Evolution Of Electron Temperature Research Articles

Downstream plasma transport and metal ionization in a high-powered pulsed-plasma magnetron

Downstream plasma transport and ionization processes in a high-powered pulsed-plasma magnetron were studied. The temporal evolution and spatial distribution of electron density (ne) and temperature (Te) were characterized with a 3D scanning triple Langmuir probe. Plasma expanded from the racetrack region into the downstream region, where a high ne peak was formed some time into the pulse-off period. The expansion speed and directionality towards the substrate increased with a stronger magnetic field (B), largely as a consequence of a larger potential drop in the bulk plasma region during a relatively slower sheath formation. The fraction of Cu ions in the deposition flux was measured on the substrate using a gridded energy analyzer. It increased with higher pulse voltage. With increased B field from 200 to 800 Gauss above racetrack, ne increased but the Cu ion fraction decreased from 42% to 16%. A comprehensive model was built, including the diffusion of as-sputtered Cu flux, the Cu ionization in the entire plasma region using the mapped ne and Te data, and ion extraction efficiency based on the measured plasma potential (Vp) distribution. The calculations matched the measurements and indicated the main causes of lower Cu ion fractions in stronger B fields to be the lower Te and inefficient ion extraction in a larger pre-sheath potential.

Temporary spectral analysis of a laser plasma of mineral coal

In this work we present results of the temporal spectral study of a plasma laser of mineral coal using the Laser-induced Breakdown Spectroscopy (LIBS) technique. The plasma was generated by focusing a laser beam of Nd:YAG laser emitting at 532 nm with energy per pulse of 35 mJ on coal target pellets. The plasma radiation was conducted by an optical fiber to the entrance slit of a spectrograph of 0.5 m, equipped with a 1200 and 2400 grooves/mm diffraction grating and an ICCD camera for registration with different delay times of the spectra in the spectral range from 250 nm to 900 nm. The temporal spectral analysis allowed the identification of the elements Al, Fe, Ca, Mg, K, and Si, and CN and C2 molecules present in natural coals. The characteristics of the spectral lines and bands were studied at different delay times obtaining the calculation of the evolution of electron temperature, electron density, and vibrational temperature of plasmas in the time. The delay times used were between 0.5 μs and 5 μs, calculating the electron temperature ranged between 5 000 K and 1 000 K.

Temperature and density evolution during decay in a 2.45 GHz hydrogen electron cyclotron resonance plasma: Off-resonant and resonant cases

Time resolved electron temperature and density measurements during the decay stage in a hydrogen electron cyclotron resonance (ECR) plasma are presented for a resonance and off-resonance magnetic field configurations. The measurements are conducted on a ECR plasma generator excited at 2.45 GHz denominated test-bench for ion-sources plasma studies at ESS Bilbao. The plasma parameters evolution is studied by Langmuir probe diagnostic with synchronized sample technique developed for repetitive pulsed plasmas with a temporal resolution of 200 ns in typical decay processes of about 40 μs. An afterglow transient is clearly observed in the reflected microwave power signal from the plasma. Simultaneously, the electron temperature evolution shows rebounding peaks that may be related to the interplay between density drop and microwave coupling with deep impact on the Electron Energy Distribution Function. The correlation of such structures with the plasma absorbed power and the coupling quality is also reported.

Investigation and optimization of the magnetic field configuration in high-power impulse magnetron sputtering

An effort to optimize the magnetic field configuration specifically for high-power impulse magnetron sputtering (HiPIMS) was made. Magnetic field configurations with different field strengths, race track widths and race track patterns were designed using COMSOL. Their influence on HiPIMS plasma properties was investigated using a 36 cm diameter copper target. The I–V discharge characteristics were measured. The temporal evolution of electron temperature (Te) and density (ne) was studied employing a triple Langmuir probe, which was also scanned in the whole discharge region to characterize the plasma distribution and transport. Based on the studies, a closed path for electrons to drift along was still essential in HiPIMS in order to efficiently confine electrons and achieve a high pulse current. Very dense plasmas (1019–1020 m−3) were generated in front of the race tracks during the pulse, and expanded downstream afterwards. As the magnetic field strength increased from 200 to 800 G, the expansion became faster and less isotropic, i.e. more directional toward the substrate. The electric potential distribution accounted for these effects. Varied race track widths and patterns altered the plasma distribution from the target to the substrate. A spiral-shaped magnetic field design was able to produce superior plasma uniformity on the substrate in addition to improved target utilization.

