Direct Ohmic Heating Research Articles

Miniature induction oven for ion sources optimisation

The space and power limitation frequently arising when inserting ovens into ion sources lead us to explore alternatives to the direct current ohmic heating. An induction oven has the following advantages over a resistance one: a) the current in the induction coil is lower; b) heating may be concentrated on the sample surface by choosing the right value of the frequency and the total power is limited; c) there is no contact between the heating coil and the heated sample; in fact a boron nitride crucible separates the sample from the coil. A tantalum enclosure minimizes EMI and coupling to the plasma. The technological and optimisation problems due to the limited size are discussed. In perspective, these devices may heat a broad range of elements up to 2000 K extending application of any ion volume sources.

Automated Gas Chromatograph/Mass Spectrometer for Routine Atmospheric Field Measurements of the CFC Replacement Compounds, the Hydrofluorocarbons and Hydrochlorofluorocarbons

An automated gas chromatograph/mass spectrometer is described for the routine field monitoring of the CFC replacement compounds, the hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs). Detection limits of 0.5 pptv are achieved by concentration of a 2-L air sample onto a three-stage adsorbent-filled microtrap. The microtrap is cooled to subambient temperatures (-50 o C) using Peltier thermoelectric devices, without a postdesorption cryofocussing step, thereby eliminating the need for liquid cryogens. The enriched air sample is thermally desorbed by direct ohmic heating of the microtrap, and compounds are separated on a 100-m, 5-μm-thick film high-resolution capillary column. Individual hydrohalocarbons are determined by selected ion masses for quantitation. The instrument is designed to operate automatically at a remote atmospheric research station providing ∼2-h replicate air and calibration analyses

Direct electric curing of concrete

Direct electric curing (DEC) of concrete is a process by which alternating electric current is passed through the bulk of freshly poured Portland cement concrete to acceleratethe curing process by means of direct ohmic heating. Electric JieM strengths, ranging approximately from 300 to 500 V/m, are required. Ris process leads to signiJicantly higher preliminary (8 hours) crushing strengths. The ultimate (28 days) strength is under suspicion, however. The Authors have investighted the influence of the maximum temperature reached, the energy input (kWh/m3), and the rate of energy input (kW/m3)on the preliminary and ultimate strengths of the concrete. It is concluded that the DEC technique is technically and economically viable, and warrants further development.

Channel probe observations of the auroral ionosphere during high‐power auroral stimulation heating experiments

Perturbations of the bottomside auroral ionosphere caused by a high‐power HF heater have been observed using a bistatically configured ionospheric channel probe. The heater transmitter power and antenna directivity resulted in a power density of the order of 1 mw/m2 at E layer heights and 0.2 mw/m2 at F layer heights. O mode polarization was used for heating at a frequency of 2.85 MHz. Effects were observed at both E and F layer heights. The F layer effects consisted of enhanced attenuation, modified group delay at frequencies above and below the heater frequency, and a change in the character of the received signal from that of a time stable specular return to a time variable irregular return. All of these effects were most pronounced at frequencies closest to the heater frequency and diminished as the frequency deviated from the heater frequency. The formation of a localized heated cavity in the F layer is thought to be responsible for the observed changes in F layer signal group delay. Some form of parametric decay instability is thought to be responsible for the irregular character of the return signal during heating and for the enhanced attenuation which primarily affected the O mode of the probing signal. Heating induced Doppler effects were also noted in a second experiment. In particular, the main Doppler component exhibited a sharp change with the onset of heating followed by a steady change in Doppler from an initially large positive value to a large negative value at the time of heater shutdown. After heater shutdown the Doppler frequency relaxed to a steady value which differed from its preheating value. The spread of the Doppler return was also observed to broaden during heating which is consistent with the irregular character of the return during heating. The E layer return consisted of normal E layer echoes at frequencies below 2.5 MHz and of weak slant sporadic E echoes at and above 2.5 MHz. The slant sporadic E echoes were strongly attenuated during heating. An additional E layer effect involving what appears to be a localized, nonblanketing enhancement of ƒ0E was observed during heating. The most likely explanation for the enhanced attenuation of the slant sporadic E is E layer heating caused by energetic electron precipitation from the heated F layer. The enhanced ƒ0E could also be attributed to high‐energy electron precipitation from the F layer but direct ohmic heating of the E layer is an equally plausible explanation.

