What are the qualifications for federal judges?

For the US Supreme Court, the only qualification is to be appointed by the President and confirmed by the Senate per Art III of the US Const. No law degree is required, although in modern practice, a law degree and extensive judicial experience is a must to prevail in the age of electronic media. For US District Courts and US Courts of Appeals, and other inferior courts, a law degree is required because those courts are established by Congress rather than the Constitution and Congress has required those judges to be lawyers.

In practice however, most, if not all federal judges are graduates of the top law schools in the US where they were top performers, served in judicial clerkships with existing federal judges, and have had distinguished careers in private practice, academia, or the government. Most have been published several times in law reviews and other peer-reviewed publications. Look at Justice Elana Kagan, she graduated at the top of her class from Harvard Law, served as Editor of the Harvard Law Review, clerked for the DC Cir Ct of Appeals, then the Supreme Court, then worked for one of the most prestigious law firms in the US before becomming a tenured professor, White House Counsel, Dean of Harvard, and Solicitor General. Along the way, she published numerous law review articles.