Ion Doppler Research Articles

Dipole trapped spheromak in a prolate flux conserver

This paper reports the observation and characterization of a spheromak formed in the Swarthmore Spheromak Experiment (SSX, [M. R. Brown, Phys. Plasmas 6, 1717 (1999)]) and trapped in a simple dipole magnetic field. The spheromak is studied in a prolate (tilt unstable) 0.4m diameter, 0.6m length copper flux conserver in SSX. This plasma does not tilt, despite the prolate flux conserver. The spheromak is characterized by a suite of magnetic probe arrays for magnetic structure B(r,t), ion Doppler spectroscopy for Ti and flow, and interferometry for ne. Three-dimensional magnetohydrodynamics simulations of this configuration verify its gross sta-bility.

Two fluid effects on three-dimensional reconnection in the Swarthmore Spheromak Experiment with comparisons to space data

Several new experimental results are reported from spheromak merging studies at the Swarthmore Spheromak Experiment [M. R. Brown, Phys. Plasmas 6, 1717 (1999)] with relevance to three-dimensional (3D) reconnection in laboratory and space plasmas. First, recent velocity measurements of impurity ions using ion Doppler spectroscopy are reported. Bidirectional outflow at nearly the Alfvén speed is clearly observed. Second, experimental measurements of the out-of-plane magnetic field in a reconnection volume showing a quadrupolar structure at the ion inertial scale are discussed. Third, a measurement of in-plane Hall electric field and nonideal terms of the generalized Ohm’s law in a reconnection volume of a weakly collisional laboratory plasma is presented. Time resolved vector magnetic field measurements on a 3D lattice [B(r,t)] enables evaluation of the various terms. Results show that the Hall electric field dominates everywhere (J×B∕ne) and also exhibits a quadrupolar structure at the ion inertial scale; resistive and electron inertia terms are small. Each of these is related to and compared with similar measurements in a solar or space context.

Ion heating during magnetic relaxation in the helicity injected torus-II experiment

Ion doppler spectroscopy (IDS) is applied to the helicity injected torus (HIT-II) spherical torus to measure impurity ion temperature and flows. [A. J. Redd et al., Phys. Plasmas 9, 2006 (2002)] The IDS instrument employs a 16-channel photomultiplier and can track temperature and velocity continuously through a discharge. Data for the coaxial helicity injection (CHI), transformer, and combined current drive configurations are presented. Ion temperatures for transformer-driven discharges are typically equal to or somewhat lower than electron temperatures measured by Thomson scattering. Internal reconnection events in transformer-driven discharges cause rapid ion heating. The CHI discharges exhibit anomalously high ion temperatures >250eV, which are an order of magnitude higher than Thomson measurements, indicating ion heating through magnetic relaxation. The CHI discharges that exhibit current and poloidal flux buildup after bubble burst show sustained ion heating during current drive.

Compact high-resolution ion Doppler spectrometer for quartz ultraviolet line emissions

A high-resolution quartz ultraviolet spectrometer based on a single compact 16-channel multianode photomultiplier tube (MAPMT) has been developed and tested on coaxial helicity injection and ohmic transformer current drive plasmas generated on the Helicity Injected Torus at the University of Washington. The use of the compact MAPMT allows a simple design with high light collection and high resolution for an ion Doppler shift diagnostic. The high quality and low cost of the design make it uniquely attractive for university scale plasma experiments. The Doppler shift of impurity ion emission lines from two chords of equal impact parameter but of opposite rotational sense is used to determine chord-averaged toroidal flow velocity. The width of the Doppler broadening is used to calculate the ion temperature. A detailed description of the ion Doppler spectrometer measurement system and some typical results are presented.

Ion Doppler Spectroscopic Measurement with a Multi-channel PMT on Spheromak Plasmas

An ion Doppler spectrometer (IDS) using a compact 16 channel photomultiplier tube (PMT) has been developed and used to measure ion temperature and toroidal rotation speed on SSPX gun-spheromaks. Doppler broadening of OV spectral line shows higher ion temperature (-500 eV) as compared to the electron temperature, which indicates anomalous ion heating due to MHD relaxation. It has been also found that the plasma rotates with a speed of 60 km/s in the direction of E×B.