Effect of an electric field on air filament decay at the trail of an intense femtosecond laser pulse

Air plasma density decay in a filament produced by an intense femtosecond laser pulse in an external electric field was investigated experimentally and theoretically. It was demonstrated by means of the terahertz scattering technique that the rate of plasma decay decreases with increasing electric field. At the electric field of 7 kV/cm the lifetime of plasma with the density above 10(16) cm(-3) was prolonged from 0.5 ns to 1 ns. Numerical simulation of electron density decay and electron temperature evolution was performed, taking into consideration dissociative and three-body electron-ion recombination as well as formation of complex positive ions. The simulation showed that under the electric field the electron temperature evolves nonmonotonically and passes through a minimum due to varying contribution of electron-ion collisions to electron heating in the field. The rate of three-body electron recombination with O(2)(+) ions of 2×10(-19)(300/T(e))(9/2) cm(6)/s was found from the experimental measurements at electron temperatures in the 0.25-0.4 eV range and electron densities in the 10(15)-10(17) cm(-3) range.

Plasma Characterization in Divertor Simulation Experiments with a V-Shaped Target on GAMMA 10/PDX

The divertor simulation experimental module (Dmodule) has been installed in the west end region in GAMMA 10/PDX. By use of Langmuir probes and spectroscopic measurement of intensity ratios of He I lines, temporal evolution of electron temperature and that of electron density of the plasma in the D-module with the V-shaped tungsten target are obtained. When the additional ICRF heating is applied to the anchor cell, the electron temperature evaluated with He I intensity ratios decreases from ~60 eV to ~25 eV and that from the probe measurement decreases from ~27 eV to ~14 eV. The difference between both measurements seems to be attributed to the difference of their measurement positions. The electron density measured by the Langmuir probe increases 2.3 times due to the RF3 power but it is rather low (< 1017 m-3). The electron density at the end region is expected to be increased by enhancement of ICRF heating and additional gas puffing at the plug/barrier cell which is the upstream cell of the end region.

Experimental investigation of electron temperature dynamics of helical states in the RFX-Mod reversed field pinch

The study of electron temperature profiles of quasi-single helical states (QSH) in RFX-Mod (and, in general, in other RFP machines) was carried out in the past mainly using Thomson scattering, thus being limited to a time resolution of the order of some milliseconds and to the collection of single, isolated cases, which displayed clear transport barriers. A new multichord double filter SXR spectrometer was developed and installed. Temperature profiles are now measured up to a frequency of some kHz, allowing one to determine the entire time evolution of electron temperature during a QSH cycle. A mapping technique, originally developed for Thomson temperature profiles, is adapted to deal with the line-integrated nature of the SXR diagnostic. In particular, temperature gradients related to the presence of transport barriers are reconstructed and analysed in a helical reference system, coherently with the underlying plasma equilibrium. The method is discussed in its capabilities and limits and then applied to a wide QSH database. As a result, a clear difference in temperature gradient behaviour between the rising phase of QSH and the saturated (or flattop) phase is observed. In the rising phase the expected correlation of temperature gradient with magnetic spectrum is confirmed, showing a positive trend between dominant mode amplitude and thermal structure dimension, as well as a negative correlation of secondary mode amplitudes and temperature gradient. On the other hand, in the flattop phase the presence of a thermal structure is intermittent, and much less influenced by the magnetic dynamics. This suggests the simultaneous presence of different mechanisms that enhance energy transport.

