Raj Singh

Cyclotron resonance heating systems for SST-1

RF systems in the ion cyclotron resonance frequency (ICRF) range and electron cyclotron resonance frequency (ECRF) range are in an advanced stage of commissioning, to carry out pre-ionization, breakdown, heating and current drive experiments on the steady-state superconducting tokamak SST-1. Initially the 1.5 MW continuous wave ICRF system would be used to heat the SST-1 plasma to 1.0 keV during a pulse length of 1000 s. For different heating scenarios at 1.5 and 3.0 T, a wide band of operating frequencies (20–92 MHz) is required. To meet this requirement two CW 1.5 MW rf generators are being developed in-house. A pressurized as well as vacuum transmission line and launcher for the SST-1–ICRF system has been commissioned and tested successfully. A gyrotron for the 82.6 GHz ECRF system has been tested for a 200 kW/1000 s operation on a water dummy load with 17% duty cycle. High power tests of the transmission line have been carried out and the burn pattern at the exit of transmission line shows a gaussian nature. Launchers used to focus and steer the microwave beam in plasma volume are characterized by a low power microwave source and tested for UHV compatibility. Long pulse operation has been made feasible by actively cooling both the systems. In this paper detailed test results and the present status of both the systems are reported.

Effect of Industrial Emission on Atmospheric Wet Deposition

In order to quantify the role of industries on atmospheric depositions, wet depositions were collected at different sites in Singrauli area of Sonbhadra district, U.P., India. Rainwater samples were collected on event basis and were analyzed for pH and concentration of H+ ion, cations and anions. The monthly variations in rainwater pH clearly indicate that early rainfall is more alkaline and as rainy season proceeds rainwater showed lower pH. The pH of rain from June to September varied from 6.6 to 4.8 at most polluted locality. Among different cations, Ca2+ showed maximum concentration followed by NH4+,Na+, Mg2+ and then K+, whereas among anions sulphate concentration was maximum followed by nitrate and then chloride.A significant positive correlation observed between sulphate and accumulated rainfall suggests that the source of sulphate is mainly the industrial activities. While the negative correlations of other cations with accumulated rain suggest their additional sources in the area. The study clearly indicates that the abundance of alkaline substances reduces the acidity of wet depositions.

Momentum mapping spectrometer for probing the fragmentation dynamics of molecules induced by keV electrons

We describe a new experimental setup for studying the fragmentation dynamics of molecules induced by the impact of keV electrons using the well-known technique of recoil ion momentum spectroscopy. The apparatus consists of mainly a time- and position-sensitive multi-hit particle detector for ion analysis and a channel electron multiplier detector for detecting the ejected electrons. Different components of the setup and the relevant electronics for data acquisition are described in detail with their working principles. In order to verify the reliable performance of the setup, we have recorded the collision-induced ionic spectra of the CO2 molecule by the impact of keV electrons. Information about the ion pairs of CO+:O+, C+:O+ and O+:O+ resulting from dissociative ionizing collisions of 20 and 26 keV electrons with a dilute gaseous target of CO2 molecules has been obtained. Under conditions of the present experiment, the momentum resolutions of the spectrometer for the combined momenta of CO+ and O+ ions in the direction of the time-of-flight axis and perpendicular to the direction of an electron beam are found to be 10.0 ± 0.2 and 15.0 ± 0.3 au, respectively.

The emission of characteristic and non-characteristic x-rays from collisions of 10–22 keV electrons with argon

Measurements of the emission cross sections of the characteristic and the non-characteristic x-rays produced from collisions of 10–22 keV electrons with Ar atoms are presented. The relative cross sections for Ar K-shell ionization as a function of impact energy are shown to yield satisfactory agreement (within the experimental uncertainty) with Born–Ochkur calculations and with results obtained from an empirical formula proposed by Hombourger (1998 J. Phys. B: At. Mol. Opt. Phys. 31 3693). Furthermore, the double differential cross sections of bremsstrahlung photons emitted from 10–22 keV e−–Ar collisions as a function of impact energy for different photon energies were measured. The shape of the experimental bremsstrahlung cross section function is found to exhibit good agreement with the theoretical calculations of Kissel et al (1983 At. Data Nucl. Data Tables 28 381) within the experimental uncertainty. In addition, the shape of the angular distributions of the bremsstrahlung is in accord with the calculations of Kissel et al and those of the modified Sommerfeld (Kulenkampff et al 1959 Z. Phys. 157 275) formula.

Kinematics and dissociation dynamics of a water molecule under the impact of 10 keV electrons

The kinematics and dissociation dynamics of a H2O molecule induced by 10 keV electrons are studied using a time-of-flight mass spectrometer in conjunction with a position-sensitive detector in multi-hit coincidence mode. Five dissociative channels arising from the complete as well as the incomplete Coulomb explosions of H2Oq+ (q = 2, 3) ions are observed and identified. The dissociation mechanisms (concerted and/or sequential) for these channels are examined. Further, the angular correlation of different fragment ions and the geometrical structure of the precursor ion are studied. The kinetic energy release distributions for the observed channels are also determined. It is found that the pure Coulomb explosion model is insufficient to explain the observed kinetic release distributions. The mean kinetic energy release for these channels is compared with the available data reported by earlier workers who have employed different charged projectiles and sources of photons.

