Xanthopoulos, P., & Jenko, F. (2006). Clebsch-type coordinates for nonlinear gyrokinetics in generic toroidal configurations. Physics of Plasmas, 13: 092301. Retrieved from http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PHPAEN000013000009092301000001&idtype=cvips&gifs=Yes.