Mechanism of central temperature peaking in impurity seeded plasmas

Formation of a centrally peaked profile of the ion temperature by impurity seeding has been observed in all the largest tokamaks: JET, JT-60U, TFTR. A possible explanation of this phenomenon is proposed, which takes into account the suppression of ion temperature gradient instability induced anomalous transport through increasing ion effective charge Zeff and Doppler shift of unstable modes, S⊥. The particular importance of the S⊥ component proportional to the temperature gradient, which stems from the pressure gradient and neoclassical poloidal rotation, is elucidated. It is demonstrated that this contribution grows significantly by impurity seeding when the rotation regime changes due to increased collisionality. An interrelation between the heating power density and Zeff necessary for the transition to a peaked ion temperature is found. The modification of radial profiles by impurity seeding is modelled numerically and the results are in agreement with the experimental data from JET.

Tomographic determination of plasma velocity with the use of ion Doppler spectroscopy

An inversion method for ion Doppler shift of spectrum lines was developed for plasma velocity measurements. By reducing the problem to the vectorial Radon transform, the characteristics of plasma ion velocity distribution such as vector potential/vorticity and velocity components were computed in two-dimensional space. Computer simulation of the reconstruction of different vector potential (vector field) models gives evidence that the reconstruction errors are acceptable, and the method can surely be used for real experimental data.

Novel spectroscopic method for analysis of nonthermal electric fields in plasmas.

In this Letter we present a simple and reliable method for the determination of the mean amplitude of the nonthermal fields and the ion Doppler temperature, based on the analysis of the Stark and Doppler broadening autocorrelation function. The method makes no assumption on the distribution function of the turbulent field or that the nonthermal fields are quasistatic. An independent ``typical field'' analysis may be performed to obtain information on the field frequency.

Doppler Measurements of the H 2O + Ion Velocity in the Plasma Tail of Comet Levy 1990c

Comet Levy 1990c was observed from 16 to 25 August 1990 at the Observatory Hoher List, Daun, Germany, using a focal reducer and a CCD camera, giving a field of view of ∼25 arcmin. The heliocentric distance of the comet was 1.46 AU; its geocentric distance was 0.47 AU. A Fabry-Perot etalon with a fixed gap of 0.9 mm was used in imaging mode to measure the Doppler shift of the emission lines of the cometary H 2O + ions at 6159 Å, allowing the determination of the line-of-sight velocity of ions in the plasma tail. Two-dimensional fields of the ion Doppler speed in the plasma tail out to 10 6 km from the nucleus have been derived for 4 nights. To compare the velocity fields with the optical appearance of the cometary plasma tail, images taken through a narrow-band interference filter at 6159 Å were also obtained, alternating with velocity measurements. The change of the appearance from one image to another has been computed using measured velocities and compared with the observed appearance. Structures were transported to the position where they were actually observed in the following image, showing that they move by real particle motions and are not caused by wave compressions. A mean H 2O + production rate of 1.5 × 10 27 sec -1 has been derived. The comparison of the derived production rate to the value expected from photoionization rates shows that only a small portion of the ions is concentrated in the inner plasma tail and visible in our groundbased observations, in agreement with results obtained by the ICE spacecraft.

Ion heating and current drive from relaxation in decaying spheromaks in mesh flux conservers

The plasma energy confinement in CTX mesh flux conservers appears to be dominated by relaxation fluctuations which drive current in the resistive plasma edge region. Strong ion heating is expected from these relaxation processes. The paper presents new data, from previously studied decaying discharges, which show impurity oxygen ion Doppler temperatures much above the Thomson scattering electron temperature. In addition, using the standard MHD equilibrium model by Knox et al., and the resistivity model previously described by Fernández et al., the contribution of relaxation fluctuations to spheromak current drive along the open magnetic field lines at the spheromak edge can be estimated.

