Calorimetric Probe Research Articles

A calorimetric probe for plasma diagnostics

A calorimetric probe for plasma diagnostics is presented, which allows measurements of the power taken by a test substrate. The substrate can be biased and used as an electric probe in order to obtain information about the composition of the total heating power. A new calibration technique for calorimetric probes, which uses monoenergetic electrons at low pressure, has been developed for an improved accuracy. The use of the probe is exemplified with an experiment where both energetic neutral atoms and ions heat the test substrate.

Comparative Study of Total Power Density at a Substrate in Pulsed DC Magnetron and Hollow‐Cathode Plasma Jet Sputtering Systems

AbstractThe pulsed DC magnetron and hollow‐cathode plasma jet sources have been studied from the point of view of total energy flux density at a “floating” substrate. Both plasma sources were operated under identical experimental conditions and differences between both systems were investigated. The aim of this work is focused on the characterization of total power density at electrically isolated substrate for various duty cycles and for different pulsing frequencies. A calorimeter probe with incorporated thermocouple junction was used for measurement of the total power density on the substrate. The main results from the calorimeter probe show clearly that total power density at the substrate is much higher for the planar magnetron operating at low duty cycle and high pulsing frequency than for the plasma jet. The total power density measured in the planar magnetron depends more strongly on the duty cycle and the pulsing frequency than measured in the plasma jet.

Physical properties of homogeneous TiO2 films prepared by high power impulse magnetron sputtering as a function of crystallographic phase and nanostructure

Optical, photo-electrochemical, crystallographic and morphological properties of TiO2 thin films prepared by high power impulse magnetron sputtering at low substrate temperatures (<65 °C) without post-deposition thermal annealing are studied. The film composition—anatase, rutile or amorphous TiO2—is adjusted by the pressure (p ∼ 0.75–15 Pa) in the deposition chamber. The different crystallographic phases were determined with grazing incidence x-ray diffractometry. The surface morphology and size of TiO2 grains/clusters were imaged with atomic force microscopy. Basic plasma parameters were determined by means of the time-resolved Langmuir probe technique. The power density influx on the substrate was estimated from calorimetric probe measurement. The data from calorimetric probe measurements and time-resolved Langmuir probe served as input parameters for the calculation of influx contributions of particular species. The band-gap energy Eg depends on the film composition and crystallographic phase. Optical parameters (refractive index n + ik, transmittance T, reflectance R and absorbance A) are measured as functions of photon energy in the UV–Vis range by spectroscopic ellipsometry. For the rutile and anatase films agreement with the respective bulk phase is found. Incident photon-current conversion efficiency determined by photo-electrochemical measurements reached the highest values (0.312) for the anatase film.

Measurement of total energy flux density at a substrate during TiOx thin film deposition by using a plasma jet system

The total energy flux density delivered to an electrically isolated substrate in a low-pressure pulsed DC hollow cathode plasma jet sputtering system during TiO2 thin film deposition has been quantified. The plasma source was operated in constant average current mode and in a mixture of argon and oxygen or only in pure argon working gas. A titanium nozzle served as the hollow cathode. The total energy flux density measurements were made using a planar calorimeter probe. The main results from the calorimeter probe showed clearly that the total energy flux density at the electrically isolated substrate decreases significantly with duty cycle from 100% (DC mode) to 10% at a given pulsing frequency 2.5kHz. A local maximum at duty cycle 60% for only pure argon operation has been observed. In addition, the voltage waveforms on the hollow cathode and before the ballast resistor have been saved for pulsed DC measurements for both pure argon and argon+oxygen mixture. A similar transient phenomenon on the cathode voltage and discharge current as observed recently in mid-frequency pulsed DC magnetron discharge has been discovered in the hollow cathode plasma jet sputtering system. We can conclude from these preliminary measurements that the main asset of the pulsed DC hollow cathode plasma jet discharge as distinct from the DC driving of the same plasma system lies in the possibility to reduce or to increase energy influx on the floating substrate within the change of duty cycle.

