-ray the intensity at r=4,2,0,-2,-4cm, respectively.
Fig.1 Time history of a pellet produced snake. In shot 5290
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