Complex but funny question: reverse current emission in a particule accelerators - Page 1
Forum section: electronics / beginners

author: matemathieu
author_rank: Contributor
timestamp: November 16, 2021, 10:58:24 am
content: Hi everyone,I have a complex case to submit to you, and if you may answer your best take on it, I would be very glad to use your suggestions to get further in this design. So thank you, if only for reading, otherwise for answering and helping.I am a beginner in electronics, not a total noob, but I lack the classical learning that you get when you go through classrooms. I design my analog signal processing for science and physics measurements, I hack a lot. I am a researcher in industry and universities with 10 years of multi-disciplinary research.This is meant to adjust your range of semantics and details in your answer and suggestions.If there is a missing element to context or information, let me know, no problem.The context of this application is very tricky, hence my request to you guys that are the proficient electronics and would add proper thinking in this issue.It is about an particle physics experiment, on a low energy accelerator that shoots protons into targets.The energy is below 20keV and the current is less than a 100μA.Our problem is that we have a reverse current emitted from the metallic target that prevents us to properly measure the current, since part of it is back-streaming from the target along the incoming beam of protons.So in short, when we shoot protons into a metal, it emits electrons that flow in the reverse of the incident beam.This is a classical issue in particle accelerators and the usual fix it to place a grid between the beam and the target, polarized to 100s of volts that force the electrons to go back to the target.I would like to create a more elegant solution to this issue. I name it a current "swallower". The principle is to polarize the target to swallow the electrons before they are emitted, carefully measuring the current produced by the polarizing device and from the target to subtract one to the other.It would require my less voltage to operate than the 100s of volts and avoid significant additional modification to our vacuum chamber, since it is all externally located.So I started to design using two options, here are my notes (in Markdown):# ObjectiveWe aim to resolve one of the experimental problem related to ion implantation: Reverse Secondary Electron Emission from the target implanted by charged ions.This tend to interact and reduce the sability of the beam during operation and measurement.# Specifications## Ideal case- Current source: 1 to 10mA- stability 1nA/V- voltage bias 0 - 300V- Stability over time, to be defined, experiments last weeks.## Attainable Specifications- 10 to 100mA- 36V per module, the modules will be placed in serie to increase the output voltage- 80nA stability per module# RequirementsThe system will need to be connected to a power supply to provide all necessary power for the operational amplifiers and electronics.# Solutions## Option 1: the simple programmable current sourceLT309 - 2200mA 2-Terminal - Programmable Current Sourcehttps://docs.rs-online.com/749d/0900766b81320b7b.pdf### Advantage:- Simple- Quick to make- Cheap- Readily available- Might be enough### Disadvantages:- Current source only- Limited in voltage amplitude- Limited choice of parameters because it depend on the intergrated circuit itself.https://docs.rs-online.com/749d/0900766b81320b7b.pdf## Option 2: the voltage regulator with current stabilisation via Op-ampBased on MAKE A PRECISION CURRENT SOURCE OR CURRENT SINK - By R. Mark Stitt (602) 746-7445https://www.ti.com/lit/an/sbva001/sbva001.pdfThis is an old technique, but it is quite valuable option considering we can adjust the parameters depending on the requirement of the application.A newer version is lengthly discribed here:https://www.maximintegrated.com/en/design/technical-documents/tutorials/3/3748.html### Advantages:- Simple- Quick to produce- Quick to answer- Effective- Versatile- Inexpensive- Wider range of parameters can be set in design### Disadvantages:- Might not be enough to do the job (V max)- Might be fragile- Require more development and time## Option 3: Commercial solution based on Calibratorshttps://www.bestech.com.au/wp-content/uploads/2020/05/4463-High-precision-calibration-source-ID-5280.pdfhttps://www.timeelectronics.com/portable-voltage-current-instruments/1021-dc-current-source/### Advantages:- Reliability- Availability- Extended purposes beyond just this application### Disadvantages:- Does not provide the guarantee of effectivness- Expensive- Availability and procurement depend on the procedure duration- limited ranges, no possibility of improvment# Development## test capabilitiesmax 3kV voltage source @10mAOsciloscope up to 700V from eevblog# Advised planStart with a system of Option 1 then 2 then 3 if required.# Additional referenceshttps://www.ti.com/lit/an/snoaa46/snoaa46.pdfhttps://www.ti.com/lit/an/slyt768/slyt768.pdfhttps://www.ti.com/lit/eb/slyc147a/slyc147a.pdfSo, am I on the right track, or completely wrong about my assumptions?Thanks for reading.mm

author: penfold
author_rank: Frequent Contributor
timestamp: November 16, 2021, 11:23:47 am
content: Just to see if I've interpreted it correctly:The target is (always?) conductive? Normally connected to ground/vacuum chamber/earth?The plan is to measure the current from the target and adjust the voltage dynamically so that the current is always zero?And the target scenario is to just make sure there's only ever as much voltage as is necessary applied to the target or grid? But otherwise, it's not critical whether there's an offset in voltage etc so long as you're keeping it to a reasonable level?

