Progress of high-power and long-pulse ECRF system development in JT-60

Abstract

A new gyrotron operation technique to increase oscillation efficiency was developed using the JT-60 electron cyclotron range of frequency (ECRF) system. In order to increase the efficiency without a significant increase in the anode leakage current by trapped electrons that limits the efficiency of gyrotrons with a collector potential depression technology, we actively optimized the electron pitch factor by controlling only the anode voltage within ∼0.1 s after the start of the gyrotron operation, and a high-efficiency oscillation in the so-called hard-self-excitation region was achieved from the start of the oscillation. As an application of this technique in the JT-60 ECRF system, the gyrotron output power of 1.5 MW for 4 s, which is the longest pulse length in the world at an output power of 1.5 MW, was recorded with a successful reduction in the collector heat load by 20% as compared with the conventional operation. The reduced collector heat load at the 1.5 MW operation was acceptable for a steady-state operation. Further progress was made with respect to the expansion of the long-pulse capability of the ECRF system. A new gyrotron with an improved mode converter was developed in order to demonstrate a reduction in the stray radiation in the gyrotron; such radiation has thus far hindered long-pulse operations by causing an unacceptable heat load. We confirmed that the stray radiation was reduced to 1/3 of that of the original gyrotron; this reduced heat load is acceptable for steady-state operation. A conditioning operation of the improved gyrotron proceeds up to 31 s at 1 MW. This progress significantly contributes to an enhancement in the high-power and long-pulse capability of the ECRF system used in JT-60SA, where a total output power of 9 MW for 100 s is planned.

Recent result of gyrotron operation in NIFS

“…That operation is conducted by changing the Va during pulse within 10 seconds under the magnetic field was set at a lower value. This method indicates another way to enter the oscillation in the hard-excitation region [2]. The oscillation region and the relationship with Va and Ibc are shown in Figure 5.…”

Research and Development of 2-frequency (110/138 GHz) FADIS for JT-60SA ECHCD system

“…In the JT-60U/SA gyrotrons, high power/efficiency oscillations have been attained in hardexcitation regions where parameter areas of the anode voltage and the cavity magnetic filed were limited in extent for regular operations with the high power output. The high power main oscillating TE 22,6 mode disappeared in the anode voltage control of ΔV a ∼2 kV, and the oscillating mode switched to the low power TE 21,6 mode in the JT-60U gyrotron [10]. This means that frequency controllability in the diplexer operation is limited into the 10 MHz level with ΔV a < 2 kV.…”

“…A frequency change 10 MHz was obtained. The anode voltage control system has been developed in the JT-60U ECHCD system to modulate the oscillating power [10]. The applied anode voltage extracts the electron beam from the gyrotron gun, and increases the beam current in gyrotrons with a triode-type magnetron injection gun.…”

Long pulse operation of a dual frequency gyrotron for JT-60SA

“…Some of the previous gyrotrons, in the world, have relief windows to extract stray rf in the gyrotron, while a large sized dc-break between collector and body electrodes was a unique outlet of the stray rf except the output window of the main rf beam in the previous 110 GHz gyrotron 5 . Thus a large relief window (diameter ~ 130 mm), three small relief windows (diameter ~ 50 mm) and dummy loads to absorb stray radiation from the relief windows were installed in the dual frequency gyrotron.…”