Plasma Lifetime Research Articles

Effect of radio frequency field on toroidal plasma parameters

Low temperature plasma parameters in a toroidal magnetic field are measured. The effect of an externally applied perpendicular electric field on the plasma parameters is studied. The lifetime of the plasma is measured in the presence and absence of the RF electric field. Decrease in the plasma lifetime in the presence of RF field is attributed to detrapping of the primary electrons to a larger volume. Plasma lifetime increases when a small vertical magnetic field is added to the toroidal magnetic field.

X-ray diagnostics for magnetic fusion (invited)

Current research efforts in magnetic fusion strongly depend upon information gained from x-ray observations of the confined plasma. Spectroscopic measurements make it possible to determine plasma temperature, density, electron energy distribution, ionization state, impurity content, and plasma velocity. X-ray imaging provides estimates of the spatial distribution of a few of these parameters, facilitates temporal studies of plasma size and shape, and indicates MHD activity. Diagnosing plasma properties in tokamaks and tandem mirrors has resulted in the development of a variety of sophisticated spectroscopy and imaging instrumentation. All share standard x-ray detection techniques and are either dispersive or nondispersive devices. To achieve optimal performance, these systems are individually tailored to specific operating regimes. Criteria such as energy, temporal and spatial resolution, plasma lifetime, x-ray emission bandwidth, system throughput, and the effects of the non-x-ray background influence the diagnostic design. X-ray diagnostics have been reviewed on numerous occasions in the context of tokamak plasmas heated by either neutral beams or lower hybrid waves. This report will focus on diagnosing tandem mirror plasmas. Detector requirements and limitations, recent instrument development and performance, and up-to-date x-ray measurements will be discussed. This work was performed for the U.S. DOE by Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48.

Time-Resolved Laser-Induced Breakdown Spectrometry for the Rapid Determination of Beryllium in Beryllium-Copper Alloys

Time-resolved laser-induced breakdown spectrometry has been combined with the long spark technique and applied to the rapid determination of beryllium in beryllium-copper alloys. A calibration curve was developed which related the beryllium concentration in a solid copper matrix to the Be(I) 234.9-nm to Cu(II) 235.7-nm intensity ratio. The beryllium concentrations ranged from 0.001 to 0.22%. For the lowest concentration the relative standard deviation of replicate samples was 7%, implying a detection limit of 0.0002% (2 ppm) at a signal-to-noise ratio of 3. The excitation temperature was determined from Boltzmann plots on Cu(I) and Cu(II), assuming local thermodynamic equilibrium. The values from the two spectra agreed well, and averaged to 13,850 K at 1 μS into the plasma lifetime.

Reversed-field pinch research in the United Kingdom

The foundations of present-day RFP research are to be found in the early results from ZETA in 1967, in which a period of improved stability was observed in discharges with a spontaneously generated reversed field, during which the magnetic field configuration was in accord with ideal-MHD stability requirements. Theoretical work at Culham subsequently established that the reversed-field configuration is a minimum magnetic energy state to which a plasma has a strong propensity to relax naturally. – HBTX1, designed to study fastprogrammed RFPs in a quartz torus, became operational during 1972; spontaneous field reversal was again demonstrated. By operating with current risetimes shorter or longer than the natural relaxation time, both reversal by strong helical instability and relaxation to reversed field more gently along a minimum energy trajectory in F-θ space were demonstrated. However, although much important information was obtained from HBTX1, including checking the predictions of relaxation and resistive instability theory, high temperatures could not be sustained,and ZETA-like quiescence was not found; this was attributed to the use of an insulating vessel. – A new assembly, HBTX1A, with a stainless-steel liner, was commissioned in 1981. By using a steady vertical field to assist in providing equilibrium, plasma lifetimes of up to 15 ms were observed with electron temperatures of up to 350 eV (at 200 kA) at densities of (1–2) × 1019 m−3. Magnetic field fluctuations with B̃/B ≥ 1–2%, seen throughout the discharge, are dominated by poloidal m = 0 and m = 1 global modes of resistive MHD origin, and localized activity in the core at higher frequencies. The m = 1 modes can contribute to anomalous energy transport, and to reversed-field penetration. The value of βθ≈ 10% varies relatively little with conditions in HBTX1 A, which is consistent with theoretical predictions of transport due to g-modes; if βθ remains constant at about 10% as the current and machine size are increased reactor conditions will be reached.

Lifetime of an electron-hole plasma of silicon in the picosecond range.

A new recombination channel for the electron-hole plasma in silicon, at carrier densities of ${n}_{e}\ensuremath{\gtrsim}{10}^{21}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, is calculated. It is shown to be important in explaining the optical response of the electron-hole plasma at different probing wavelengths generated by picosecond pulses at very large fluences sufficient to melt the sample.

