We could only consider a few options for the heater source, because many alloys melt at these temperatures. Since we expect thermionic emission at 1000K and above, more commonly available options to us are tungsten (thoriated, iridium coated, or bare), neodymium (Rutgers' design) and nichrome (Niell's design).
With 2 filament leads fabricated from 1/10" aluminium rods housed in ceramic beads, a current of 2.5A and resistance of 20+ ohms should get up to heat in 20s~. (Specific heat capacity * mass of filament / power dissipated from Joule heating). This is assuming a constant resistance and no heat loss to the surroundings. This seemed rather slow, so increasing the current is necessary. But at the same time, I'm not sure if subjecting an overly large current to the filament is a good idea. I have no knowledge of melting filaments whatnot - although a discussion with my team mate yielded the possibility that the filament has a 'cut-off temperature', where the resistance gets overly large, and the current decreases proportionally. So I^2 * R heating is essentially held off.
That left me to consider how to put such a current through the filament, while maintaining the filament at negative bias. For convenience, I considered a series of batteries (recharging issue) and car battery, but learned that the power is too low (meaning the voltage is acceptably high, but the current it too low). The next in line for convenience is the power grid, but getting a negative bias out of an AC sounded troublesome to me. From what we knew, the best technique would be to step down the voltage with a transformer, then rectify with a breakdown diode, and use a capacitor to 'smoothen' the current supplied.
Simple, Fernando has a 30V, 12.5A DC power supply that can be plugged into 230Vac/50Hz mains. It doesn't take much thinking to figure out the variable resistor to use in line with the power supply to offer us some control over the power dissipated in the filament, and hence time taken for the filament to get up to heat.
The hydrogen source is giving me a headache. The Swagelok feedthrough seems like the most convenient. Swagelok only makes its tubings, and most of its fittings, in stainless steel, and 20 foot lengths! 6m of tubing is too much!
With 2 filament leads fabricated from 1/10" aluminium rods housed in ceramic beads, a current of 2.5A and resistance of 20+ ohms should get up to heat in 20s~. (Specific heat capacity * mass of filament / power dissipated from Joule heating). This is assuming a constant resistance and no heat loss to the surroundings. This seemed rather slow, so increasing the current is necessary. But at the same time, I'm not sure if subjecting an overly large current to the filament is a good idea. I have no knowledge of melting filaments whatnot - although a discussion with my team mate yielded the possibility that the filament has a 'cut-off temperature', where the resistance gets overly large, and the current decreases proportionally. So I^2 * R heating is essentially held off.
That left me to consider how to put such a current through the filament, while maintaining the filament at negative bias. For convenience, I considered a series of batteries (recharging issue) and car battery, but learned that the power is too low (meaning the voltage is acceptably high, but the current it too low). The next in line for convenience is the power grid, but getting a negative bias out of an AC sounded troublesome to me. From what we knew, the best technique would be to step down the voltage with a transformer, then rectify with a breakdown diode, and use a capacitor to 'smoothen' the current supplied.
Simple, Fernando has a 30V, 12.5A DC power supply that can be plugged into 230Vac/50Hz mains. It doesn't take much thinking to figure out the variable resistor to use in line with the power supply to offer us some control over the power dissipated in the filament, and hence time taken for the filament to get up to heat.
The hydrogen source is giving me a headache. The Swagelok feedthrough seems like the most convenient. Swagelok only makes its tubings, and most of its fittings, in stainless steel, and 20 foot lengths! 6m of tubing is too much!
The Swagelok fitting is very ingenious, IMO. I'd love to work with their engineers.First, fixing the tubing issue, I realized that standardised vacuum tubing has the same dimensions, does not leak, and is definitely able to withstand the same pressures. So, I can buy these in increments of a fractional foot.
Second, the stainless steel fittings would distort the magnetic field. So... I needed a certain length of copper or brass pipe - which do not have a standardised fit to Swagelok. Moreover, galvanic corrosion takes place significantly between stainless steel and copper/brass.
Left with no choice: I carried out an extensive research on plumbing, and I found out that they usually use 'dielectric couplings' for dissimilar metals, basically, junctions which use rubber seals to prevent metal-metal contact. The problem is, these couplings for gas usage are difficult to find, and probably costly. However, there's one possible machining technique to join these dissimilar metals - brazing. It's basically soldering with silver, which does not have a high outgassing rate. Which is why wire feedthroughs to vacuum tube instruments are brazed on.
That gave me the idea. I decided to use a silver alloy which does not exhibit much corrosion with copper/brass as the flux, and braze on a copper pipe onto the stainless steel pipe (which is in turn directly connected to the feedthrough).
Second, the stainless steel fittings would distort the magnetic field. So... I needed a certain length of copper or brass pipe - which do not have a standardised fit to Swagelok. Moreover, galvanic corrosion takes place significantly between stainless steel and copper/brass.
Left with no choice: I carried out an extensive research on plumbing, and I found out that they usually use 'dielectric couplings' for dissimilar metals, basically, junctions which use rubber seals to prevent metal-metal contact. The problem is, these couplings for gas usage are difficult to find, and probably costly. However, there's one possible machining technique to join these dissimilar metals - brazing. It's basically soldering with silver, which does not have a high outgassing rate. Which is why wire feedthroughs to vacuum tube instruments are brazed on.
That gave me the idea. I decided to use a silver alloy which does not exhibit much corrosion with copper/brass as the flux, and braze on a copper pipe onto the stainless steel pipe (which is in turn directly connected to the feedthrough).
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