Sustained Spheromak Physics Experiment Research Articles

Ultrashort Pulse Reflectometry (USPR) diagnostic for EAST

Ultrashort Pulse Reflectometry (USPR) is a plasma diagnostic technique involving the propagation of ultrashort duration (∼few nsec) chirps which contain frequency components spanning large portions of the plasma density profile. Upon reflection, each frequency component reflects from a distinct density layer. The reflected wave packet is down-converted and passed through a multi-channel filter bank, with time-of-flight (TOF) measurements made on each of the filtered wave packets which are then used to reconstruct the electron density profile. Applied previously to the Sustained Spheromak Physics Experiment, UC Davis is updating this technique with higher power (>10X) sources, enhanced channel counts, and high speed customized TOF electronics. A 32 channel USPR system is being developed for the Experimental Advanced Superconducting Tokamak (EAST) device with frequencies spanning 52 to 92 GHz, reflecting from the right-hand plasma cut-off layer. The system will employ three monostatic transmit/receive horns each covering a different waveguide band. Each of the two higher frequency bands will have 8 distinct frequency channels, with the lowest frequency band, spanning 52 to 65 GHz, to have 16 receiver channels to better resolve the EAST scrape-off layer. With a pulse repetition rate of 1 MHz and simultaneous acquisition of all 32 TOF signals, electron density profiles may be obtained with time resolutions of as short as 3–6 µsec.

A next generation ultra short pulse reflectometry (USPR) diagnostic.

Ultrashort Pulse Reflectometry (USPR) is a plasma diagnostic technique involving the propagation and reflection of ultrashort duration (∼few ns) chirps. The reflected packets pass through a multichannel filter with time-of-flight measurements performed on each of the filtered packets. A next generation USPR system is under development, spanning 28-75GHz, for use on compact, short duration, magnetically confined fusion devices. This system presents a dramatic increase in performance compared with an earlier USPR system employed on the LLNL Sustained Spheromak Physics Experiment device more than a decade ago. The new system replaces upconverting mixers with higher power active multiplier chains to generate mm-wave transmitter chirps, with custom time-of-flight electronics reducing the time per measurement by a factor of 3X. Finally, the system is equipped with a field programmable gate array for data acquisition and analysis.

Simulation of multi-pulse coaxial helicity injection in the Sustained Spheromak Physics Experiment

Nonlinear, numerical computation with the NIMROD code is used to explore magnetic self-organization during multi-pulse coaxial helicity injection in the Sustained Spheromak Physics eXperiment. We describe multiple distinct phases of spheromak evolution, starting from vacuum magnetic fields and the formation of the initial magnetic flux bubble through multiple refluxing pulses and the eventual onset of the column mode instability. Experimental and computational magnetic diagnostics agree on the onset of the column mode instability, which first occurs during the second refluxing pulse of the simulated discharge. Our computations also reproduce the injector voltage traces, despite only specifying the injector current and not explicitly modeling the external capacitor bank circuit. The computations demonstrate that global magnetic evolution is fairly robust to different transport models and, therefore, that a single fluid-temperature model is sufficient for a broader, qualitative assessment of spheromak performance. Although discharges with similar traces of normalized injector current produce similar global spheromak evolution, details of the current distribution during the column mode instability impact the relative degree of poloidal flux amplification and magnetic helicity content.

Experiment of Double-pulsing Helicity Injection on Helicity Injected Spherical Torus Plasmas

Coaxial Helicity Injection (CHI) is one of useful methods for start-up to spherical torus (ST) and spheromak plasmas. The new approach of CHI so called the Multi-pulsing CHI operation has been proposed for the purpose of achieving the quasi-steady-state plasma and demonstrated in the sustained spheromak physics experiment (SSPX) gun-spheromak device for the first time. We conducted the initial experiment with the first application of double-pulsing CHI for the ST configurations. The experimental results showed flux/current amplification and sustainment of the plasmas. We observed repeating the driven phase and the decay phase from change of profile of force-free parameter λ. By the double-pulsing CHI operation, the sustainment time had increased up to about 8 ms which was longer than that in the single discharge. We investigated dependency of plasma current, density and the current density profile on the external toroidal field strength.

