H.S. McLean

Tungsten spectroscopy relevant to the diagnostics of ITER divertor plasmas

The possibility of using extreme ultraviolet emission from low charge states of tungsten ions to diagnose the divertor plasmas of the ITER tokamak has been investigated. Spectral modelling of Lu-like W3+ to Gd-like W10+ has been performed by using the Flexible Atomic Code, and spectroscopic measurements have been conducted at the Sustained Spheromak Physics Experiment (SSPX) in Livermore. To simulate ITER divertor plasmas, tungsten was introduced into the SSPX spheromak by prefilling it with tungsten hexacarbonyl prior to the usual hydrogen gas injection and initiation of the plasma discharge. The tungsten emission was studied using a grazing-incidence spectrometer.

Simulation of spheromak evolution and energy confinement

Simulation results are presented that illustrate the formation and decay of a spheromak plasma driven by a coaxial electrostatic plasma gun, and model the plasma energy confinement. The physics of magnetic reconnection during formation is also illuminated. The simulations are performed with the three-dimensional, time-dependent, resistive magnetohydrodynamic NIMROD code [C. R. Sovinec, A. H. Glasser, T. A. Gianakon, D. C. Barnes, R. A. Nebel, S. E. Kruger, D. D. Schnack, S. J. Plimpton, A. Tarditi, and M. S. Chu, J. Comput. Phys. 195, 355 (2004)]. The simulation results are compared to data from the Sustained Spheromak Physics Experiment (SSPX) [E. B. Hooper, L. D. Pearlstein, and R. H. Bulmer, Nucl. Fusion 39, 863 (1999)]. The simulation results are tracking SSPX with increasing fidelity (e.g., improved agreement with measured magnetic fields, fluctuation amplitudes, and electron temperature) as the simulation has been improved in its representations of the experimental geometry, the magnetic bias coils,...

Design and operation of a passively switched repetitive compact toroid plasma accelerator

The design and operation ofa spheromak-like compact toroid (SCT) plasma accelerator is described. As an example application, some principles are presented for using the device as a plasma injector to fuel a tokamak plasma. The device forms and accelerates an SCT plasma. The SCT is a self-contained structure of plasma with embedded poloidal and toroidal magnetic fields and their associated currents that provide plasma confinement and structural integrity. The SCT is formed in a magnetized coaxial plasma gun and then accelerated within coaxial electrodes. The typical mass of an SCTfor tokamak fueling is from several tens to several hundreds of micrograms and is accelerated up to a velocity of ∼2 × 10 5 m/s. Larger-mass SCTs can be produced, and higher velocities are possible. This is important for other applications such as space propulsion, X-ray generation, fast-opening plasma switches, and low-temperature high-density plasma simulators. The novel features of the device are as follows: (a) it can be operated in a repetitive mode, (b) the high-energy capacitor bank to form the SCT is switched by initiating breakdown with fast gas injection, (c) the required delay between formation and acceleration is achieved passively with saturable inductors that switch the high-energy accelerator capacitor bank, and (d) a drift section has been added within the toroidal field region to study cross-field propagation prior to tokamak penetration. With the device installed on the Davis Diverted Tokamak (DDT), measurements are taken to study tokamak fueling. Typical rep-rated parameters are as follows: the SCT poloidal magnetic field at the outer electrode = 0.4 T, the stored formation bank energy = 1600 J, and the stored accelerator bank energy = 3600 J. The lower bound on SCT kinetic energy leaving the accelerator = 40 J (inferred from electron line density measurements). Typical SCT velocity is 15 to 20 cm/μs. The maximum rep-rate achieved so far with the device is 0.2 Hz and is currently limited by vacuum pumping capacity. Reliable operation has been demonstrated for 1000 consecutive shots. Higher-energy single shots have also been taken to study SCT propagation through an open guide tube and to study penetration of the SCT into the DDT tokamak vessel with both tokamak plasma discharges and vacuum toroidal magnetic field only.

