By John Green­wald, Prince­ton Plasma Physics Laboratory

Corkscrew-shaped insta­bil­i­ties called “snakes” have long been a com­mon fea­ture of the hot, elec­tri­cally charged plasma gas that fuels fusion reac­tions, which could pro­vide a future source of clean and abun­dant energy for gen­er­at­ing elec­tric­ity. Such snakes trap impu­ri­ties released from the plasma-facing walls of exper­i­men­tal fusion ves­sels called toka­maks, and these impu­ri­ties radi­ate away copi­ous amounts of energy, cool­ing the plasma to tem­per­a­tures below those required to cre­ate fusion reac­tions. Under­stand­ing the for­ma­tion and sur­vival of snakes can thus be essen­tial to elim­i­nat­ing the prob­lem so that fusion can take place.

New exper­i­men­tal data reported today in Phys­i­cal Review Let­ters sheds light on how snakes form and sur­vive in fusion plas­mas. The paper, whose lead author is Luis Delgado-Aparicio of the U.S. Depart­ment of Energy’s Prince­ton Plasma Physics Lab­o­ra­tory (PPPL), cites recent exper­i­ments on the Alca­tor C-Mod toka­mak at the Mass­a­chu­setts Insti­tute of Tech­nol­ogy Plasma Sci­ence and Fusion Cen­ter (MIT-PSFC). The find­ings com­piled by a mul­ti­dis­ci­pli­nary team show that the for­ma­tion of snakes can­not be explained, as pre­vi­ously thought, by plasma pres­sure alone. Instead, the for­ma­tion reflects com­plex inter­ac­tions among phe­nom­ena that include the sep­a­rately evolv­ing plasma den­sity and tem­per­a­ture con­di­tions that pro­duce the plasma pres­sure. This sep­a­rate evo­lu­tion of den­sity and tem­per­a­ture also enables the snakes to sur­vive peri­odic relax­ations of plasma pres­sure known as saw­tooth instabilities.

Left: Time sequence of a snake, in red, depicted by x-ray detec­tors. The saw­tooth crash occurs halfway through the sequence and barely per­turbs the snake.
Right: X-ray recon­struc­tion of cross-section of cres­cent snake inside Alca­tor C-Mod.
Credit: Luis Delgado-Aparicio

Read the abstract.

Cita­tion:
Delgado-Aparicio, Luis; Linda Sugiyama, MIT; Robert Granetz, MIT; David Gates, PPPL; John Rice, MIT; Matthew Reinke, MIT; Man­fred Bit­ter, PPPL; Eric Fredrick­son, PPPL; Chi Gao, MIT; Mar­tin Green­wald, MIT; Ken­neth Hill, PPPL; Amanda Hub­bard, MIT; Jerry Hughes, MIT; Earl Mar­mar, MIT; Novimir Pablant, PPPL; Yuri Pod­paly, MIT; Steven Scott, PPPL; Randy Wil­son, PPPL; Steve Wolfe, MIT; and Steve Wuk­itch, MIT. 2013. For­ma­tion and sta­bil­ity of impu­rity “snakes” in toka­mak plas­mas. Phys­i­cal Review Let­ters 110, 065006.

This work was per­formed under U.S. DOE con­tracts includ­ing DE-FC02-99ER54512 and oth­ers at MIT and DE-AC02-09CH11466 at PPPL. Com­pu­ta­tional sup­port was pro­vided by the National Energy Research Sci­en­tific Com­put­ing Cen­ter under DE-AC02-05CH11231.