As of yet there's no formal electrical design info on this trap yet (though we've spent nearly $100k on it this last week!). That of course was all on the custum vacuum system (oof!), turbo pumps, valves, etc, (which I must confess, is much more my field)...
Up until now, we 've been operating a hyperbolic ion trap (toroid with two hyperbolic endcaps makes the quadrupole field), and that was pretty easy in terms of circuits, since you just have to put RF on the toroid (~ 10MHz, 2kV pk-pk) which we do with a resonant circuit, and DC on the electrodes (just 1 dc supply, and only about 10V). Though I admit that I've actually melted some air-toroids that I've made...
I just posted some more details below on the circuit, but I assume that I don't have to "buy" a crap load of bias-tees, and that I should be able to make my own with inductors and caps, since good bias-tees (minicircuits?) seem to be expensive.
Here's a paper about a similar system, though there are some distinct differences (they're capturing 60keV ions, so they float their whole trap at 60kV!!!). We don't have a beam, only a cloud.
I am not sure about that- Linear (IIRC) has at least one clever app note where they show how to easily adapt one of their miniature boost regulators to buck-boost mode. This is done by placing a P-MOSFET between the boost output and load and then using the LBI/LBO logic and FET as hysteretic controller when the converter goes into buck mode- boost SW off and normally problematic Vin feedthrough directly to FET.
Having had some experience in this area, I have a few comments.
I prefer to separate the resonating coil from the RF transformer. That's because these perform difference roles, and benefit from being separately optimized. For example, a high-Q coil is not bad, it's good, provided its inductance value is reasonably-steady. A small high-permeability pot core with a precision-ground air gap has a temperature-independent value of A_L, and it lets you to make a high-Q resonating coil that can be tuned by means of a small screw tuning slug that's adjusted inside the gap (this is probably more convenient than using a tuning capacitor). Pot cores are easy to wind, and overall are superior to anything a toroid can offer, at least for your frequency and voltage range. :>)
The capacitor C is selected taking the coax capacitance in mind. It's best to minimize the total resonator capacitance, because that reduces the circulating current. It's possible in some cases that the coax capacitance alone will be sufficient for C.
. simple, precision low-power RF electrode drive scheme . ____________ . G n1:n2 ,-----+------------+---+--)___________}---| . __|\\__ T | | | | coax electrode . | |/ | || # # _|_ | 1pF __ . Vac # || # # L C --- '-||--)__ coax carries RF . |______| || # # | voltage-monitor . gapless | | Cdc | sig to cap. divider . pot core '--+--+---||--(#)--+-- gnd . | \\ \\ . | gapped current transformer . dc bias --\\/\\/--' pot core or small sense resistor
A high-Q resonator allows the transformer T to control the voltage across the electrodes without requiring it to carry the high RF resonator currents. The transformer should be a tightly-coupled (i.e. low leakage inductance), but with a magnetizing inductance that's high compared to L, so it's not too much a part of the resonating circuit. Its windings will contribute to C.
The amplifier G should have a low output impedance, to force the output voltage amplitude, independent from any slight mistuning of the coil. It won't be supplying much power, assuming a high-Q coil. A very important point: trying to enforce a 50-ohm pathway is not advantageous to the primary goal of enforcing a set output voltage. I like APEX PA09 hybrid amplifiers, which are happy delivering 10W at 1MHz, but simple class-A emitter-followers with CS pulldown can also work well and are much cheaper.
Cdc is a 200V ceramic capacitor to isolate the DC bias voltage. It's physically small but looks like a short at RF.
(BTW, An appropriately-designed RF balun transformer can be used to modify the above scheme for balanced RF electrode drive.)
I use a small cap to sample the RF output voltage, this cap is the top part of a capacitive divider that includes some shield and coax capacitance. I usually give up on trying to make the capacitance divider a certain ratio, like 100:1, and settle on getting an RF monitoring voltage somewhere below my goal, say 700mV or so, and follow this with a trimpot cal adjustment on the necessary 50-ohm output buffer amplifier. All this is located near the resonator.
Most CFB-type opamps are happy with this circuit's 100-ohm load.
It's useful to have a current-sampling resistor or current transformer to allow for phase-checking the resonance tuning at any point in time, although simply peaking the RF output voltage during tuneup procedures works fine; the subsequent drift will be small and its effect largely overcome by the amplifier's low Z-out and the transformer's low L-ell.
BTW, I do prefer toroid cores for making RF current transformers. :>)
Well, spring is here and it's planting time for our garden, so I'm going outside and switching to dirt-man mode.
Correction: The dc-bias resistor for the opamp above needs to be at least 50k, to allow RF measurements down to ~100kHz for the capacitive-divider values shown, 1k was far too small. Some CFB opamps have low bias currents on the + input, or use a high-performance JFET opamp.
I just wanted to say thanks to all of you guys who gave me some help. Got a good start and some work to do now! (Stupid springtime makes it hard for grad students to work in their basements...)
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