Direct electron heating experiment on the Aditya tokamak using fast waves in the ion cyclotron resonance frequency range

Second harmonic heating experiments using fast waves are carried out on the Aditya tokamak in the ion cyclotron resonance frequency (ICRF) range with the help of a 200?kW, 20?40?MHz RF heating system, which is developed indigenously. Significant direct electron heating is observed in a hydrogen plasma. The rise in electron temperature is prompt with the application of RF power and the increment in electron temperature increases linearly with RF power. A corresponding increase in plasma beta and hence an increase in stored diamagnetic energy are also observed in the presence of RF power. The low-Z impurity radiation and electron density do not increase significantly with RF power. The direct electron heating by fast wave in Aditya is also predicted by the ion cyclotron resonance heating code TORIC.

Ion cyclotron resonance heating system on Aditya

An ion cyclotron resonance heating (ICRH) system has been designed, fabricated indigenously and commissioned on Tokamak Aditya. The system has been commissioned to operate between 20.0 and 47.0 MHz at a maximum power of 200 kW continuous wave (CW). Duration of 500 ms is sufficient for operation on Aditya, however, the same system feeds the final stage of the 1.5 MW ICRH system being prepared for the steady-state superconducting tokamak (SST-1) for a duration of 1000 s. Radio frequency (RF) power (225 kW) has been generated and successfully tested on a dummy load for 100 s at 30.0 MHz. Lower powers have been coupled to Aditya in a breakdown experiment. We describe the system in detail in this work.

Differential partial ionization cross sections for 10–24 keV electrons colliding with helium and neon atoms

The relative differential partial ionization cross sections of helium and neon atoms for 10–24 keV electron impact have been measured by using an ejected electron–ion coincidence technique. Measurements were accomplished by employing a time-of-flight mass spectrometer for charge analysis of the ions and a channel electron multiplier working in a pulse-counting mode for detecting the slow electrons of non-discriminated energies ejected at 90° to the incident electron beam direction. The charge state fractions Fn of ions with charge states n are found to be nearly invariant with the impact energy. The present results show that the multiply charged ions of the target atoms are produced by different mechanisms, namely by a direct multiple ionization or by inner shell ionization followed by non-radiative transitions, for instance, by an Auger process combined with shake-off transitions. A good agreement is obtained between our experimental results and the calculations based on inner shell ionization cross sections folded with charge state abundances, which result from the decay of the corresponding initial inner shell vacancies.

Study of K-line radiation of thick titanium produced in collisions of keV electrons.

The characteristic K-line yields Y(E₀) of a pure thick titanium (Z=22) element target are measured for 8-18 keV electron impact and compared with the simulation calculations using PENELOPE code. A fair agreement between experiment and simulation results is found within the existing experimental uncertainty of measurements. The ratio F of indirectly produced characteristic Ti K X-ray yield to its total (directly+indirectly) yield is determined by employing an approximate analytical formulation of Hanson and Cowan (Hanson, H.P., Cowan, D.J., 1961. Phys. Rev. 124, 22-26). It is found that F changes strongly with impact energy E₀ for normal angle of incidence in contrast to a mild change predicted by the simulation calculations. Furthermore, experimental and simulation results for peak to effective continuum ratio R of Ti K-line are compared and discussed for the energy range of impact of the present investigation.

Conditioning technique for high power RF vacuum transmission line components using multipactor plasma

Multipactor is a low power, electron multiplication based resonance breakdown phenomenon in vacuum often observed in radio frequency (RF) and microwave systems. A multipactor discharge is often undesirable as it can create a reactive component that detunes the resonant cavities, generates noise in communication system and induces gas desorption from the conductor surfaces. Multipactor breakdown on dielectric surface is also a major concern for failure of vacuum window in klystrons, cyclotrons and accelerators. Despite of these, the multipactor discharge is not absolutely undesirable. Its usefulness is being explored in electron gun technology, plasma display technology, ICRH antenna conditioning etc. Since multipactor is a pure electron resonance phenomenon, it can happen without any gas being present in the system. Nevertheless, the massive electron bombardment on conductor surfaces removes substantial amount of adsorbed gas species and increases the neutral pressure. In the presence of neutrals these resonant electrons ionize the gaseous atoms and forms the plasma known as multipactor plasma. Many of the high power RF components used in the vacuum transmission line interface sections of ICRH system are having restricted access to surface condition them especially the inner conductors of transmission line sections and dielectric material of vacuum window. By suitably choosing the operating frequency and a minimum pressure, it is possible to form a multipactor plasma in the above components to condition them. Such a conditioning technique is adopted for conditioning of vacuum window and vacuum transmission line of ICRH system. In this paper, an overview of multipactor plasma, a brief description of the test set up, testing conditions and conditioning results are presented.