Measurement of energy transfer at an isolated substrate in a pulsed dc magnetron discharge

The power density delivered by particles to an electrically isolated substrate in an asymmetric bipolar pulsed dc unbalanced magnetron has been quantified. The plasma source was operated in argon with a titanium target, and measurements were made using both a calorimeter probe and time-resolved Langmuir probe incorporated into a specially made substrate holder. The main results from the calorimeter probe show clearly that with increased pulse frequency (from dc to 350kHz) and reduced duty cycle (90%–50%), the particle power density (from ions, electrons, sputtered Ti, and backscattered Ar) at the substrate increases significantly. For instance, at 350kHz and 60% duty cycle, the total power density is 83mW∕cm2, about 60% higher than in dc mode for the same time-average discharge power. However, from an inventory of the individual particle contributions to the total power density derived from time-resolved Langmuir measurements and a simple model of the substrate sheath and plasma internal processes, we predict values of power density much lower than those measured. The measured and calculated values are in close agreement for the results obtained in dc mode but diverge at high frequencies. It is believed that this is due to the Langmuir probe measurements being unable to observe the presence of high-energy ions, created during the transient peaks in the electron temperature at the transitions from on off and off on [J. W. Bradley et al., Plasma Sources Sci. Technol. 11, 165 (2002)] which subsequently bombard the substrate. This paper shows conclusively the benefit of pulsing the magnetron over and above dc operation for enhancing the ion power per depositing neutral in the ion assisted deposition process.

End-pointing chamber clean by calorimetric probing of process effluent

The semiconductor industry employs gas-phase cleaning widely to remove materials deposited on the chamber walls during thin-film deposition processes. Chamber clean end-pointing—i.e., terminating the process when the chamber is clean—is desirable to manage cost of ownership and environmental impact. Existing end-pointing methods tend to rely on changes of plasma characteristics as the in situ plasma removes the deposit in time. Chamber clean technology is moving towards remote generation of plasma species for cleaning. In this arrangement, the chamber is located downstream from the plasma source. Because the etching reaction occurs ex situ, there are no relevant changes occurring in the plasma characteristics, and the effectiveness of many existing methods decreases. We report the development of a calorimetric probe for chamber clean end-pointing. The probe has an all solid-state construction and is engineered to be immersed in the process effluent during end-pointing operation. When the probe is operated at constant temperature, the probe power is closely related to the energy flux carried by the effluent, which in turn correlates with chamber conditioning. We examine probe response to NF3 plasma and etching of silicon specimens in a laboratory setting, and demonstrate successful end-pointing for both in situ and remote chamber clean on production tools. The probe results compare favorably with other coinstalled end-pointing solutions.

Application of a downstream calorimetric probe to reactive plasma

We explore the application of calorimetric probe located downstream from the plasma zone. By positioning the probe in a downstream location, the probe signal can be correlated with the integral energy flux carried by the effluent species as they propagate downstream from the plasma zone. Because the integral energy flux channeled downstream depends on the plasma conditions, it is possible to infer the plasma conditions from the probe response even though the probe is located away from the plasma zone. A calorimetric probe, based on resistance-temperature detector principle, adequate for fluorine plasma exposure is constructed. Probe operation is demonstrated for NF3–Ar plasmas.

Calorimeter probe for the DIII-D divertor

Heat flux measurements of the DIII-D divertor plate have been obtained with 6 mm spatial resolution using a calorimeter probe. These measurements complement the infrared camera system normally used for heat flux measurements on DIII-D but at higher-spatial resolution. The calorimeter probe is inserted into the tokamak from below to a position which is flush with the lower divertor plate tiles using the divertor materials experimental station (DiMES). The DiMES mechanism allows for retraction of the probe behind a gate valve and removal from the tokamak for modification or calibration. A 6 mm diameter insulated graphite cylinder for collecting energy is mounted within a standard DiMES sample. A 0.8 mm diameter thermocouple, installed 4 mm below the surface, provides a measurement of the temperature during and after the plasma exposure. The 80 ms time constant for the measurement is fast enough to determine heat flux changes during the 5 s plasma discharge and heat flux profiles have been obtained using both fixed strike points and slow strike point sweeps across the calorimeter. Special electronics and isolation is necessary as the sample is in direct electrical contact with the plasma. The calorimeter observes approximately 100 °C temperature rise over one tokamak discharge. The thermocouple signals are typically less than 1 mV and must be amplified near the vacuum feedthrough, passed through a low-pass filter to eliminate magnetic pickup, isolated, and sent to the data acquisition system approximately 8 m away. Initial measurements are included.

