Journal of the Korean Physical Society, Vol. 64, No. 9, May 2014, pp. L1244∼L1247

Letters

Transient Snakes in an Ohmic Plasma Associated with a Minor Disruption in the HT-7 Tokamak Songtao Mao, Liqing Xu,∗ Liqun Hu and Kaiyun Chen Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China (Received 13 December 2013, in final form 28 March 2014) A transient burst (∼2 ms, an order of the fast-particle slowdown timescale) of a spontaneous snake is observed for the first time in a HT-7 heavy impurity ohmic plasma. The features of the low-Z impurity snake are presented. The flatten electron profile due to the heavy impurity reveals the formation of a large magnetic island. The foot of the impurity accumulation is consistent with the location of the transient snake. The strong frequency-chirping behaviors and the spatial structures of the snake are also presented. PACS numbers: 52.35.Py, 52.70.La, 52.55.Tn, 52.55.Fa Keywords: Snake, HT7 Tokamak, Frequency chirping, Ohmic plasma DOI: 10.3938/jkps.64.1244

I. INTRODUCTION The snake, including a pellet induced one [1–5] and an intrinsic impurity spontaneous accumulation driven one [6,7], can lower the plasma energy confinement, slow the plasma toroidal rotation and trigger the annoying neoclassical tearing mode (NTM). On the other hand, the snake can as serve a probe of the radial location of the q = 1 surface, as a flag of central impurity accumulation, and as a ruler of plasma rotation in the q = 1 surface. Thus, understanding the formation and controlling of snake is very important to tokamak (like international thermonuclear experimental reactor (ITER)) operation. In a discharge with a central safety factor slightly above unity, the broad current profile, which is due to the accumulation of impurities brings about an overlapping of adjacent magnetic islands and results in an ergodization of magnetic surfaces, hence leading to a minor or major disruption [8,9]. In this research the transient snake (T-snake) instability associated with a minor disruption was observed for the first time in a HT-7 heavy impurity ohmic plasma. The mode structures are located inside the q = 1 flux surface. The internal mode appears in the soft X-ray emission in the plasma is center when the intensity of the line radiation from the impurity reaches a critical value. The mode lasts about several milliseconds, which is an the order of the timescale for the slow down of fast electrons. This letter is organized as follows: Section II describes the impurity in the minor disruption. In Section III, an overview of the experimental observations of the T-snake ∗ E-mail:

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Fig. 1. (Color online) Typical waveforms of an ohmic discharge which experiences several minor disruptions followed by a major disruption in the HT 7 tokamak. Ip is the plasma current, ne0 is the central electron density from the HCN interferometer, Te0 is the central electron temperate from the ECE, XUV is the central line-integrated extreme ultraviolet radiation, SXR is the core soft X-ray radiation, ILII is the line radiation of lithium, Hα is the line emissions from hydrogen, and δBθ dt magnetic fluctuations from the edges of the Mirnov coils.

is presented. Finally, we summarize our main points in Section IV.

II. IMPURITY IN A MINOR DISRUPITON Figure 1 shown several typical minor disruptions. The minor disruptions are always accompanied by complex magnetohydrodynamics (MHD) activates. The spikes in

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Transient Snakes in an Ohmic Plasma Associated with a Minor Disruption· · · – Songtao Mao et al.

Fig. 2. (Color online) Yields of photo-neutrons matched with the line radiations from impurity ions during a minor disruption.

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Fig. 4. (Color online) Possible q profile at time t = 0.537 s (left) and the spatial structure of the m = 1 and the m = 2 mode obtained by using of SVD method. The superposition of the m = 1 and the m = 2 modes is clear from the right frames and is consistent with the ECE measurements in Fig. 3.

q = 1 surface, obtained from the soft X-ray system, is also about 0.3a, Due to the low edge safety factor qa , the impurity accumulation due to the shrinkage of the current channel leads to a broad current profile with a central q above unity. As shown in Fig. 4, q0 = 1.005, and qa is only 3.75.