An investigation of the temporal evolution of plasma potential in a 60 MHz/2 MHz pulsed dual-frequency capacitively coupled discharge

Using an electron-emitting probe, time-resolved plasma potential (Vp) measurements are carried out in a pulsed dual-frequency 60 MHz/2 MHz capacitively coupled discharge. The discharge is produced using argon gas at two chosen pressures of 20 and 40 mTorr, a duty ratio of 50% and pulse powers ranging from 100 to 500 W. The pulsing frequency is set at 1 kHz. The plasma potential measurements are carried out at 10 mm above the centre of the substrate.It is observed that Vp follows the source discharge pulse voltage and remains positive during the whole pulse cycle. Without substrate biasing, the prominent features observed in the Vp profile, under all the operating conditions, remain similar; however, the magnitude of Vp increases with the applied RF source power. For further analysis, three distinguishable features in the Vp profile, a transient spike at the beginning of the discharge pulse, a stable ‘on-phase’ and a ‘stable-off’ phase, are identified. For typical operating conditions (20 mTorr and 500 W), the transient spike in Vp of ∼30 V appears for 30 µs, then it attains a stable value of ∼12 V during the rest of the pulse on-period. Vp decreases up to ∼3 V as the pulse is switched off.It is also observed in this study that a continuous wave RF biasing of the substrate significantly modulates the plasma potential evolution, specifically when the pulse is switched off and the magnitude of modulation depends on the substrate biasing power. The temporal evolution of electron temperature derived from the plasma and floating potentials is also reported.

Model for Pedestal Transport Based on Suppression of Anomalous Transport Using ωE ×B Flow Shear and Magnetic Shear

A model for describing a pedestal transport in H -mode tokamak plasmas is developed and tested in BALDUR integrated predictive modelling code. The transport model is calculated based on the suppression of an anomalous core transport due to the ω E × B flow shear and magnetic shear. Because of the reduction of transport in the edge region, the pedestal can be formed and evolved. In this work, an anomalous transport is computed using a semi-empirical mixed Bohm/gyro-Bohm anomalous core transport model. The BALDUR code with both core and pedestal transport is used to simulate the time evolution of plasma current, ion and electron temperature, and particle and impurity density profiles for 10 DIII-D H -mode discharges in various plasma scenarios (i.e., gyro-radius scan, density scan, power scan, and elongation scan). It is found that the formation of the pedestal can be observed in all simulations, in which the L – H transition in the simulations is consistent with the L – H transition power threshold model. ...

Classical scaling and the correspondence between the coupled rate equation and molecular dynamics models for the evolution of ultracold neutral plasma

Coupled rate equation calculations demonstrate that the variational rate theory models for the three-body electron–electron–ion scattering dynamics associated with ultracold plasma evolution scale with density in accordance with the classical mechanics of a Coulomb system. Completeness of scaling requires a broad domain of Rydberg levels over which a quasi-equilibrium can be established. For given initial conditions, the predicted evolution of electron temperature with time changes with the choice of nmax, i.e. the highest Rydberg orbital populated by three-body recombination. But, if the criterion that defines nmax scales with density, then the results scale. Molecular dynamics calculations distinguish bound Rydberg states by an electron–ion distance criterion, Rmax, which is represented in scaled coordinates by Rmax = rmaxaws, where aws represents the Wigner–Seitz radius and rmax is chosen arbitrarily. The rate equation calculations best conform with the output of molecular dynamics simulations for values of nmax defined by a thermal criterion, . In a hydrogenic limit, the thermal criterion equates with a scaled distance criterion when rmax = Γe/2, where Γe = e2/4πϵ0awskBTe describes the degree of electron correlation. Molecular dynamics simulations conventionally attenuate the Coulomb potential by means of a term, C, . Reference to rate equation calculations suggests a connection between the soft-core term, C, and nmin, the lowest Rydberg level considered in the relaxation of the plasma to a quasi-steady state.

