Note that the U.S. holds fixed-interest debt. If interest rates go up it will be forced to refinance at higher rates, but it will happen over some period of time. The weighted average maturity of the debt right now is over 5 years, so it's not like it'll be an extra $700 billion all of the sudden.
In any case, a worrying level of debt does not make something a Ponzi scheme...
In any case, a worrying level of debt does not make something a Ponzi scheme...