Ring structures can be seen on the emission patterns of a ZrO/W (100) thermal field emission cathode, when the operating condition is not adequate. The rings collapse with operating time so that the collapsing has a tight relation to the emission current fluctuations at very low frequencies. The collapsing rates of the ring structures are measured under several oxygen partial pressures and cathode temperatures. The results show that the collapsing rate does not change monotonically with the cathode temperature, but it showes an U shaped change with increasing cathode temperatures. The U shaped curves shift to the lower temperature when oxygen partial pressure is decreased and/or the applied voltage is increased.
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