Mirror waves driven by newborn ion distributions

Ring‐type ion distributions, such as are found in the vicinity of comets, are shown to drive mirror waves unstable. Major differences are found between mirror waves driven by anisotropic bi‐Maxwellian distributions and mirror waves driven by ring‐type distributions; in particular, for pure ring distributions the maximum mirror wave growth rate occurs for quasiparallel wave propagation. The growth rate for ring‐type driven mirror waves is approximated analytically and calculated numerically. For pure ring ion distributions, the analytic approximations are quite good. For ring‐beam distributions, the mirror waves appear with a real frequency approximately equal to the ring ion Doppler frequency; the analytic approximations agree with the numerical results when the real frequency is small compared to the proton gyrofrequency so that there is little proton Landau damping. The analytic approximations provide a simple explanation for the growth dependences on propagation angle and wavelength, and lead to a simple (approximate) expression for the most unstable wavelength. Resonant instabilities can also be driven by ring‐type distributions; growth rates for the nonresonant mirror wave instability are comparable to growth rates for resonant instabilities. In the presence of a ring‐type distribution, the mirror waves remain nonoscillatory (ωr = 0) when viewed in the frame of the newborn ions.

On the light force upon a single trapped three-level ion

Heating spectra of a single Ba + ion in an rf trap are derived, by suitable normalisation, from spatially selective recordings of the excitation spectra of its resonance fluorescence. They reveal two novel types of light force ascribed to (i) stimulated two-photon transition followed by (spontaneous) electronic Raman scattering, and (ii) all-stimulated parametric action. The latter one is analogous to free-electron laser action, with the electrons' relativistic frequency shift replaced by the ion's Doppler shift.

High resolution, cw laser induced fluorescence study of the A 2Πu–X 2Σ+g system of N+2

Laser induced fluorescence detection of the N+2 ion has been accomplished using a single mode, cw dye laser, thereby demonstrating the feasibility of continuous ion monitoring and measurement of ion Doppler profiles. A detailed spectral analysis has been performed for 173 transitions of the 4–0 2Πu↔2Σ+g Meinel band system of N+2 . Molecular constants for both the X and A states are obtained. By combining the present results with earlier optical emission work on other vibrational transitions of the Meinel system, equilibrium molecular constants are obtained.

Capped probe for ion temperature diagnostic in a magnetized plasma

A simple ion temperature measurement method in a magnetic field using an electrostatic capped probe is presented and tested. The probe with the axis oriented parallel with the magnetic field is a small cylinder whose flat tip is insulated with a cap to inhibit electron inflow parallel to the field. The ion collector current, essentially less affected, becomes comparable to or larger than the saturation electron current, and the usual retarding potential technique can be used to measure ion temperature. Measurements were made in a diffused helium plasma with the density 1010 ∼ 1013 cm−3 at the gas pressure ∼mTorr and the field 0.8 ∼ 3.5 kG. Ion temperatures obtained are 0.8 ∼ 2 eV, which nearly equal electron temperatures, and are compared with data from the ion probe and Doppler profile measurements. Availabilities of the method and limiting conditions for the temperature determination are also discussed.

Coulomb-collision corrected ion-acoustic line profiles

Complete profiles of Coulomb-collision corrected ion-acoustic lines from near resonance to zero frequency shift have been computed. The results are compared with those predicted by collisionless plasma theory. The resonant frequency is found to be shifted to a higher frequency from the ion Doppler frequency whereas the width of the resonant peak is reduced by collisional effects. The intensity of the resonant peak is significantly enhanced and a dip is found to occur between resonance and zero frequency. These results are presented graphically as a function of normalized collision frequency. Possibilities of observing these collisional effects experimentally on spectra of Thomson-scattered light and using these results for plasma diagnostics are discussed.

Interaction of Electromagnetic Waves with Quantum and Classical Plasmas. Correlation Effects

A quantum statistical approach is used to study the interaction of electromagnetic (EM) waves with a hot plasma. The quantum calculation takes into account the correlation effects. The Coulomb interaction between the charged particles is renormalized so as to take care of the collective effects. This renormalization also reduces the number of diagrams to be considered to obtain any desired accuracy. The results for a classical plasma are obtained by passing to the limit $\ensuremath{\hbar}\ensuremath{\rightarrow}0$. Some applications are considered for a classical hot plasma. In the damping of EM waves, the contributions of correlations are more important, at the limits of high and low frequencies, than the Landau contribution, which prevails at plasma resonances. Also, the appearance of the ion Doppler spread, instead of the electron Doppler spread, in the data for the scattering of EM waves observed in the ionosphere backscattering experiments, is explained in terms of correlation effects.