Flute instability of plasma in a gas-dynamic trap

The flute instability of a plasma confined in a gas-dynamic trap where the mean free path of the particles λ is small compared with the trap length L is examined. In analysing the instability the influence on the oscillations both of the energy and particle injection which maintains the system in steady state and also of the damping of perturbations which results from plasma flowing out through the mirrors is included. An expression is obtained for the growth rate of the flute instability which is used in formulating the necessary and sufficient criterion of stability. It is found that in certain conditions, even when the flute instability has been suppressed, an instability with a characteristic growth rate of the order of the reciprocal of the plasma life-time can still develop in the trap. The effects examined in the paper are also of interest for open traps with λ > L.

The new generation of highly-charged ion sources

Introduction. Highly charged ion sources are major components in heavy-ion accelerators employing classical systems, and they ultimately determine design features such as the size, mass, cost, and facilities. Up to the 1970s, virtually the only type of ion source for heavy-ion accelerators was based on the Penning reflective discharge with oscillating electrons (PIG). Most accelerators were equipped with these sources, and the ongoing upgrading of them has meant that the intensities and charges of the beams they provide have attained the theoretical limits realted to the restricted plasma temperature, density, and lifetime.

Metabolic heterogeneity of post-lipolysis rat mesenteric lymph small chylomicrons produced in vitro.

The study was undertaken to investigate the metabolic rat of post-lipolysis mesenteric lymph small chylomicrons produced in vitro. Small chylomicrons doubly labeled with [3H]cholesterol (more than 70% in cholesteryl esters) and [14C]palmitate-labeled triglycerides were collected from rat mesenteric lymph during periods of fasting. Lipolysis was performed in vitro with lipoprotein lipase purified from bovine milk. More than 98% of the chylomicron-triglycerides could be hydrolyzed to fatty acids. Post-lipolysis chylomicrons were separated by zonal ultracentrifugation, characterized, and tested for biological behavior in intact rats. Following lipolysis the lipoproteins lost nearly all their triglycerides, apoA-I, and apoC, and were relatively enriched with cholesteryl esters, unesterified cholesterol, phospholipids, and apoB. Three preparations were tested for biological behavior: pooled (total) post-lipolysis chylomicrons (diameter approximately 250 A); particles at the ascending part of the zonal effluent (diameter approximately 300 A), and at the descending part (diameter approximately 200 A). After intravenous injection to intact rats, [3H]cholesteryl ester decay was very rapid with pooled lipoproteins and the 300-A preparation (t1/2 = 5-10 min). The 200-A preparation in contrast stayed in circulation much longer (t1/2 = 60-90 min). The study thus demonstrated metabolic heterogeneity of post-lipolysis small chylomicrons and indicated that some may form an LDL-like subpopulation with a plasma lifetime slower than "remnants" but faster than LDL.

Investigations of the magnetic structure and the decay of a plasma-gun-generated compact torus

The results of a series of experimental measurements of compact toroidal (CT) plasmas produced by a magnetized coaxial plasma gun injecting into a flux-conserving metallic liner are reported. The experiments were performed on the Beta II facility at Lawrence Livermore National Laboratory. The magnetic equilibria are well described by a force-free eigenmode structure that results from an extension of Taylor’s theory of the reversed-field pinch. Consideration of helicity conservation during relaxation of the composite plasma-gun flux-conserver system to the final state equilibrium yields theoretical expressions that are compared with the experiment. In particular the CT poloidal flux (ψpol) and the overall electrical efficiency for producing the CT are predicted to be functions of the plasma gun inner-electrode flux (ψgun) and the volt-seconds input to the gun discharge (∫∞0 V dt). Away from a cutoff at too low values of ∫∞0 V dt or too high values, ψgun ,ψpol scales linearly with the square root of the product of ψgun and ∫∞0 V dt, whereas the electrical efficiency equals about 13% for ∫∞0 V dt/ψgun ≊10. For an electrical energy input Win =45 kJ, CT’s are produced with poloidal plus toroidal field energy up to WB =8 kJ and toroidal plasma current Itor =330 kA. The chord-averaged plasma density is 2–4×1014 cm−3, and the plasma volume equals 150 liters. The radius of the flux conserver is 37.5 cm, and the axial length is 40 cm. If a bias flux ψb is superimposed on the flux conserver, n=1 tilting is observed when ψb/ψpol exceeds a ratio of about 0.20 to 0.25. Impurity radiation measured by a pyroelectric detector accounts for all of the plasma magnetic energy if uniform volume emission of radiation is assumed. The dominant impurities observed are carbon and oxygen. Helium-like lines are not observed, indicating that the plasma has not ‘‘burned through’’ the low electron temperature radiation maxima. The experimentally observed decay times (defined by the e-folding time of plasma magnetic fields) are 80 to 160 μsec—consistent with Zeff =2 and Te in the range 5–10 eV if classical resistivity is assumed. A zero-dimensional rate equation model of impurity radiation loss gives a reasonably good account of the experimental observations and predicts that the carbon concentration must be reduced to the level of a few percent to allow burnthrough of the low-Te carbon radiation barrier. Glow discharge cleaning of the gun electrodes and flux conserver resulted in a 20% increase of the e-folding time of plasma magnetic fields (from an average value 115 to 140 μsec). The CT plasma density was observed to scale linearly with the electrical energy input to the gun discharge and to be only weakly dependent on the filling pressure and timing of pulsed deuterium gas valves. It seems likely that further improvements in increasing plasma lifetime can be made by improving the vacuum conditions and discharge cleaning methods and experimenting with the gun electrode materials.

