So...who is buying up the bond market to keep all the balls in the air? Here are some clues. The following information was taken directly from the website of U.S. DEPARTMENT OF THE TREASURY
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Exchange Stabilization Fund
Introduction
The Exchange Stabilization Fund (ESF) consists of three types of assets: U.S. dollars, foreign currencies, and Special Drawing Rights (SDRs), which is an international reserve asset created by the International Monetary Fund. The financial statement of the ESF can be accessed at "Reports" or "Finances and Operations."
The ESF can be used to purchase or sell foreign currencies, to hold U.S. foreign exchange and Special Drawing Rights (SDR) assets, and to provide financing to foreign governments. All operations of the ESF require the explicit authorization of the Secretary of the Treasury ("the Secretary").
The Secretary is responsible for the formulation and implementation of U.S. international monetary and financial policy, including exchange market intervention policy. The ESF helps the Secretary to carry out these responsibilities. By law, the Secretary has considerable discretion in the use of ESF resources.
The legal basis of the ESF is the Gold Reserve Act of 1934. As amended in the late 1970s, the Act provides in part that "the Department of the Treasury has a stabilization fund …Consistent with the obligations of the Government in the International Monetary Fund (IMF) on orderly exchange arrangements and an orderly system of exchange rates, the Secretary …, with the approval of the President, may deal in gold, foreign exchange, and other instruments of credit and securities.
Basically, if my interpretation is correct, they can do anything they want to do.