Low frequency fluctuations of a plasma in the central cell of the GAMMA10 tandem mirror have been observed with electrostatic probes (ESPs) installed at the peripheral region of the plasma. Typical plasma parameters for a hot ion mode operation are ne∼2×1012 cm−3, Ti∼4 keV, Te∼100 eV, B=4 kG, and rp=18 cm. Although the ESP is an effective tool to measure local plasma parameters and fluctuations, it can be damaged with the higher electron temperature. Here, a new visible light detector was introduced, which could measure the plasma fluctuations without contacting the plasma. The obtained results were compared with those of ESPs. It is found that the visible light detector is applicable when the electron temperature becomes much higher in the future experiment.
The formation of eigenmodes with the m = 1 fast Alfvén waves in the ioncyclotron range of frequency are investigated in the axisymmetric central cell of the GAMMA 10 tandem mirror. When the fast waves with frequencies near the fundamental ion-cyclotron frequency have been used for the plasma production, the saturation in the density has been observed. The spatial structure of the excited wave field is calculated in the central cell using a two-dimensional full wave code. The results of numerical analysis indicate that the increase in plasma density depends strongly on the eigenmode formations associated with the boundary conditions. The results of numerical analysis are compared with the results of measurements of the waves with magnetic probes. A very good degree of agreement is found between the theoretical results and the experimental results. It is suggested that the simultaneous excitation of several radial eigenmodes with high-harmonic fast waves is effective for higher density plasma production.
The effects of irradiation on the fatigue properties of stainless steel were studied under the condition of in situ irradiation or after irradiation using 20 MeV protons. Strain controlled fatigue tests using a resonant and a drive force DF at constant were measured in situ during the test. Whereas the present damage rate was very low, the apparent effects of in situ irradiation on the fatigue properties were observed, e.g. the elongation
Alfvén eigen modes were observed in the Large Helical Device for various magnetic field strengths and plasma densities. It was found that Alfvén eigen modes could be excited at much higher frequencies (0.5 ∼ 2.5 MHz) and higher magnetic fields (1.5 ∼ 2.83 T) than previously reported. A part of the estimated Alfvén velocities (2 ∼ 6 × 10 7 m/s) exceeded the thermal velocity of injected neutral beams (∼ 4.1 × 10 6 m/s). and Alfvén eigen modes (AEs) are also observed with NB injection in many fusion-oriented devices including the LHD [2, 3]. High energy ions produced by ion cyclotron heating (ICH) can also excite these fluctuations. Investigating these fluctuations is important because they degrade the fusion reaction rate and neutron yield.In the present experiment, magnetic probes (MP) were employed to observe these fluctuations excited by NB and/or high power ICH injection in the LHD. Signals from MPs were recorded by a sampling oscilloscope (250 Ms/s). So far, fluctuations were observed in NB-sustained plasmas with field strength B on a magnetic axis ranging from 1.0 to 2.8 T, when the plasma density is lower than ∼ 10 19 m −3 . An example is shown in Fig. 1 for B = 2.75 T. Shown in Fig. 1 (a) is the plasma density n e near the magnetic axis, which was estimated by the measured line density. In Fig. 1 (b), the contour plot of the fast Fourier transformed (FFT) power spectrum obtained from a MP signal is shown with the frequency resolution ∆ f ∼ 15 kHz. The darker tone indicates the higher power. A FFT power spectrum is plotted in Fig. 1 (c) at 0.92 sec. The observed frequency can be fitted well with the n −1/2 e fitting curve as shown in Fig. 1 (d).Accumulating these fluctuation signals at various B and n e , it was found that the frequencies of observed fluctuations were proportional to B and n −1/2 e as shown in Fig. 2 (a). Dashed lines are n −1/2 e fitting curves for the data. For the sake of clarity, observed frequencies are nor-
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