Despite his statement, I'm sure that Chairman Powell isn't actually worried about the national debt. As measured in gold, the national debt peaked in 2001 and has declined by about 25% since then. Although it ends in 2015, this chart shows some of the decline: http://pricedingold.com/us-federal-debt/. I've also done some research that implies it has fallen even further since then.

The national debt is a problem for Treasury bond holders and people who have cash sitting in a bank account. Those people suffer when the gold-value of the national debt declines. But because the government does not suffer any negative consequences when it devalues the national debt, it has no incentive to ever stop. So I don't believe Powell when he says he is concerned about it.

He is basically just trying to convince Treasury bond holders not to sell. That way, he and the rest of the US Government can continue to transfer the wealth of these people to the government to spend buying votes. This is nonsense.