Extreme ultraviolet spectroscopy and modeling of Cu on the SSPX Spheromak and laser plasma “Sparky”

Impurities play a critical role in magnetic fusion research. In large quantities, impurities can cool and dilute plasma creating problems for achieving ignition and burn; however in smaller amounts the impurities could provide valuable information about several plasma parameters through the use of spectroscopy. Many impurity ions radiate within the extreme ultraviolet (EUV) range. Here, we report on spectra from the silver flat field spectrometer, which was implemented at the Sustained Spheromak Physics experiment (SSPX) to monitor ion impurity emissions. The chamber within the SSPX was made of Cu, which makes M-shell Cu a prominent impurity signature. The Spect3D spectral analysis code was utilized to identify spectral features in the range of 115-315 Å and to more fully understand the plasma conditions. A second set of experiments was carried out on the compact laser-plasma x-ray∕EUV facility "Sparky" at UNR, with Cu flat targets used. The EUV spectra were recorded between 40-300 Å and compared with results from SSPX.

Diagnostics for Lab and Astrophysical Plasmas

To monitor effects of vacuum conditioning, three radiation diagnostics are being built: a Helium–Neon (HeNe) heterodyne interferometer, a Bolometer, and a Hydrogen-alpha (Hα) detector. The interferometer will measure the line-average refractive index of the plasma, enabling us to obtain the line average density. The bolometer is a soft X-ray/UV detector (similar to those used on the Sustained Spheromak Physics Experiment (McLean et al. in Rev. Sci. Instruments 72(1):556–561, 2001) and the High-Beta Tokamak Experiment (Xiao and Navratil. in Rev. Sci. Instrument 67(9):3334–3335, 1996) and is used to directly measure the radiation loss from photons. The Hα detector will detect the amount of Hα being emitted by the plasma as a function of time, thus gauging the neutral density. These same concepts are also applied to astrophysical plasmas, with slightly different approaches. A brief overlap between diagnostics/detectors for laboratory and astrophysical plasmas is discussed.

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 Electron Density Profile Reconstruction on Ultrashort-Pulse Reflectometry

For the reconstruction of electron density profiles in the Sustained Spheromak Physics Experiment plasma, ultrashort-pulse reflectometry signals are numerically generated solving a one-dimensional wave equation by means of the central difference scheme. The signals cover an O-mode cutoff frequency range of 33–93 GHz. The lower frequencies (6–18 GHz) of a chirped waveform are up-converted to higher frequencies using four mixers. The reflected signals are down-converted to the lower frequency signals, thereby permitting band-pass filters to compute the time delays of different frequency components. A variety of density profiles are assumed to examine the performance of density profile reconstruction.

Aspect-ratio effects in the driven, flux-core spheromak

Resistive magnetohydrodynamic simulations are used to evaluate the effects of the aspect ratio A (length to radius ratio) in a spheromak driven by coaxial helicity injection. The simulations are benchmarked against the Sustained Spheromak Physics Experiment (SSPX) [R. D. Wood et al., Nucl. Fusion 45, 1582 (2005)]. Amplification of the bias (“gun”) poloidal flux is fitted well by a linear dependence (insensitive to A) on the ratio of gun current and bias flux above a threshold dependent on A. For low flux amplifications in the simulations, the n=1 mode is coherent and the mean-field geometry looks like a tilted spheromak. Because the mode has relatively large amplitude the field lines are open everywhere, allowing helicity penetration. Strongly driven helicity injection at A≤1.4 in simulations generates reconnection events which generate cathode-voltage spikes, relaxation of the symmetry-breaking modes, and open, stochastic magnetic field lines; this state is characteristic of SSPX. The time sequences of these events suggest that they are representative of a chaotic process. Near the spheromak tilt-mode limit, A≈1.67 for a cylindrical flux conserver, the tilt approaches 90°; reconnection events are not generated up to the strongest drives simulated. Implications for spheromak experiments are discussed.

Spectroscopy of multiply charged titanium ions in high-density magnetic fusion plasmas

The M-shell line emission from multiply charged titanium ions has been investigated at the Sustained Spheromak Physics Experiment in Livermore. Titanium was introduced into the relatively low-temperature, high-density magnetically confined spheromak plasmas using a titanium gettering system. The measurements were done using a high-resolution grazing-incidence spectrometer with a 1200 lines/mm grating and a Photometrics charged-coupled device camera. Spectral lines from the transition array 3s23pk - 3s23pk-13d in argon-like Ti4+, chlorine-like Ti5+, and sulfur-like Ti6+ have been observed in the 240 - 370 Å interval.

