• K C Mittal

Articles written in Pramana – Journal of Physics

• RF properties of 700 MHz, $\beta = 0.42$ elliptical cavity for high current proton acceleration

BARC is developing a technology for the accelerator-driven subcritical system (ADSS) that will be mainly utilized for the transmutation of nuclear waste and enrichment of U233. Design and development of superconducting medium velocity cavity has been taken up as a part of the accelerator-driven subcritical system project. We have studied RF properties of 700 MHz, $\beta = 0.42$ single cell elliptical cavity for possible use in high current proton acceleration. The cavity shape optimization studies have been done using SUPERFISH code. A calculation has been done to find out the velocity range over which this cavity can accelerate protons efficiently and to select the number of cells/cavity. The cavity's peak electric and magnetic fields, power dissipation $P_{c}$, quality factor 𝑄 and effective shunt impedance $ZT^{2}$ were calculated for various cavity dimensions using these codes. Based on these analyses a list of design parameters for the inner cell of the cavity has been suggested for possible use in high current proton accelerator.

• Measurement of high-power microwave pulse under intense electromagnetic noise

KALI-1000 pulse power system has been used to generate single pulse nanosecond duration high-power microwaves (HPM) from a virtual cathode oscillator (VIRCATOR) device. HPM power measurements were carried out using a transmitting–receiving system in the presence of intense high frequency (a few MHz) electromagnetic noise. Initially, the diode detector output signal could not be recorded due to the high noise level persisting in the ambiance. It was found that the HPM pulse can be successfully detected using wide band antenna, RF cable and diode detector set-up in the presence of significant electromagnetic noise. Estimated microwave peak power was $\sim 59.8$ dBm ($\sim 1$ kW) at 7 m distance from the VIRCATOR window. Peak amplitude of the HPM signal varies on shot-to-shot basis. Duration of the HPM pulse (FWHM) also varies from 52 ns to 94 ns for different shots.

• Plasma-ﬁlled rippled wall rectangular backward wave oscillator driven by sheet electron beam

Performance of the backward wave oscillator (BWO) is greatly enhanced with the introduction of plasma. Linear theory of the dispersion relation and the growth rate have been derived and analysed numerically for plasma-ﬁlled rippled wall rectangular waveguide driven by sheet electron beam. To see the effect of plasma on the TM01 cold wave structure mode and on the generated frequency, the parameters used are: relativistic factor $\gamma = 1.5$ (i.e. $v/c = 0.741$), average waveguide height $y_0 = 1.445$ cm, axial corrugation period $z_0 = 1.67$ cm, and corrugation amplitude $\epsilon = 0.225$ cm. The plasma density is varied from zero to $2\times 10^{12}$ cm-3. The presence of plasma tends to raise the TM01 mode cut-off frequency (14 GH$_z$ at $2 \times 10^{12}$ cm-3 plasma density) relative to the vacuum cut-off frequency (5 GH$_z$) which also causes a decrease in the group velocity everywhere, resulting in a ﬂattening of the dispersion relation. With the introduction of plasma, an enhancement in absolute instability was observed.

• Thermal conductivity of large-grain niobium and its effect on trapped vortices in the temperature range 1.8–5 K

Experimental investigation of the thermal conductivity of large grain and its dependence on the trapped vortices in parallel magnetic ﬁeld with respect to the temperature gradient $\nabla T$ was carried out on four large-grain niobium samples from four different ingots. The zero-ﬁeld thermal conductivity measurements are in good agreement with the measurements based on the theory of Bardeen–Rickayzen–Tewordt (BRT). The change in thermal conductivity with trapped vortices is analysed with the ﬁeld dependence of the conductivity results of Vinen et al for low inductions and low-temperature situation. Finally, the dependence of thermal conductivity on the applied magnetic ﬁeld in the vicinity of the upper critical ﬁeld $H_{c2}$ is ﬁtted with the theory of pure type-II superconductor of Houghton and Maki. Initial remnant magnetization in the sample shows a departure from the Houghton–Maki curve whereas the sample with zero trapped ﬂux qualitatively agrees with the theory. A qualitative discussion is presented explaining the reason for such deviation from the theory. It has also been observed that if the sample with the trapped vortices is cycled through $T_c$, the subsequent measurement of the thermal conductivity coincides with the zero trapped ﬂux results.

• Planar electron beams in a wiggler magnet array

Transport of high current (∼kA range with particle energy $\sim 1$ MeV) planar electron beams is a topic of increasing interest for applications in high-power (1–10 GW) and high-frequency (10–20 GHz) microwave devices such as backward wave oscillator (BWO), klystrons, gyro-BWOs, etc. In this paper, we give a simulated result for transport of electron beams with velocity $V_{b} = 5.23 \times 10^{8}$ cm s-1 , relativistic factor $\gamma = 1.16$, and beam voltage = ∼80 kV in notched wiggler magnet array. The calculation includes self-consistent effects of beam-generated fields. Our results show that the notched wiggler configuration with ∼6.97 kG magnetic field strength can provide vertical and horizontal confinements for a sheet electron beam with 0.3 cm thickness and 2 cm width. The feasibility calculation addresses to a system expected to drive for 13–20 GHz BWO with rippled waveguide parameters as width $w = 3.0$ cm, thickness $t = 1.0$ cm, corrugation depth $h = 0.225$ cm, and spatial periodicity $d = 1.67$ cm.

• # Pramana – Journal of Physics

Volume 94, 2019
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019