How expensive is it to service the national debt? : A battle between interest rates and growth rates

The U.S. federal debt has been rising steadily since the Great Recession and is currently 103 percent of GDP. So let’s enlist FRED to help us study the sustainability of this debt by looking at how much it costs to service it.
Neil Mehrotra recently described the cost of servicing public debt as dependent on the gap between the real interest rate on debt and the growth rate of real GDP: This gap captures the difference between the interest the government must pay to its lenders, in real terms, and the pace at which U.S. income increases. If U.S. income increases more rapidly, then interest payments on U.S. debt shouldn’t be a major burden.
The graph plots this measure of the cost of servicing the debt. (Here, the growth rate of real GDP is the sum of real GPD per capita growth and population growth, and the real interest rate is the difference

Federal Reserve Source