Toroidal confinement experiment featuring direct ohmic heating of the ions

Recently a toroidal plasma device with the confinement being provided by surface multipole magnetic cusps generated by permanent magnets has been operated at UCLA. The cusps are oriented in the poloidal direction and have a maximum field strength of 1.3 kG. A toroidal current is driven by a transformer; the loop voltage is about 100 V. It is found that for a hydrogen plasma a current of 3 to 6 kA is driven and that the current is carried by the ions. This gives direct ion heating to temperature of 40 to 100 eV in a 10 13 cm −3 density hydrogen plasma. The electron temperature is lower, being in the 20–40 eV range. The β of the plasma is high, in the 20–30% range. We have simulated the operation of this device using a 2 1 2 dimensional electromagnetic particle code. The model duplicates many of the features of the experiment, including the restraint of electron current by the cusps while the ions flow relatively freely, the focussing of the streaming ions to the center of the chamber by the magnetic lens effect of the cusps, and the pinch effect. A simple theoretical treatment explains both the experiments and the simulations, although the detailed simulation results indicate the need for a more complete theory.

The direct Ohmic heating of positive ions in tokamak plasmas

A universal Ohm's law is derived for the ion bootstrap current relative to the impurity ions. The contribution driven by qir, the neoclassical plus electrostatic part of the ion heat flux, is found to be Jiz = nie (V̄i‖−V̄z‖) = 1.5 e2qirBθ/miTi(νiz + 2νii) and the frictional heating of the ions is JizEϕ[1−(1/Zeff)], which typically can be 10% to 15% of the total Ohmic heating when nzZ2⩾ni.

Temperature jump method in molecular beam relaxation spectrometry applied to catalytic decomposition of CH 3OH on polycrystalline Ni foil

A new modification of molecular beam relaxation spectrometry (MBRS) is described: the temperature jump method for studying catalytic surface processes on metal foils. The temperature of the catalyst foil is maintained by direct ohmic heating; a constant particle beam is directed towards the catalyst surface. A jump of the surface temperature caused by a high current pulse generates a response of the fluxes of desorption. The decay of the desorption intensity after the temperature jump contains the relaxation times of the elementary steps involved. The mathematical treatments of unimolecular and bimolecular surface reactions, of sequences of two and three unimolecular steps and of a sequential reaction accompanied by the redesorption of the reactant are given. The application of the new method is shown by a study of the catalytic decomposition of CH 3)OH on polycrystalline Ni: CO and H 2 are the sole reaction products. The limit of the catalytic activity — apart from the low sticking probability of the reactant — must be seen in the abstraction of the first methyl hydrogen from the transient methoxy species. In the temperature range between 320 and 550 K the reaction mechanism can be described as follows: ▪ Rate constants in dependence from surface temperature T are: k 1 = 4.2 × 10 4 exp ( −22.4 RT kJ mol ) s −1; k 3 = 2.4 × 10 9 exp ( −75 RT kJ mol ) s −1; k 4 = 1.2 × 10 13 exp ( −104 RT kJ mol ) s −1; η = 0.2. Typical surface residence times of the intermediates are: 110 ⩾ τ 1 ⩾ 15 ms at 320 ⩽ T ⩽ 450 K; 210 ⩾ τ 3 ⩾ 6 ms at 450 ⩽ T ⩽ 550 K; 98 ⩾ τ 4 ⩾ 6 ms at 450 ⩽ T ⩽ 500 K.

Method for heat injection into a liquid

A novel technique of heat injection into a liquid is presented. Advantages of the technique (based on direct ohmic heating) include the ease with which the apparatus can be set up and its simplicity of operation. Very small heat sources can be fabricated to closely approximate point sources.

Direct ohmic heating of ions in an ionizing shock

An experimental study of the structure of a normal ionizing shock in helium suggests that the shock is essentially resistive with the major dissipative mechnism being ohmic heating of ions and neutrals by ion currents.

Direct High-Temperature Ohmic Heating of Metals as Liquid Pipes

When a sufficiently high electric current is passed through a liquid metal, the electromagnetic pressure pinches off the liquid metal and interrupts the flow of current. For the first time the pinch effect has been overcome by use of centrifugal acceleration. By rotation of a pipe of liquid metal, tin or bismuth or their alloys, at sufficiently high speed, it can be heated electrically without intermission of the electric current. One may now heat liquid metallic substances, by resistive (ohmic) heating, to 5000 degrees K and perhaps higher temperatures.

Preparation and Characterization of Evaporated Boron Films

A method of preparing high purity, evaporated boron films by direct Ohmic heating of elemental boron filaments in ultrahigh vacuum is described. Electron micrography, electron diffraction, and B.E.T. (Brunauer-Emmett-Teller) surface area measurements yield some insight on the nature of these films.