Characteristics of heat flux of [formula omitted] gas-arc discharge for the production of fullerenes

Heat flow of J× B gas-arc discharge in helium gas is investigated because this discharge was developed for the efficient production of fullerenes, in which the control of the hot-gas reaction is an important issue. Heat flux around the arc is measured as functions of position, magnetic field and discharge parameters by using a water-cooled calorimetric probe. As a result, heat flux from the arc is found to concentrate in the J× B direction, and it is strongly increased by the magnetic field. In addition, heat flux increases with increasing gas pressure and gap distance.

Diagnosis of Asymmetric Thermal Plasma Jet Using Computer Tomography Technique

In order to clarify the feasibility of application of the computer tomography technique for the diagnosis of an asymmetrical plasma jet, the heat transfer flux of the jet is measured using a water-cooled calorimetric probe. The asymmetric plasma jet is generated by injecting the feed gas into the jet perpendicular to the jet axis. The algebraic reconstruction technique (ART) is used as the reconstruction method. With increasing the feed-gas flow rate, the asymmetry of the heat transfer flux profile of the jet is found to increase, by means of the computer tomography technique.

An AC calorimeter probe for a closed-cycle cryogenic station

The AC temperature technique is used to develop a calorimeter probe for measuring the specific heat of small samples. A commercial closed-cycle refrigerator operating in the 8 - 300 K temperature range was used as a cryogenic station. A theory of operation is presented and the experimental set-up is discussed. The calorimeter has been used to study the nature of the magnetic and order - disorder phase transitions of several materials. Specific heat data taken from a sample of chromel alloy (90% Ni - 10% Cr) and a single-crystal sample of show that the sensitivity of the apparatus to relative variations in the specific heat is about 0.1%.

Studies on plasma-surface interaction in the stellarator W7-AS

The analysis of long term collector samples has given information on wall erosion by ion sputtering in W7-AS. There is strong indication that this erosion occurred during high - t-discharges which are separatrix-dominated by 5/n island chains. Langmuir and calorimeter probe measurements have demonstrated that in this case particle and heat fluxes leave the plasma locally with high densities. Erosion of limiter surfaces measured post-mortem can be satisfactorily correlated with results from measurements with flush mounted limiter probes. Measurements with a moveable collector probe have shown that the plasma impurity contamination by limiter material is higher after wall boronization in limiter-dominated discharges. This is explained by less contamination of the limiter surface after removal of the boronization layer from the limiter surface. On the basis of a simple 1D-impurity transport model a comparison of Langmuir and collector probe measurements with spectroscopic measurements in the core plasma was made, supporting this hypothesis.

Modular enthalpy probe and gas analyzer for thermal plasma measurements

The enthalpy or calorimetric probe is a water-cooled stagnation/sampling probe for studying the temperature, velocity, and composition of hot-gas flow fields. In order to derive the thermodynamic properties of complex flow fields such as plasma arc jets or high-velocity oxygen fuel jets, the specie concentration must be known accurately. To this end a differentially pumped quadrapole mass spectrometer has been integrated with a fully automated enthalpy probe system. An inexpensive modular probe is described along with the system and its theory of operation. Calibration and error are also discussed. Typical results are presented for the system operating in an argon/helium plasma arc jet in atmospheric pressure air. The maximum temperature measured is 13434 K on the center line of the jet, 5 mm from the exit, with a corresponding velocity of 1295 m/s. The utility in integrating the mass spectrometer to the enthalpy probe system is not only an accurate measurement of the gas mixture components for obtaining the correct property information, but also valuable information can be obtained about demixing diffusion and chemical reaction taking place in the plasma plume. The relative amount of argon to helium is shown to deviate from the nominal mixture by as much as 40% at the center of the plume.

Heat flux measurements in the ASDEX edge plasma

A calorimeter probe used in combination with a Langmuir double probe for studies of the plasma boundary layer of ASDEX is described. The scope and the limitations of the device are discussed.The combined probe is designed for measuring the energy flux parallel to the magnetic field, on the ion and the electron drift side simultaneously. In addition the electron temperature and the density can be determined. First experimental results from discharges employing different heating methods, i.e. OH, LHRH and NBI are presented.Besides yielding decay lengths for P, Te and n, the asymmetry of the energy flux during neutral beam injection indicates a toroidal plasma rotation in the case of tangential neutral beam injection.

