Jesse Wodin wrote ...
Incidentally, in case anyone is interested, we're a neutrino physics
group at Stanford (
http://www-project.slac.stanford.edu/exo/). I'm
on a team building a linear ion trap (for Barium actually), which is
basically a set of 4 segmented rods (15 segments each) that are driven
at about 1MHz@100Vpk-pk with a DC offset that needs to be controllable.
By changing the DC offset, you can move on ion around wherever you
need it. After we trap an ion, we hit it with 493nm and 650nm lasers
(650nm is an off the shelf diode, 493 is a doubled diode using KTP) to
drive it's levels, and we look for the fluoresence. And, though I'm a
physics grad, I unfortunately have WAY less electronics experience than
I should (other than the super basics -- opamps and stuff).
J Wodin also wrote ...
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.
[email protected] continued ...
To be more detailed, the trap is 4 "rods", where each "rod" is actually
15 rod segments, each electrically isolated from the other, but very
close together.
Cross section:
o o
o o
Side view:
___ ___ ___
|___| |___| |___|
___ ___ ___ ...
|___| |___| |___|
each rod segment is ~ 3cm long, with a diameter of ~6mm. All rods have
to be driven at 1MHz, 100Vpkpk, and each set of 4 rods (e.g. the 4 in
the cross section view) are going to be at the same DC potential.
In order to drive the RF, I just built the following extremely simple
circuit
G1|\
+--|/--- -------------------
| ( ( | |
Vac ) ) --- ---
| ( ( ---Cmatch ---Ctrap
| ) ) | |
-------- -------------------
| N1 N2 |
GND L1 L2 GND
(wound toroid)
Where
(1) Vac is just an HP fuction generator (needs 50 Ohm output)
(2) G1 is a 50dB honkin RF amplifier (50 Ohm output)
(3) N1/N2 = 20:5 wound micrometals toroid transformer
(4) Cmatch variable cap for making sure that Ztot=50Ohm as seen by G1
(5) Ctrap is the total capacitance of the rod segments (~500pF max)
I'll probably have to add a small resistor (10 Ohm?) somewhere in the
right hand side to lower the Q of the transformer, which is too high
right now. The tranformer does 2 things for me. First, it matches the
impedance so that the right hand side looks like 50Ohm to G1 (tweakable
by Cmatch) and it gives me voltage gain of ~ 4, so that if I want to
run Ctrap at 100Vpk-pk, then the power dissipated in this circuit is
only 12.5^2/50=3W.
Now, the obvious question is how to insert my DC supply (I'm building
it right now...) so that Ctrap sees the DC potential, but also so that
the RF doesn't go back into the DC circuit to kill it. I'm not
really sure how to do this yet... Possibly a bias-tee, but that's my
only idea.
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.
.. --+------ 100V RF
.. | ~0.7V
.. | ~1pF ___6" coax / __ 2.0V 1.0V
.. '--||--)___)--+----+-----|+ \ / 50 / 1/100
.. about _|_ | | >----+---/\/\---- calibrated
.. 130pF --- 1k ,-|-_/ | RF-monitor to
.. total | | | 1.0k | 50-ohm term.
.. gnd gnd +--/\/\----'
.. '--/\/\--/\/\--- gnd
.. 249 500 trimpot
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.