Electron—Hole plasma in CuCl after subpicosecond photoexcitation

The formation of a superdense electron—hole plasma in CuCl is reported for injected carrier densities n > 10 20 cm -3. The plasma lifetime is determined to be τ ≲ 10 −11 s.

Measurement of the Plasma Width in a Ring Cusp

A pure, fully ionized, ${\mathrm{D}}_{2}$ plasma of 2\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ions, generated from free-falling ${\mathrm{D}}_{2}$ pellets with use of a 100-J Nd-glass laser pulse followed by a 1000-J C${\mathrm{O}}_{2}$ laser pulse, is used to fill ($\ensuremath{\beta}=1$) a cusp-configured magnetic field. The width of the plasma escaping through the ring cusp is determined by laser scattering, chamber wall light and chemical deposits, and plasma lifetime. The ring cusp is an ion gyroradius wide, in contrast to recent experiments that have indicated a considerably narrower cusp width.

Vacuum ultraviolet spectroscopic apparatus for space- and time-resolved measurements on a single Tokamak plasma discharge

The apparatus described operates in the range 10 and 220 nm. By use of a rotating mirror working at grazing incidence, several spatial profiles (between five and ten, depending on plasma lifetime and mirror rotation speed) of two impurity lines are obtained during a single plasma discharge.

Plasma and recombination effects in the fission fragment pulse height defect in a surface barrier detector

The pulse height defects for fission fragments separated according to their kinetic energies and masses are determined for a heavy ion silicon surface barrier detector operated at various applied biases. A discussion of the results in LSS dimensionless units is included. Several versions of a model are developed to relate the field dependent contribution of the defect due to charge recombination to the lifetimes of the charge plasmas formed in the wake of the fragments. Estimates are hence made from our defect data and plasma lifetime data (obtained elsewhere) of the plasma column initial radius (3 μm) and of the charge recombination lifetime in the track vicinity (20–50 μs at low charge densities). The recombination defect expressed as a fraction of the fragment energy is for heavy fragments up to three times that for light fragments. The resulting low prediction for the recombination lifetime may be correlated with the greater crystal damage created by heavy fragments.

Local regions of high-pressure plasma in a vacuum spark

The formation of local regions of the high-temperature, high-pressure plasma, 'hot-spots', in a vacuum spark is studied using X-ray spectroscopy and holography. Measurements of the dimensions of the hot-spots, of the absolute flux of radiation and of the spectral composition of the X-ray emission, have been analysed to give order-of-magnitude values for the local electron density, temperature and plasma lifetime. It appears that the plasma pressure is >or approximately=107 atm and that the formation of the hot spots requires intense heating rates more commonly associated with focused laser beams, i.e. >or approximately=1015 W cm-3. It is probable that in the vacuum spark electron beams are responsible for plasma heating. Possible mechanisms for concentrating energy deposition in localised regions are discussed.

Transient confinement of a high-density plasma in a multiple-mirror magnetic-field configuration

Multiple-mirror magnetic confinement is investigated experimentally with a pulsed hydrogen plasma produced by a theta-pinch source. The gun output at the end of the guide field between the theta-pinch coil and the multiple-mirror region was a sharp plasma pulse of strength ∼1014 cm−3 with a ratio of drift to thermal velocity close to unity (∼ 10 eV). The seven multiple-mirror cells were stabilized with a linked quadrupole field. Transient experiments were performed with hydrogen in the favourable multiple-mirror regime (n ∼ 1013 cm−3, T ∼ 10 eV) where the ion mean free path is comparable to a cell length. The quadrupole field achieved partial plasma stabilization and the plasma lifetime inside the device was increased from 10–30 μs in the unstabilized simple mirror geometry to 50–80 μs at high values of the stabilizing current. The plasma pulse was successfully injected into the multiple-mirror region but was mainly trapped in the first half of the device, because of a non-uniform introduction region of two cells and because of the low value of the pulse drift velocity. The main features of the plasma build-up and decay were studied analytically and numerically with a simple simulation of the experiment. The numerical results were in qualitative agreement with the density profiles observed experimentally inside the device. The strong density gradients, obtained at the large values of the quadrupole current for which the flux surfaces are strongly elliptical, greatly enhanced the classical radial losses, particularly from ion-ion collisions. When included in the numerical model, the theoretical radial loss rates indicated a significant (up to 30%) decrease of the plasma lifetime inside the multiple mirror for the large values of the quadrupole current necessary for stability, in reasonable agreement with the experimental results.