Factors affecting heat transfer between gas-fluidized beds and immersed surfaces

Heat transfer coefficients were measured between gas-fluidized beds and spherical calorimetric probes of different sizes and to a water-cooled horizontal tube. The bed materials used were alumina and sand of narrow size ranges. Operating temperatures extended up to 980 °C and some estimate of the magnitude of the radiant heat transfer component was obtained by comparing the results for surfaces of oxidized copper (high emissivity) and polished fine gold (low emissivity). Tests with small spherical calorimetric probes immersed in beds of large mean particle diameter showed that there is an effect of relative heat capacity leading to enhanced bed-to-surface heat transfer coefficients if the heat capacity of the ‘heat transfer surface’ is less than an order of magnitude greater than that of the bed particles. Relative bed/surface temperature affects the heat transfer coefficient in two ways: through the temperature influence on gas thermal conductivity affecting the particle convective component of the coefficient, and through the rate of cooling of material directly adjacent to the transfer surface affecting the radiant component of heat transfer. Bed-to-tube coefficients are lower than those to a small calorimetric probe because of the sustained low temperature of a coolant tube and the obstruction that it presents to the circulation of the fluidized solids. The predictive capabilities of the published correlations are poor over the range of operating temperatures tested.

Beam properties of a high-perveance single-aperture dc extraction system

In continuous operation a positive hydrogen ion beam up to 240 mA has been extracted at 50 kV out of a single aperture, corresponding to a perveance of 2.1×10−8 A/V3/2. The effective mass of the extracted ion beam was measured to be about 1.8 amu. The current density distribution of the extracted ion beam was measured with a small calorimetric probe. It was found that the beam distribution strongly depends on the matching between the plasma current density and the extraction voltage. At overmatched conditions the ion beam may even become hollow. The distribution of the extracted ion beam depends on the pressure along the beam path. At pressures above 3×10−4 Torr H2, the distribution was observed to be Gaussian. At a pressure 2×10−5 Torr H2, the intensity in the wings of the beam is relatively high with respect to the intensity in the beam center. The beam divergence decreases with the square root of the extraction voltage. The space-charge potential of the neutralized beam is estimated to be about 5.6 V at 2×10−5 Torr decreasing to 1.6 V at 10−3 Torr independent of the beam energy.

Heating of solid surfaces by an electric arc

We have developed a calorimetric probe for measuring unsteady heat fluxes with a resolving time Δt < 10− sec. We have determined the flux to solid surfaces from an electric arc stabilized by a rotating cylinder.

Distribution of the heat flux to the calorimetric probe in a multiarc plasma reactor

The article presents the distribution of the heat flux to the calorimetric probe in a multiarc plasma reactor.

Particle and energy transport in the plasma scrape-off zone and its impact on limiter design

The design of limiters for fusion devices depends critically on the transport properties of the plasma edge. Recent data from Langmuir probe experiments and calorimeter probe experiments on the poloidal divertor experiment (PDX) have provided information about the transport of particles and energy in the edge region of a plasma. These data have been used as input to a simple one dimensional model in which the particle and thermal diffusion coefficients are permitted to be functions of the density and temperature. The transport coefficients deduced from the data are then applied to the cases of the TFTR device and a device like FED/INTOR to predict the optimum design for limiters or pump limiters. The results indicate the optimum limiter should have a concave shape when viewed from the plasma. The effect of the uncertainties in the deduced coefficients on the optimum design performance are also discussed.

Calorimeter probe studies of PDX and PLT

Using a combined electrostatic-calorimeter probe, a comprehensive survey of energy flux in the edge plasmas of low field ohmic and beam-heated PDX and PLT discharges has been made. Ions are shown to carry the majority of the power to floating probes. The intercepted energy is found to increase nearly linearly with applied heating power. The scrape-off distances, ranging from 0.5 cm for a 4 null diverted discharge to 2.5 cm for inside dee diverted discharge, are independent of heating power. Direct evidence is presented for the prompt loss to the walls and limiters in PDX of energetic beam ions.