The electric field inside the dees of a cyclotron is zero. How is a charge particle accelerated?

To know about the acceleration of positively charged particles by Cyclotron, first you would have to know about the working principle of it. By it, you will get your answer.
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https://www.meritnation.com/ask-answer/question/explain-with-diagram-how-is-the-working-of-a-cyclotron/electric-charges-and-fields/1870690