Why is a vacuum diode one-way conductive?

The anode is connected to the positive pole of the current source and the cathode to the negative. The created external field accelerates the electrons ejected from the cathode, and they reach the anode. If the anode is connected to the negative pole, and the cathode is connected to the positive, then under the action of the external field, the electrons will return to the cathode and there will be no current in the circuit.

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