Electron temperature and density profile evolution during the edge-localized mode cycle in ohmic and electron cyclotron-heated H-mode plasmas in TCV

The dynamics of electron temperature and density profiles during quasi-stationary H-mode phases in the TCV tokamak has been investigated for type III and type I edge-localized modes (ELMs) using electron cyclotron heating (ECH) to vary the collisionality. At heating power levels close to the threshold for the L–H transition, ELMs of type III were observed, with an energy loss per ELM below 10% of the total plasma energy. Electron temperature and pressure showed no significant increase during the phase before the ELM crash. ELMs of type I were characterized by a lower repetition rate, but caused fractional energy losses reaching 20%. For this ELM type, a clear increase in pedestal pressure and pressure gradient was observed before the collapse associated with the ELM event. The rapid drop in electron temperature also affected the plasma core explaining the rather large energy losses per ELM. Ideal MHD stability calculations of the edge pedestal with the KINX code showed that high-n ballooning modes restricted the achievable pressure gradient at high collisionality and ELMs of type III. With additional heating and type I ELMs, stability limits were set by medium-n kink-ballooning modes. Comparing the values of the pressure gradients and current densities obtained from experimental data with those of ideal MHD stability calculations showed good agreement. The model, however, needs to account for the radial displacement of the location of maximum pressure gradient during the ELM cycle.

Optimizing efficiency of Ti ionized deposition in HIPIMS

The evolution of plasma composition, electron temperature and deposition rates of high-power impulse magnetron sputtering (HIPIMS) of a titanium target have been studied by optical emission spectroscopy and deposition on a crystal oscillator, for pulse lengths of 50 µs and 100 µs over a range of voltages. The observations indicate a strong power dependence of the populations of Ti1+ ions with respect to the Ti neutrals. A clear saturation point for Ti1+ is seen, while the more highly ionized species increase, even at a constant electron temperature. The creation of the more highly ionized species follows the current, increasing throughout the pulse, and is attributed to ionization by collisions with energetic secondary electrons. Within the HIPIMS regime, deposition energy efficiency correlates positively with the emission from Ti1+ ions and negatively with emission from more highly ionized species.

Thermal analysis of intense femtosecond laser ablation of aluminum

This paper numerically simulates the process of ablation of an aluminum target by an intense femtosecond laser with a fluence of 40 J/cm2 based on the two-temperature equation, and obtains the evolution of the free electron temperature and lattice temperature over a large temporal and depth range, for the first time. By investigating the temporal evolution curves of the free electron temperature and lattice temperature at three representative depths of 0, 100 nm and 500 nm, it reveals different characteristics and mechanisms of the free electron temperature evolution at different depths. The results show that, in the intense femtosecond laser ablation of aluminum, the material ablation is mainly induced by the thermal conduction of free electrons, instead of the direct absorption of the laser energy; in addition, the thermal conduction of free electrons and the coupling effect between electrons and lattice will induce the temperature of free electrons deep inside the target to experience a process from increase to decrease and finally to increase again.

Effect of Radiation Power Loss due to Impurity Gas Puff on Divertor Plasma

In order to examine the effect of radiation power loss due to impurity gas puff in divertor plasma, we calculated time evolution of impurity ion densities and electron temperature for several cases with Ne gas puff rates 1012 and 1013 cm−3 s−1 during 0.01 s and initial electron temperature 10-300 eV for a plasma with electron density 1013 cm−3 with a one zone model. We found that the electron temperature decreases less than 1 eV for the cases with gas puff rate 1013 cm−3 s−1, independent of initial electron temperature and in this case we expect plasma detachment. Detailed conditions for plasma detachment should also depend on time history of radiation power.