Experimental demonstration of compact torus compression and acceleration

Tests of compact torus (CT) compression on the RACE device [Phys. Rev. Lett. 61, 2843 (1988)] have successfully demonstrated stable compression by a factor of 2 in radius, field amplification by factors of 2–3 to 20 kG, and compressed densities exceeding 1016 cm−3. The results are in good agreement with two‐dimensional magnetohydrodynamic simulations of the CT dynamics. The CT is formed between a pair of coaxial conical conductors that serve as both a flux conserver for stable, symmetric formation and as electrodes for the compression and acceleration phases. The CT is compressed by J×B forces (poloidal current, toroidal field) when a 120 kV, 260 kJ capacitor bank is discharged across the electrodes. The CT reaches two‐fold compression to a radius of 8 cm and a length of 20–30 cm near the time of peak current, 10 μsec (many Alfven times) after the accelerator fire time, and is subsequently accelerated in a 150 cm straight coaxial section to velocities in the range 1.5–6.5×107 cm/sec. A new set of accelera...

Time-resolved compression of a capsule with a cone to high density for fast-ignition laser fusion

The advent of high-intensity lasers enables us to recreate and study the behaviour of matter under the extreme densities and pressures that exist in many astrophysical objects. It may also enable us to develop a power source based on laser-driven nuclear fusion. Achieving such conditions usually requires a target that is highly uniform and spherically symmetric. Here we show that it is possible to generate high densities in a so-called fast-ignition target that consists of a thin shell whose spherical symmetry is interrupted by the inclusion of a metal cone. Using picosecond-time-resolved X-ray radiography, we show that we can achieve areal densities in excess of 300 mg cm(-2) with a nanosecond-duration compression pulse--the highest areal density ever reported for a cone-in-shell target. Such densities are high enough to stop MeV electrons, which is necessary for igniting the fuel with a subsequent picosecond pulse focused into the resulting plasma.

Plasma diagnostics for the sustained spheromak physics experiment

In this article we present an overview of the plasma diagnostics operating or planned for the sustained spheromak physics experiment device now operating at Lawrence Livermore National Laboratory. A set of 46 wall-mounted magnetic probes provide the essential data necessary for magnetic reconstruction of the Taylor relaxed state. Rogowski coils measure currents induced in the flux conserver. A CO2 laser interferometer is used to measure electron line density. Spectroscopic measurements include an absolutely-calibrated spectrometer recording extended domain spectrometer for obtaining time-integrated visible ultraviolet spectra and two time-resolved vacuum monochrometers for studying the time evolution of two separate emission lines. Another time-integrated spectrometer records spectra in the visible range. Filtered silicon photodiode bolometers provide total power measurements, and a 16 channel photodiode spatial array gives radial emission profiles. Two-dimensional imaging of the plasma and helicity injec...

Energy confinement and magnetic field generation in the SSPX spheromak

The Sustained Spheromak Physics Experiment (SSPX) [Hooper et al., Nuclear Fusion 39, 863 (1999)] explores the physics of efficient magnetic field buildup and energy confinement, both essential parts of advancing the spheromak concept. Extending the spheromak formation phase increases the efficiency of magnetic field generation with the maximum edge magnetic field for a given injector current (B∕I) from 0.65T∕MA previously to 0.9T∕MA. We have achieved the highest electron temperatures (Te) recorded for a spheromak with Te>500eV, toroidal magnetic field ∼1T, and toroidal current (∼1MA) [Wood et al., “Improved magnetic field generation efficiency and higher temperature spheromak plasmas,” Phys. Rev. Lett. (submitted)]. Extending the sustainment phase to >8ms extends the period of low magnetic fluctuations (<1%) by 50%. The NIMROD three-dimensional resistive magnetohydrodynamics code [Sovinec et al., Phys. Plasmas 10, 1727 (2003)] reproduces the observed flux amplification ψpol∕ψgun. Successive gun pulses are...

