A simple replacement of tungsten filament hot cathodes by DC heated LaB6 rod and its noise characteristics with laser-induced fluorescence

Abstract

Hot cathode discharges are common plasma sources for fundamental plasma physics studies and other applications due to their quiescent and relatively simple properties, and tungsten filaments are commonly used for the ease of heating them. Recently, tungsten filaments are increasingly being replaced by less luminous alternatives, such as barium oxide or lanthanum hexaboride. These materials can emit electrons at temperatures close to 1000 K lower than tungsten, greatly reducing their blackbody radiations. This results in significant improvement in signal recovery for active spectral diagnostic, such as laser-induced fluorescence. However, these less luminous cathodes often come in vastly more complicated designs than those of tungsten hot cathodes and are much more expensive to procure and difficult to operate. In this paper, we present a simple, low cost direct current heated design of a LaB6 cathode that is manufactured at suitable dimensions and make a comparison of the laser-induced fluorescence (LIF) signal-to-noise ratio of this LaB6 hot cathode discharge with that of a typical tungsten filament discharge, revealing that LaB6 has, indeed, an improved LIF signal-to-noise ratio compared with the tungsten filament.

Direct measurement of ion and electron flux ratio at their respective sheath-edges and absence of the electron Bohm criterion effects

“…In addition, the ratio of electron/ion flux density into their respective sheaths is the critical condition with which electron sheath can form near an electrode immersed in a plasma [7]. This makes this ratio a significant parameter in the design of asymmetric probes [10,11], satellite and spacecraft based Langmuir probes [12,13], and Langmuir probes in small devices [14][15][16]. In magnetized [17] and/or ion-neutral collisional [18] plasmas, the plasma loss to the wall is also restricted by magnetization and collisional effects, thus the 'unperturbed' electron/ion flux ratio becomes an important benchmark parameter before we can better investigate these phenomena.…”

Recent development of LIF diagnostics in ASIPP

“…With a feed forward functionality and a delicate control of an ECDL's operating environment, ECDLs with a tunable range ∼80 GHz has been previously reported [52]. Exploring the control limits of a Wavicle ECDL 668 nm Littrow cavity diode laser paired with a 668 TA tapered amplifier system reproduces similar frequency tuning range >100 GHz via tailoring the feed forward input [50]. The tuning range has been recorded with both an iodine spectrum and with an F-P etalon as shown in figure 3.…”

“…The use of the LaB 6 cathode instead of tungsten filaments, although being space charge limited [49], facilitates smaller blackbody radiation, longer cathode life, and higher plasma densities. At a discharge current currently limited to 0.6 A restricted by chamber heating issues, plasmas with densities ranging from 10 8 to 10 10 cm −3 and electron temperatures in the range of 1-5 eV can be produced at an operating pressure range spanning from 10 −2 Pa to 10 −1 Pa [50]. With recent advances in hot cathode discharge techniques [51] and improved vacuum chamber cooling, higher discharge currents and therefore higher plasma densities can be readily achieved.…”

Improving discharge performance of space-charge limited hot cathodes via local neutral collisionality

“…With recent development of LaB 6 manufacturing capabilities, a simple DC heated LaB 6 hot cathode has been developed to directly replace tungsten filaments [14,15]. These rod shaped LaB 6 hot cathodes can be manufactured as thin as 2 mm, which makes them compatible with DC heating simply using copper filament holders installed onto standard electrical vacuum feedthroughs, and can be heated by a low voltage (∼5V) and high current (∼100A) DC heating power supply.…”

“…These rod shaped LaB 6 hot cathodes can be manufactured as thin as 2 mm, which makes them compatible with DC heating simply using copper filament holders installed onto standard electrical vacuum feedthroughs, and can be heated by a low voltage (∼5V) and high current (∼100A) DC heating power supply. This LaB 6 cathode benefits from the much lower emission temperature and is found to produce much lower blackbody radiation than standard tungsten filaments and is shown to produce better signal-to-noise ratio even with weak active spectroscopies like laser induced fluorescence (LIF), and its particularly advantageous over tungsten filament cathodes for experiments with a higher density plasma [14]. However, compared to tungsten and tantalum wires, the larger dimensions of LaB 6 cathodes remains also much larger than the plasma Debye length along any direction, making them spacecharge limited (SCL) hot cathodes [12,16,17] which limited their emission characteristics, as opposed to temperature limited (TL) hot cathode from with which emission is expected to be limited by the cathode temperature.…”