Boronization on NSTX using deuterated trimethylboron

Blanchard, W.; Gernhardt, Robert; Kugel, H.; LaMarche, P. H.

doi:10.1109/fusion.2002.1027681

Cited by 4 publications

(3 citation statements)

References 9 publications

(15 reference statements)

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“…Likewise researchers at Los Alamos National Laboratory exposed a B powder compaction to Mg vapour[42]. This same approach could be applied to the formation of a MgB 2 film on the surface of an RF cavity previously coated with B using established CVD technology[43,45]. In the field of tokomak-fusion, plasma-vessel walls of stainless steel or Mo are coated with B in order to getter oxygen or other impurities.…”

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“…The B is deposited on the vessel wall by decomposition of various Bcontaining gaseous mixtures under the action of conventional glow discharge. Gaseous mixtures used in this CVD approach are: (i) D 2 + He + vaporized decaborane (B 10 H 14 )[43], (ii) He + borane (H 2 B 6 )[30], or (iii) D 2 + He + trimethylboron (TMB)[45]. One or other of these techniques could be employed to deposit the initial B coating, after which the heated cavity could be exposed to Mg vapour in order to form the final MgB 2 layer.…”

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Magnesium diboride superconducting RF resonant cavities for high energy particle acceleration

Collings

Sumption

Tajima

2004

Supercond. Sci. Technol.

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Arguments in support of any particular superconducting coating must be framed in terms of its fundamental thermodynamic properties. The superconducting transition temperature, Tc, determines the surface resistance, and thus the Q of the cavity. This must remain sufficiently high that the system can be driven at the required field gradients and frequencies without leading to excessive power loss. In this regard the 39 K Tc of MgB2 is advantageous. With an anticipated maximum accelerating field, EaccMAX, of 77 MV m−1 and a BCS surface resistance, RsBCS (4 K, 500 MHz), of 2.5 n Ω as discussed later, MgB2 represents an interesting possibility as a coating for SRF cavities. In addition, the higher Hc2 of MgB2 than Nb results in a slightly lower estimated trapped flux sensitivity. Recent measurements of an MgB2 film at the Los Alamos National Laboratory (LANL) have shown an RF surface resistance lower than that of Nb at 4 K, which is proof-of-principle evidence of the attractiveness of MgB2. Our calculations are based conservatively on 4 K operation at 500 MHz. However, with a Tc of 39 K, MgB2-coated cavities should be less susceptible to thermal breakdown than low-Tc ones. Superconducting materials for use at GHz frequencies at voltage gradients >40 MV m−1, a recently cited target, will require both low Rs (high Tc) and high Hsh values. With a Tc of 39 K, MgB2 clearly has the potential to reduce RsBCS if the films are well prepared and free from defects and particles. Additionally, while the Hc1 for MgB2 is relatively low, the superheating critical field, Hsh, is higher than that of Nb. Currently, there is some debate about the exact roles of Hc1 and Hsh in the determination of Eacc limits. However, the higher values of Hsh for MgB2 do suggest the possibility of enhanced Eacc values. The exact roles of Hc1 and Hsh should be further investigated. Techniques exist that may enable cavity-like structures to be internally coated with a MgB2 film.

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Magnesium diboride superconducting RF resonant cavities for high energy particle acceleration

Collings

Sumption

Tajima

2004

Supercond. Sci. Technol.

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“…This system was used for Helium Glow Discharge Cleaning (HeGDC) between plasma discharges and for boronization using a mixture of 5% deuterated-trimethyl boron in 95% helium. Two DC-biased, 60 Hz AC heated filaments near each of these anodes provided preionization both to assist the HeGDC startup between discharges and to provide preionization for the startup of all NSTX plasmas [2][3][4][5][6]. These biased preionization filaments facilitate breakdown of the glow discharge at the actual operating pressure, voltage, and current, thereby greatly simplifying process control [1].…”

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Design and Performance of NSTX Movable GDC Probe

Kugel

Maingi

Bell

et al. 2007

2007 IEEE 22nd Symposium on Fusion Engineering

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Abstract--The NSTX GDC system has been improved by replacing one of the two fixed anodes with a Movable GDC Probe (MGP) anode that can be inserted 1.2 m to about midway between the inner and outer vessel walls. The purpose was to provide more spatially uniform HeGDC for improving discharge stability and reliability. The MGP has been used reliably between every discharge during the last two NSTX experimental campaigns. It has also been used to apply HeGDC assisted boronization, and more recently, HeGDC assisted lithiumization. The MGP has contributed to improved NSTX performance during long pulse and H-mode discharges, and enabled a faster discharge repetition rate.

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