Magnetic Reconnection During Flux Conversion in a Driven Spheromak

During buildup of a spheromak by helicity injection, magnetic reconnection converts toroidal flux into poloidal flux. This physics is explored in the resistive magnetohydrodynamic code, NIMROD [C.R. Sovinec, A.H. Glasser, T.A. Gianakon, D.C. Barnes, R.A. Nebel, S.E. Kruger, D.D. Schnack, S.J. Plimpton, A. Tarditi, and M.S. Chu, J. Comp. Phys., 195, 355-386 (2004)], which reveals negative current sheets with {lambda} = {mu}{sub 0}j {center_dot} B/B{sup 2}reversed relative to the applied current. The simulated event duration is consistent with magnetic diffusion on the sheet thickness and is accompanied by cathode voltage spikes and poloidal field increases similar to those seen in the Sustained Spheromak Physics Experiment, SSPX [E. B. Hooper, L. D. Pearlstein, and R. H. Bulmer, Nucl. Fusion 39, 863 (1999)]. All magnetic fieldlines are open during reconnection and their trajectories are very sensitive to their starting points, resulting in chaos. The current sheets are most intense inside the separatrix near the X-point of the mean-field spheromak, suggesting that the reconnection occurs near fieldlines which are closed in the azimuthal average.

Fast electron temperature and conversion efficiency measurements in laser-irradiated foil targets using a bremsstrahlung x-ray detector

Measurements of fast electron temperature and conversion efficiencies using bremsstrahlung x-rays emitted from laser-produced, fast electrons are presented. Experiments were carried out using the Titan laser (150 J, 1.5 ps) at the Lawrence Livermore National Laboratory. The maximum intensity was 2 × 1020 W/cm2 on 250 μm thick silver foil targets. The emission of bremsstrahlung x-rays from the fast electrons in the target was measured using a filter-stack based detector. The conversion efficiency of laser energy into fast electrons and the electron temperature were studied as a function of incident laser energy. Several models of the electron divergence angle were investigated, and the effect of the assumed divergence angle on the inferred conversion efficiency was quantified. This allows for upper and lower bounds on the conversion efficiency to be established for a range of possible divergence angles. The value for upper bound is 60% (from a 75° divergence angle model) and for the lower bound is 25% (fro...

Controlled and spontaneous magnetic field generation in a gun-driven spheromak

In the Sustained Spheromak Physics Experiment, SSPX [E. B. Hooper, D. Pearlstein, and D. D. Ryutov, Nucl. Fusion 39, 863 (1999)], progress has been made in understanding the mechanisms that generate fields by helicity injection. SSPX injects helicity (linked magnetic flux) from 1 m diameter magnetized coaxial electrodes into a flux-conserving confinement region. Control of magnetic fluctuations (delta B/B similar to 1% on the midplane edge) yields T-e profiles peaked at > 200 eV. Trends indicate a limiting beta (beta(e)similar to 4%-6%), and so we have been motivated to increase T-e by operating with stronger magnetic field. Two new operating modes are observed to increase the magnetic field: (A) Operation with constant current and spontaneous gun voltage fluctuations. In this case, the gun is operated continuously at the threshold for ejection of plasma from the gun: stored magnetic energy of the spheromak increases gradually with delta B/B similar to 2% and large voltage fluctuations (delta V similar to 1 kV), giving a 50% increase in current amplification, I-tor/I-gun. (B) Operation with controlled current pulses. In this case, spheromak magnetic energy increases in a stepwise fashion by pulsing the gun, giving the highest magnetic fields observed for SSPX (similar to 0.7 T along the geometric axis). By increasing the time between pulses, a quasisteady sustainment is produced (with periodic good confinement), comparing well with resistive magnetohydrodynamic simulations. In each case, the processes that transport the helicity into the spheromak are inductive and exhibit a scaling of field with current that exceeds those previously obtained. We use our newly found scaling to suggest how to achieve higher temperatures with a series of pulses.