An off-axis sawtooth oscillation is excited by electron cyclotron heating in a lower hybrid current drive plasma in the WT-3 tokamak. The internal structure of the plasma is investigated using computer tomography with five detector arrays after the application of singular value decomposition to the soft X-ray detector signals. It is found that the precursor oscillation of the off-axis sawtooth collapse is the m = 2 mode. On the other hand, the double m = 2 mode structure is observed as a postcursor of the collapse.Index Terms-Computer tomography (CT), off-axis sawtooth oscillation, singular value decomposition (SVD), tokamak. S AWTOOTH oscillations are commonly observed in tokamaks. Recently, another kind of sawtooth oscillation has been observed in reversed magnetic shear plasmas [1], [2]. It had only a small perturbation at the plasma center during the collapse. Therefore, it was called "off-axis sawtooth oscillation." In [1], a nonlinear magnetohydrodynamics simulation showed that the off-axis collapse was due to the double-tearing magnetic reconnection. The appearance of the double m = 2 mode was predicted as the precursor of the collapse in the simulation [1]. However, the existence of the double m = 2 mode has not yet been identified in those previous experiments. For this purpose, the soft X-ray (SXR) computer tomography (CT) with sufficient numbers of detector arrays is very useful. On the WT-3 tokamak (R 0 = 65 cm, a = 20 cm, and B φ < 1.75 T), five SXR detector arrays have been installed to surround a minor cross section [3]. In WT-3, an off-axis sawtooth oscillation is found to be excited by electron cyclotron heating (ECH) in a lower hybrid current drive (LHCD) plasma. In this paper, we present the result of the investigation of the off-axis sawtooth collapse by the CT method [4] after the application of the singular value decomposition (SVD) [5], [6] to the SXR detector signals.The outline of the SVD is as follows [6]. We consider a scalar quantity y(x, t) (in our case, SXR signals) whose time Fig. 1. Reconstructed local SXR signal at (R, z) = (70.4 cm, 0.0 cm).evolution is measured simultaneously at M different spatial locations. The signal is sequentially sampled, and the data are assembled into an N × M matrix Y i,j . The SVD method consists in expanding the discrete spatio-temporal data Y i,j = y(x j , t i ) into a unique set of modes that are orthogonal in time and space, i.e.,The weights of modes A k are either positive or equal to zero, and it is conventional to sort the series in a decreasing weight order. The spatial eigenmodes Φ k (x j ) will thereafter be called topos, and the temporal eigenmodes Ψ k (t i ) will be called chronos.An LHCD (2 GHz, 90 kW) plasma with the safety factor of q = 5 at the limiter is used for the target plasma of the ECH (56 GHz, 50 kW) experiment. Another kind of sawtooth oscillation is excited when the resonance layer of ECH R ECH is located in the range of |R ECH − R C | = 4.5−5.5 cm, where R C is the horizontal position of the plasma center. Fig. 1 shows the ex...