III. OBSERVATIONS OF T-SNAKE IN AN OHMIC PLASMA Fig. 3. (Color online) Electron temperature and density profiles during one of the minor disruptions. The foot of impurity accumulation is about 0.3 ∼ 0.5a, where a is the minor radius of the HT-7 tokamak. The notch in the electron temperature profiles is due to the formation of m = 1 heavyimpurity-driven islands

the extreme ultra-violet (XUV), hydrogen alpha emission (Hα ) and lithium emission (ILII ) followed by the transient bursts of minor disruptions indicate heavy impurity accumulation during disruption. The yield of the neutron flux is increased due to the impurities just prior to the burst of the 1/1 mode, as shown in Fig. 2. Most generated neutrons are photon-neutrons. Because of the cooling due to the accumulation of impurities, the plasma becomes cold regions where the impurities accumulate. As shown in Fig. 3 a notch (flatten region) in the electron temperature profiles is observed during minor disruptions, which suggests the formation of a big magnetic island, as shown in the Fig. 3. Limited by the spatial resolution of the hydrogen cyanide (HCN) system in a HT-7 tokamak, we do not see the influence of the impurity on the electron density. The blue shaded region in Fig. 3 indicates that the foot of impurity accumulation is about 0.3 ∼ 0.5a, where a is the minor radius of the HT-7 tokamak. Noticeably, the position of

As shown in the Fig. 4, the 1/1 internal mode in the core region is observed during minor disruptions. The superposition of the m = 1 and the m = 2 modes is also seen by means of multi-channel soft X-ray system. Notably, the superposition of the m = 1 and the m = 2 modes is also evidenced by the flatten region in the electron cyclotron emission (ECE) profiles (Fig. 3). This 1/1 mode lasts about 2∼3 milliseconds (Fig. 5(A(b))), which is in an order of the timescale for the slow down of fast electrons. The mode has strong frequency-chirping behavior as shown in the Fig. 5(A(c)). The frequency of this mode drops from about 4 kHz to 2 kHz. The Choi-Williams distribution (CWD) method is used for the time-frequency analysis [10]. The loss of impurities is responsible for the destabilization of this 1/1 mode, as shown in the Figs. 5(A(b)) and (B). Unlike the typical electron fishbone, just before the 1/1 mode burst, we do not find sharp spikes in the hard X-ray signal, which implies the dynamics of fast electron. No runaway electrons are observed during this 1/1 mode process, as shown in Fig. 6. Thus, clearly this 1/1 mode is not an electron fishbone. Based on the good agreement between the impurity loss and the bust of 1/1 mode, we put forward that this 1/1 mode is an impurity ion snake. The spatial structure of the 1/1 kink is obtained by using a high-resolution multi-array soft X-ray (SXR) to-

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Journal of the Korean Physical Society, Vol. 64, No. 9, May 2014

Fig. 6. (Color online) No hard X-ray radiation on run-away electrons are seen during the T-snake process.

Fig. 5. (Color online) (A) Occurrence of a typical T-snake during three minor disruptions. The time span of the T-snake chirping is about 2−3 ms, which is an order of the time scale of fast electron slowing down. The loss of impurity ions due to the T-snake is clearly shown in (B).

mography system [11]. The results are shown in Fig. 7. Clearly, the 1/1 mode is located inside the q = 1 surface. More interesting, the stronger m = 2 component of the SXR signals appears after the minor disruption. The superposition of the m = 1 and the m = 2 modes shown in the reconstructed tomography pictures and the spatial structures shown in fig. 4 are both indicate the coupling of these two mode. The higher frequency of the m = 2 mode in Fig. 7 the yellow shaded region, is due to the appearance of a higher MHD mode, such as the 3/2 mode.

IV. SUMMARY A T-snake is observed for the first time in a HT7 ohmic plasma. Unlike the electron fishbone or ionfishbone, the T-snake is driven by impurity ions rather than fast electrons or fast ions. The impurity accumu-

Fig. 7. (Color online) Accumulation of impurities and the synchronism of the 1/1 and the 2/1 mode before minor disruption. Appearance of the m = 2 component in the core SXR signal prior to the crash. The rampant and compact Mirnov signal indicates a nonlinear MHD growth of higher mode numbers.

lates in the 0.3 ∼ 0.5a (a is the minor radius of HT-7 Tokamak) region of the plasma. Many questions regarding the complex MHD activities during minor and major disruptions remain. For instance, the role of the LHCD in the T-snake is still unclear. We will leave these questions for further study on the EAST Tokamak.

ACKNOWLEDGMENTS This work was supported by the JSPS-NRF-NSFCA3 Foresight Program in the field of Plasma Physics (NSFC) with contract No. 11261140328.

Transient Snakes in an Ohmic Plasma Associated with a Minor Disruption· · · – Songtao Mao et al.

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