Moment approach to deriving parallel heat flow for general collisionality

In the moment approach, a parallel electron heat flux density is obtained for arbitrary collisionality in the transport ordering. The parallel moment equations are derived from the drift kinetic equation with the exact linearized Landau operator and analytically solved for the heat flux in integral form. Quantitative analysis of the integral heat flux for sinusoidal temperature profiles shows that the number of moments required for convergence increases as collision length increases. The integral heat flux well agrees with the Braginskii heat flux and the collisionless heat flux for high and low collisionalities, respectively. Incorrect application of the Braginskii heat flux in moderately collisional or nearly collisionless plasmas often leads higher heat flux than the integral closure. This fact is consistent with numerical studies of electron heat confinement in the sustained spheromak physics experiment [Hooper et al., Nucl. Fusion 39, 863 (1999)] which show a modest (6%) increase in core temperature when the integral heat flux is adopted.

Improved magnetic field generation efficiency and higher temperature spheromak plasmas

New understanding of the mechanisms governing the observed magnetic field generation limits on the sustained spheromak physics experiment has been obtained. Extending the duration of magnetic helicity injection during the formation of a spheromak and optimizing the ratio of injected current to bias flux produce higher magnetic field plasmas with record spheromak electron temperatures. To explore magnetic field buildup efficiency limits, the confinement region geometry was varied resulting in improved field buildup efficiencies.

EUV spectroscopy on the SSPX spheromak

Extreme ultraviolet (EUV) plasma spectroscopy is one the diagnostics implemented at the Sustained Spheromak Physics Experiment (SSPX) at the Lawrence Livermore National Laboratory. A grating spectrometer covering the spectral region of 25 - 450 Å with a resolution of 0.3 Å was used as an impurity diagnostic to monitor the plasmas and to carry out atomic physics research. Several low-Z impurities have been found in the spheromak, notably B, C, N, and O. Of the heavier elements, Ti, Cu, and W were found in the plasmas. As a relatively dense and low-temperature laboratory plasma device, SSPX served as an excellent radiation source for investigation of atomic spectra in a regime not readily attained in other devices. We have injected atomic titanium and tungsten hexacarbonyl into the spheromak under different operating conditions. We also report on electron temperature and electron density measurements based on the Kα lines from B IV at 60 Å.

Grazing-incidence spectrometer on the SSPX spheromak

The silver flat field spectrometer (SFFS) is a high-resolution grazing-incidence diagnostic for magnetically confined plasmas. It covers the wavelength range of 25-450 A with a resolution of Delta lambda=0.3 A full width at half maximum. The SFFS employs a spherical 1200 lines/mm grating for flat-field focusing. The imaging is done using a backilluminated Photometrics charge-coupled device camera allowing a bandwidth of around 200 A per spectrum. The spectrometer has been used for atomic spectroscopy on electron beam ion traps and for plasma spectroscopy on magnetic confinement devices. Here we describe the design of the SFFS and the spectrometer setup at the sustained spheromak physics experiment in Livermore.

Extreme ultraviolet spectroscopy of low-Z ion plasmas for fusion applications

The study of impurities is a key component of magnetic fusion research as it is directly related to plasma properties and steady-state operation. Two of the most important low-Z impurities are carbon and oxygen. The appropriate method of diagnosing these ions in plasmas is extreme ultraviolet (EUV) spectroscopy. In this work the results of two different sets of experiments are considered, and the spectra in a spectral region from 40 to 300 A are analyzed. The first set of experiments was carried out at the Sustained Spheromak Physics Experiment at LLNL, where EUV spectra of oxygen ions were recorded. The second set of experiments was performed at the compact laser-plasma x-ray/EUV facility "Sparky" at UNR. In particular, Mylar and Teflon slabs were used as targets to produce carbon, oxygen, and fluorine ions of different ionization stages. Nonlocal thermodynamic equilibrium kinetic models of O, F, and C were applied to identify the most diagnostically important spectral features of low-Z ions between 40 to 300 A and to provide plasma parameters for both sets of experiments.

