Question: The Department of Energy sponsors a laboratory that is currently working on magnetic fusion devices based on what principle?

<P0-S0> A magnetic nozzle is a convergent-divergent magnetic field that guides, expands and accelerates a plasma jet into vacuum for the purpose of space propulsion. <P0-S1>  The magnetic field in a magnetic nozzle plays a similar role to the convergent-divergent solid walls in a de Laval nozzle, wherein a hot neutral gas is expanded first subsonically and then supersonically to increase thrust. <P0-S2>  Like a de Laval nozzle, a magnetic nozzle converts the internal energy of the plasma into directed kinetic energy, but the operation is based on the interaction of the applied magnetic field with the electric charges in the plasma, rather than on pressure forces acting on solid walls. <P0-S3>  The main advantage of a magnetic nozzle over a solid one is that it can operate "contactlessly," i.e. avoiding the material contact with the hot plasma, which would lead to system inefficiencies and reduced lifetime of the nozzle. <P0-S4>  Additional advantages include the capability of modifying the strength and geometry of the applied magnetic field in-flight, allowing the nozzle to adapt to different propulsive requirements and space missions. Magnetic nozzles are the fundamental acceleration stage of several next-generation plasma thrusters currently under development, such as the helicon plasma thruster, the electron-cyclotron resonance plasma thruster, the VASIMR, and the applied-field magnetoplasmadynamic thruster. <P0-S5>  Magnetic nozzles also find another field of application in advanced plasma manufacturing processes, and their physics are related to those of several magnetic confinement plasma fusion devices. <SEP> <P1-S0> The National Spherical Torus Experiment (NSTX) is an innovative magnetic fusion device based on the "spherical tokamak" concept. <P1-S1>  It was constructed by the Princeton Plasma Physics Laboratory (PPPL) in collaboration with the Oak Ridge National Laboratory, Columbia University, and the University of Washington at Seattle. <SEP> <P2-S0> Oak Ridge National Laboratory (ORNL) is an American multiprogram science and technology national laboratory sponsored by the U.S. Department of Energy (DOE) and administered, managed, and operated by UT-Battelle as a federally funded research and development center (FFRDC) under a contract with the DOE. <P2-S1>  ORNL is the largest science and <SEP> <P3-S0> Neutral beam injection (NBI) is one method used to heat plasma inside a fusion device consisting in a beam of high-energy neutral particles that can enter the confinement magnetic field. <P3-S1>  When these neutral particles are ionized by collision with the plasma particles, they are kept in the plasma by the confining magnetic field, and can transfer most of their energy by further collisions with the plasma. <P3-S2>  By tangential injection in the torus, neutral beams provide also momentum to the plasma and current drive, one essential feature for long pulses of burning plasmas. <P3-S3>  Neutral beam injection is a ﬂexible and reliable technique, which has been the main heating system on a large variety of fusion devices. <P3-S4>  To date, all NBI systems were based on positive precursor ion beams. <P3-S5>  In the 90s there has been impressive progress in negative ion sources and accelerators with the construction of multi-megawatt negative ion based NBI systems at LHD (H, 180 keV) and JT-60U (D, 500 keV). <P3-S6>  The NBI designed for ITER is a substantial challenge (D, 1MeV, 40A) and a prototype is being constructed to optimize its performance in view of the ITER future operations. <P3-S7>  Other ways to heat plasma for nuclear fusion include RF heating, electron cyclotron resonance heating (ECRH), and ion cyclotron resonance heating (ICRH).