Two-dimensional boundary-value problem for ion-ion diffusion

Like-particle diffusion is usually negligible compared with unlike-particle diffusion because it is two orders higher in spatial derivatives. When the ratio of the ion gyroradius to the plasma transverse dimension is of the order of the fourth root of the mass ratio, previous one-dimensional analysis indicated that like-particle diffusion is significant. A two-dimensional boundary-value problem for ion-ion diffusion is investigated. Numerical solutions are found with models for which the nonlinear partial differential equation reduces to an ordinary fourth-order differential equation. These solutions indicate that the ion-ion losses are higher by a factor of six for a slab geometry, and by a factor of four for circular geometry, than estimated from dimensional analysis. The solutions are applied to a multiple mirror experiment stabilized with a quadrupole magnetic field which generates highly elliptical flux surfaces. It is found that the ion-ion losses dominate the electron-ion losses and that these classical radial losses contribute to a significant decrease of plasma lifetime, in qualitiative agreement with the experimental results.

Magnetic surfaces in a dc toroidal octupole field under the influence of magnetic field perturbations

Calculations are presented which show the effects of magnetic field perturbations on toroidal multipole fields used for plasma confinement. The field topology was destroyed, with the attendant formation of magnetic surfaces. In contrast to earlier calculations, exact equations are solved numerically and the results apply to actual experiments in which the plasma lifetime was seriously decreased by the perturbing field.

Finite-Larmor-radius stabilization of the m = 2 instability in the Isar T1-B high-beta stellarator

The m = 2 mode stability is investigated in the ISAR T1-B toroidal high-beta stellarator (major diameter 2.7 m, capacitor bank energy 0.5 or 1.5 MJ) for a great variety of plasma parameters using deuterium filling gas (0.5 < ωL · τii < 200; 0.1 < rL/a < 0.35; ωL = ion Larmor frequency; rL = ion Larmor radius; a = plasma radius; τii = ion self-collision time). The ion mass dependence was checked by a small number of discharges with hydrogen. For rL/a < ηcrit, ηcrit = 0.14, fast-growing m = 2 modes were found, the limit ηcrit being essentially independent of the collision frequency within the accessible range (2 ≲ ωL · τii ≲ 20). For rL/a > 0.14 no fast-growing m = 2 modes were observed, the plasma lifetime being limited by a m = 1, k ≈ 0 mode. The normalized growth rate, γ̃ = γ/(Vi/a) = 0.15 ± 0.05, (Vi = ion thermal speed) was about half the sharp boundary value and slightly higher than that calculated for a diffuse pressure profile. The experimentally determined minimum value of rL/a, for which the m = 2 mode is stabilized, is by about a factor of three smaller than Freidberg's estimation using a sharp-boundary Vlasov model. Advanced calculations in a sharp boundary model by Lewis and Turner deviate in the critical limit only by a factor of about 1.5 and recent results using a diffuse pressure profile by Herrnegger and Schneider and by Freidberg and Hewett agree quite well with the experimental data.

Endloss from a slender high beta plasma column contained in a linear solenoid

A model is presented to simulate endloss from a high beta plasma contained laterally in a straight solenoid. For slender plasma columns, the plasma lifetime depends on both the acoustic transit time (time for a sound wave to traverse one-half of the plasma length), and the characteristic radial diffusion time (diffusion of plasma across the magnetic field). In the limit of strong diffusion (which occurs for narrow high beta plasma columns), the plasma lifetime equals the hybrid lifetime which is proportional to the geometric mean of the acoustic and diffusion times. This predicted behavior stands in marked contrast with conventional endloss theories which have confinement time proportional to acoustic time alone. It is also shown that the endloss process may be accelerated considerably if the ratio of plasma radius to magnet radius is not small, due to inverse mirroring effects in the streaming plasma.

Pulsed-gas-valve measurements of impurity ion concentrations and energies in the 2XII mirror device

Measurements of impurity ion concentrations and energy distributions in the 2XII mirror device are desaibed. The measurements were made using a fast-pulsed gas valve and a charge-exchange analyser that employs both electrostatic and magnetic separation to obtain energy spectra of separate particle species. The predominant high-Z material detected wascarbon, having a concentration of ∼2% and a mean energy of 3-6 keV. Smaller amounts of oxygen were the only materialother than hydrogen detected. These energetic impurity levels are determined to be contained against self-scattering into themirror loss cones and to have only minor influences on plasma life-time and on mean ion and electron energy.