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 demonstrated to maintain the magnetic field in a quasisteady state against resistive decay. Initial measurements of neutral particle flux in multipulse operation show charge-exchange power loss <1% of gun input power and dominantly collisional majority ion heating. The evolution of electron temperature shows a distinct and robust feature of spheromak formation: A hollow-to-peaked Te(r) associated with q∼1∕2.

NIMROD resistive magnetohydrodynamic simulations of spheromak physics

The physics of spheromak plasmas is addressed by time-dependent, three-dimensional, resistive magnetohydrodynamic simulations with the NIMROD code [C. R. Sovinec et al., J. Comput. Phys. 195, 355 (2004)]. Included in some detail are the formation of a spheromak driven electrostatically by a coaxial plasma gun with a flux-conserver geometry and power systems that accurately model the sustained spheromak physics experiment [R. D. Wood et al., Nucl. Fusion 45, 1582 (2005)]. The controlled decay of the spheromak plasma over several milliseconds is also modeled as the programmable current and voltage relax, resulting in simulations of entire experimental pulses. Reconnection phenomena and the effects of current profile evolution on the growth of symmetry-breaking toroidal modes are diagnosed; these in turn affect the quality of magnetic surfaces and the energy confinement. The sensitivity of the simulation results addresses variations in both physical and numerical parameters, including spatial resolution. There are significant points of agreement between the simulations and the observed experimental behavior, e.g., in the evolution of the magnetics and the sensitivity of the energy confinement to the presence of symmetry-breaking magnetic fluctuations.

Measurements and phenomenological modeling of magnetic flux buildup in spheromak plasmas

Internal magnetic field measurements and high-speed imaging at the Sustained Spheromak Physics Experiment [E. B. Hooper, L. D. Pearlstein, and R. H. Bulmer, Nucl. Fusion 39, 863 (1999)] are used to study spheromak formation and field buildup. The measurements are analyzed in the context of a phenomenological model of magnetic helicity based on the topological constraint of minimum helicity in the open flux before reconnecting and linking closed flux. Two stages are analyzed: (i) the initial spheromak formation, i.e., when all flux surfaces are initially open and reconnect to form closed flux in the toroidal average sense, and (ii) the stepwise increase of closed flux when operating the gun on a new mode that can apply a train of high-current pulses to the plasma. In the first stage, large kinks in the open flux surfaces are observed in the high-speed images taken shortly after plasma breakdown, and coincide with large magnetic asymmetries recorded in a fixed insertable magnetic probe that spans the flux c...

Spectroscopy on magnetically confined plasmas using electron beam ion trap spectrometers

Multiple spectrometers originally designed for and used at the University of California Lawrence Livermore National Laboratory’s electron beam ion trap have found use at various magnetically confined plasma facilities. Three examples will be described. First is a soft X-ray/EUV grating spectrometer (6–150 Å), which is operating at the National Spherical Torus Experiment. Second is an EUV spectrometer with wavelength coverage up to 400 Å, which has just recently started operating at the Sustained Spheromak Physics Experiment. The last is a high-resolution transmission-grating spectrometer for visible light that has been used at the Compact Toroid Injection Experiment and is currently at the Alcator C-Mod tokamak.PACS Nos.: 39.30.+w, 52.55.–s, 32.30.Rj, 07.60.Rd, 52.70.La

The Search for Reconnection and Helicity During Formation of a Bounded Spheromak

Recent results from investigations using insertable magnetic probes at the Sustained Spheromak Physics Experiment (SSPX) [E. B. Hooper et al., Nucl. Fusion 39, 863 (1999)] are presented. Experiments were carried out during pre-programmed, constant amplitude coaxial gun current pulses, where magnetic field increases stepwise with every pulse, but eventually saturates. Magnetic traces from the probe, which is electrically isolated from the plasma and spans the flux conserver radius, indicate there is a time lag at every pulse between the response to the current rise in the open flux surfaces (intercepting the electrodes) and the closed flux surfaces (linked around the open ones). This is interpreted as the time to buildup enough helicity in the open flux surfaces before reconnecting and merging with the closed ones. Future experimental and diagnostic plans to directly estimate the helicity in the open flux surfaces and measure reconnection are briefly discussed.