Spectroscopic analysis of temperature and density of Sn plasma produced by a CO2 laser

The temporal and spatial evolution of electron temperature and electron density from Sn plasma produced by a CO2 laser has been investigated in vacuum using spectroscopic methods. The plasma parameters were inferred by the Boltzmann plot method from experimentally observed line profiles of singly ionized Sn and Stark broadened profiles. At a laser intensity of 1010 W/cm2, electron temperature and density were measured to be within 1.13 eV to 0.53 eV and 5.3×1016 cm−3 to 1.4×1016 cm−3, respectively, for delay times between 200 ns and 1100 ns, and at distances up to 5 mm along the target normal. The results show the electron temperature and density from Sn plasma produced by a CO2 laser with wavelength of 10.6 μm to be lower than previously reported results using a 1064 nm laser in a similar parameter regime. The lower temperature in the region far away from the target surface confirms the smaller interaction region for CO2 laser as compared with that of neodymium-doped yttrium aluminum garnet laser.

Harmonic generation upon mixing an s-polarised wave and a wave heating a metal

It is shown that when an s-polarised wave is mixed with a wave of the same frequency, which is incident along the normal to the metal surface, three frequency-doubled waves are generated. Two of them are p-polarised and one is s-polarised. The effect of the electron and lattice temperature evolution on the SHG efficiency is described quantitatively. An anomalously strong change in the generation efficiency of an s-polarised harmonic is found during temperature variations, which leads to a noticeable change in the electron collision frequency.

Advances of Thomson scattering diagnostic on the TEXTOR tokamak

A Thomson scattering (TS) diagnostic based on the laser multi-pass intra-cavity plasma probing has been developed and successfully implemented in the TEXTOR tokamak [1, 2]. The laser system operates in a burst mode delivering up to 45 laser pulses 50 J of probing energy each at 5 kHz frequency. The diagnostic has provided the measurements of fast evolution of electron temperature and density at accuracy of 2% and 1% correspondingly at a spatial resolution <1 cm along the whole plasma diameter during a 9 ms time interval. The developed TS diagnostic combines unique features: large amount of collected photons, a high repetition rate of the measurements, a high spatial resolution along the whole plasma diameter and a high spectral resolution with hundred spectral points. Some of new advances of the TS diagnostics and its possible applications on the TEXTOR tokamak are briefly discussed in the report.

Evolution of electron temperature in low pressure magnetized capacitive plasma

The evolution of electron temperature in a low pressure magnetized capacitive discharge was investigated under the collisionless electron heating regime. The results showed that while the electron temperature increases monotonously with the magnetic field in previous study [Turner et al., Phys. Rev. Lett. 76, 2069 (1996)], the electron temperature in our experiment exhibited nonmonotonic evolution behavior with the magnetic field. This nonmonotonic evolution of the electron temperature with the magnetic field was shown to be a combined effect of suppressing electron resonance heating and enhancing collisional heating while increasing the magnetic field.

Runaway electrons behaviors during ion cycolotron range of frequency and lower hybrid wave plasmas in the HT-7 Tokamak

HT-7 Tokamak is equipped with a lower hybrid wave (LHW) system and an ion cyclotron range of frequency (ICRF) system. ICRF can accelerate ions effectively, while LHW can accelerate electrons effectively. The generation of runaway electrons during the LHW and ICRF plasmas, as well as the time evolution of electron temperature during the ICRF and LHW plasmas was investigated in this paper. The runaway critical energy for runaway electrons was also calculated according to the experimental data. It was observed that the combination of ICRF and LHW can produce a higher heating efficiency and a better coupling between ICRF and plasmas if the power of LHW exceeds a critical value. Therefore, the generation of runaway electrons and fusion neutrons are affected by ICRF.

Initial Electron Bernstein Wave Emission Measurements on the TJ-II Stellarator

Thermal electron emission at 28 GHz has been measured on the TJ-II stellarator. The emission from neutral beam-heated overdense plasmas, where the plasma density is greater than the ordinary-mode (O-mode) cutoff density, is consistent with electron thermal emission from mode-converted electron Bernstein waves (EBWs) via the Bernstein wave to extraordinary mode to ordinary mode scenario (B-X-O). Emission from underdense plasmas without neutral beam injection is consistent with the measurement of oblique electron cyclotron emission. Electron Bernstein wave emission measurements are being made to determine the optimum launch angle for planned EBW heating experiments and also to provide an indication of electron temperature evolution in overdense plasmas on TJ-II.