Threefold Increase of the Bulk Electron Temperature of Plasma Discharges in a Magnetic Mirror Device P. A. Bagryansky, 1,2,* A. G. Shalashov, 1,3,4,† E. D. Gospodchikov, 1,3,4 A. A. Lizunov, 1 V. V. Maximov, 1,2 V. V. Prikhodko, 1,2 E. I. Soldatkina, 1,2 A. L. Solomakhin, 1,2 and D. V. Yakovlev 2 1 Budker Institute of Nuclear Physics, 11 Lavrentieva ave., 630090 Novosibirsk, Russia 2 Novosibirsk State University, 2 Pirogova str., 630090 Novosibirsk, Russia 3 Institute of Applied Physics, Russian Academy of Sciences, 46 Ulyanova str., 603950 Nizhny Novgorod, Russia 4 Lobachevsky State University of Nizhny Novgorod, 23 Gagarina ave., 603950 N. Novgorod, Russia (Received 18 November 2014; published 18 May 2015) This Letter describes plasma discharges with a high temperature of bulk electrons in the axially symmetric high-mirror-ratio (R ¼ 35) open magnetic system gas dynamic trap (GDT) in the Budker Institute (Novosibirsk). According to Thomson scattering measurements, the on-axis electron temperature averaged over a number of sequential shots is 660 Æ 50 eV with the plasma density being 0.7 × 10 19 m −3 ; in few shots, electron temperature exceeds 900 eV. This corresponds to at least a threefold increase with respect to previous experiments both at GDT and at other comparable machines, thus, demonstrating the highest quasistationary (about 1 ms) electron temperature achieved in open traps. The breakthrough is made possible by application of a new 0.7 MW=54.5 GHz electron cyclotron resonance heating system in addition to standard 5 MW heating by neutral beams, and application of a radial electric field to mitigate the flute instability. DOI: 10.1103/PhysRevLett.114.205001 PACS numbers: 52.55.Jd, 52.50.Gj, 52.50.Sw Open magnetic systems for plasma confinement have a number of potential advantages for fusion reactors with various thermonuclear applications starting from neutron sources with a thermonuclear gain factor Q< 1 [1,2] and ending with power plants with Q ≫ 1 [3,4]. In addition to the simplicity of their design, the advantages of open systems are inherent steady-state operation, proven capability of high-β plasma confinement (β is the ratio of the plasma pressure to the magnetic field pressure), no disruptions because there is no plasma current, a relatively low wall loading by plasma heat and radiation, natural diverters with a large area to absorb power, and the possibility of directly converting plasma “exhaust” to electricity. Axisymmetric magnetic mirrors provide addi- tional advantages: no neoclassical radial transport, high field magnets enable simple tandem mirror power plants [5], maintenance and upgrades are easier, and thick-liquid walls become feasible, reducing or eliminating issues of neutron damage to materials. Alternative magnetohydro- dynamic (MHD) stabilization techniques will likely be needed for axisymmetric open system power plants, but there are a number of candidates [6]. The hot ion component with the energy optimal for fusion applications is commonly sustained in such systems by high-power neutral beam injection (NBI). In turn, the electrons are heated by collisions with NBI-driven ener- getic ions. Electrons with their superior mobility carry most of the heat flux, which flows mainly along magnetic field lines and hits the end plates outside the magnetic mirrors. Because of the higher electron mobility, the electron temperature (T e ) is significantly lower than the mean energy of fast ions. The energy confinement time of fast ions in a plasma with relatively cold electrons is determined by the electron-ion Coulomb collisions (electron drag), τ h ∝ T 3=2 e . For this reason, the electron temperature is the main factor limiting the confinement time of fast ions and, thus, the power efficiency of a beam-driven fusion reactor based on a magnetic mirror. Widely believed estimates based on classical (Spitzer) electron thermal conductivity to end walls show that the heat flux along the magnetic field is proportional to T 7=2 e [7], which would prevent any thermonuclear power appli- cation of mirror traps due to a poor quality of energy confinement. This, together with many experiments that never demonstrated an electron temperature higher than 280 eV [8], led to a judgment that fusion reactors based on magnetic mirrors were not feasible. As a result, much of the research activity in this field was discontinued. However, plasma self-organization in a region behind the magnetic mirrors can lead to significant suppression of the longitudinal electron heat flux [9]. More recent theoretical work [10] has shown that electrons can be decoupled from the end walls (thereby, eliminating electron thermal con- ductivity to end walls) by expanding the magnetic field from that at the mirrors by at least the square root of the ion to electron mass ratio. This prevents secondary electrons generated at the end wall from reaching and cooling the hot plasma; in addition, the vacuum must be maintained at a sufficiently high level to keep ionization of gas to a minimal level. PRL 114, 205001 (2015) PHYSICAL REVIEW LETTERS week ending 22 MAY 2015 0031-9007=15=114(20)=205001(5) 205001-1 © 2015 American Physical Society