It seems “core hours” are best place 11-3 on TuTh. In general, plan to be in the lab during that time when possible. Otherwise, work at your convenience. Elizabeth has been promoted to “independent contractor,” working for Bernhard through LBL and reassigned here to steal our secrets, which were sharing with Bernhard anyway.
Despite my best efforts, the vacuum is back up and running on the test stand after the BPM installation. If it holds, Phil can give it his blessing ritual and we’ll be ready to actually see the beam on the scope.
It’s been brought to my attention that the focus voltage may be some appreciable fraction of the acceleration potential. As such, it will probably require another HV supply. There is a +-5 kV supply in the rack, which may or may not be sufficient. We may also use the resistor-stack-with-alligator-clips idea, making a crude potentiometer with a total resistance at least 200 Mohm. It should be a lot more obvious what is going on with the focus after we get the BPM installed.
The electron source is set up in the test stand and capable of running, pending getting the final paperwork in place to run. It needs to be in writing that we are not a danger to ourselves or others. Here’s a rundown of what all the knobs and buttons are and what they do, roughly in the order they should be turned on.
screen voltage, a.k.a. extraction voltage, grid voltage: Manipulated by the insulated knob sticking through the cage, it controls the negative bias on the Pierce electrode. The important function of this voltage is to control the beam current. Attempting to read Pierce’s book to determine why the electrode is designed the way it is only lets me know how he might have figured it out but doesn’t really explain the details. Once the beam is extracted, it could be accelerated over 0 V, 20 kV, or 3500 kV and the beam is mostly the same.
cathode heater: Running a steady 450 mA (about 12 V, but variable depending on how much the cathode feels like resisting), it heats the barium oxide cathode matrix to around 1100 degC. Electrons boil off the hot surface (thermionic emission) and zip around in the cool vacuum that surrounds the cathode. This gets set up once and run at a steady state the entire time. To shut down, just turn everything off.
acceleration potential: This is the high voltage source and what gives the beam the bulk of its energy. The source itself is designed for 20 kV. Many, many HV sources inhabit the lab. The current one is a 60 kV source. It was the most appropriate HV source, given the bias has to be negative and not all the power supplies are reversible. Avoid over-biasing the source. There is also a very useful quantity displayed on the HV supply: the source current is the amperage drawn by the HV source. It is showing all the current leaving the HV stand. That current can be leaving as the beam, leakage across the insulator standoffs, leakage across the source acceleration gap insulator, or pretty much any other way a net charge could be leaving the HV stand. It can be more precisely “measured” by using the current-limit knob and checking for a lack of response from the ammeter needle, essentially finding the point where the supply switches from current-limited to voltage-limited. Alternately, there is a 0-10 VDC current indicator output lug in the back that can be connected to a multimeter.
focus voltage: Probably another insulated knob, this controls the voltage on an electrostatic focuser. Basically, it is a series of three rings with the beam passing through the center. The middle of the three rings is biased to some voltage and the other two are at source potential. Pierce’s book explains this simple focusing mechanism quite well. There is no good guess as to the “best” focus voltage and it’s the main beam profile tuning parameter. Likely, we will be fiddling with it a lot. At this point, we have the focus voltage tied to the screen voltage.
Faraday cup current: This is the 200 microA-FS ammeter alligator-clipped to the instrument rack. It is measuring the current across the Faraday cup, or effectively the beam current when the FC is in the beam and the beam is aimed well enough to hit it. It is also possible to catch the (former) beam electrons in your hand by grabbing the hot lead of the FC cable while installing the ammeter with the beam on, though this is inadvisable.
So, stuff to do with the source:
1) Measure the beam current and beam efficiency (beam current over source current) for different values of the extraction voltage at a fixed focus voltage. Using the extraction voltage as the focus voltage seems to be an OK starting point.
2) See if there is a focus voltage range over which there is any change in efficiency at any beam current up to 100 microA.
Other, non-graphical, tasks include routing the cabling for the installation of the BPM and learning how to communicate with the BPM preamp and controller, so the output can be seen on an oscilloscope. I think we’re 9 pins away from true happiness on that front. In other words, it will take some homebrew serial connectivity engineering to get the job done, and you kids will appreciate the concept of plug-n-play a lot more afterward.
Despite the radio silence, I have been here and moderately productive for the duration. The fact is it’s actually been quite busy down here so I have less time to waste on this “new media” business.
I removed the gate valves and blanked off the open ports of the Faraday cup T. To assure the cleanliness of the new components, I baked some delicious flange-pies in the oven before installing them. The system immediately pumped down below 10^-6 torr and was below 3e-8 torr 15 hours later. That confirms my suspicion that those gate valves aren’t meant to be used as UHV vacuum-breaks and are instead more of an equipment safety-only function. In any case, as soon as we get ourselves an insulator stand for the source and figure out all three of the inputs, we ought to be able to shoot some electrons.
I managed to get out source test setup turbo pump spinning at full speed without tripping the over-power protection on the controller. It seems to be a matter of getting the pump to ramp up more slowly than its programmed to do right now. This can be accomplished by stopping the pump, flipping the controller into programming mode, and changing the appropriate settings. Alternately, you can get the system running at 3 krmp, let it pump itself out as much as possible, then hit the LS (low speed) button and the START/STOP button. It may trip a few times on its way up to the low-speed setting of 28 krpm. Once it gets there, let it pump out and disengage the low-speed setting. Then hit START/STOP again. My first attempt made it to full speed, 42 krpm, from 28 krpm drawing about 220 W, which is well below the over-power trip level.
It’s not an elegant solution. My guess is the controllers are configured to work in the original Rapiscan setup, which we can tell by looking was not short on pumping power. They probably wanted to be able to pump their system down as fast as possible, so the ramp speed is tuned to that, rather than our less-timely experimental speed.
I managed to actually make the turbo pump spin at 3k RPM by mashing the “Low Speed” button as it came on and it seems stable for now. As Jimmy Carter says, let’s build on that.
The vintage 1960 Texas Nuclear Corp. Mr. Fusion D-T neutron generator will be running Tuesday, 11/24 for the NE 180/280 classroom demonstration at 330 pm. If you find any stray neutrons in the lab, please return them to the TNC enclosure. There is likely to be some sparking and cursing during the attempted operation.
The source is here. Let the games begin.