author: Capernicus
author_rank: Frequent Contributor
timestamp: November 16, 2021, 11:33:06 am
content: What horribly world destructive technology are u interested in?

author: Terry Bites
author_rank: Super Contributor
timestamp: November 16, 2021, 01:23:02 pm
content: Current source or sink? Deliver current from 300v to GND or sink from 300V ptential to ground? Thats a Gohm of output impedance at DC! What about a Gohm resistor? Current source or sink? Exotic. Can it be done in analog alone? Well bootstrap in for this scorching 20Gohm design . Ay caramba! CERNhttp://cds.cern.ch/record/643294/files/cer-002399331.pdfI'm sure you have these guys phone numbers. Buy them a beer, I'm sure they'll sort you out.

author: Capernicus
author_rank: Frequent Contributor
timestamp: November 16, 2021, 05:03:50 pm
content: Your on the right track, just believe in yourself. (and that means sometimes not to listening to what other people think.)Maybe this amazing technology stuff, is not so impossible to understand or get going, just takes a bit of work.

author: matemathieu
author_rank: Contributor
timestamp: November 17, 2021, 04:26:22 pm
content: Thanks for your questions!The target is metallic, so always conducive (sea of electrons).It is not connected to ground, but floating. However, if you connect it with the BNC plug outlet, into a current meter, it will be connected to ground through a low impedance electric path of the shunt.That is the original way we use to measure current, but it is not satisfying for the aforementioned reasons, i.e. secondary electron backstreaming.I plan to use the system as a current sink, this way I polarize the target to avoid emission of electrons.In the great realm of 4-quadran it would be a voltage source with a current sink.The voltage adjustment idea is very good, but I want to keep things simple for initial attempts. The idea of a voltage variation relates to use identical modules that you can stack-up to increase the bias and make sure no back-streaming would occur if needed.As for the technology, it is just classical ion accelerator stuff.Thank you.

author: penfold
author_rank: Frequent Contributor
timestamp: November 17, 2021, 04:48:12 pm
content: Any chance you could draw a diagram? Just like a quick sketch of how you envisage the current sink attaching etc,I may be being a bit slow, but surely whether you apply a voltage or current, something in the system is going to end up at a potential that achieves the desired effect either way?

author: TimFox
author_rank: Super Contributor
timestamp: November 17, 2021, 05:28:17 pm
content: The "Faraday cup" is the usual method to avoid errors due to secondary emission from a target.See:https://en.wikipedia.org/wiki/Faraday_cupNote that the Wikipedia editors admonished the writer for being too technical, and asked him to revise the entry for "non experts".

author: dmills
author_rank: Super Contributor
timestamp: November 17, 2021, 05:59:58 pm
content: Ok, current sources in series are living in sin, you basically cannot do that and have things work properly (It is the equivalent of voltage sources in parallel)....I might suggest that a precision current source with a high voltage mosfet as a cascode might be a reasonable approach here.Another thought, but there is a honking great mass ratio in play, so if exposed to a magnetic field the electrons will bend one hell of a lot more then the proton beam does, I would be tempted by a magnet and a couple of bits of metal with holes in them.

author: matemathieu
author_rank: Contributor
timestamp: November 18, 2021, 03:59:34 pm
content: Great to read you all again!I will try to avoid stacking up then! Thanks for the tip.Should I read cathode rather than cascode? I guess it is a typo issue, or something I don't know, tell me please @dmillsI thought about many things, especially these vacuum lamp amplifiers, they love high voltage, they can have very good stability. But I am such a rookie in this, that I get immediately outside my zone of confidence.But if you say that a precision current source with a high voltage mosfet would do the job, I am all in favor of this. Can you give more details about it? How should integrate it?The electrons are meant to avoid escaping the metallic target, we are shooting positive ions. This is why I called it a funny question, because we think in opposite charge when we deal with most of particle accelerators. Their mass is so large in comparison, that the added magnetic field will have close to no influence on the path trajectory of the beam.The idea is really about trying to drag the electrons produced by the impacts toward ground (and accounting them) rather than letting them escape and get away from the target (and loose their track).The way it is usually dealt with, is we use a polarized grid few milimeters before the target, hence the emitted electrons will be forced to get back to the target by this opposing electric field.Another problem, is that your grid acts as a mask and prevent some of the beam to reach certain zones of the target, and that is not good for what I study.We use Faraday cups every time we have to align the beam. For electron beams, there are some kind of special design to make sure that some emission of electrons (backscattered) will avoid escaping, the angled cup shape being the one they use the most. But for ions, we don't have such problem of them backscattering, you basically stuff them in your target like candles in a cake.However, the implantation will produce secondary electrons and for this we cannot change the orientation or make a cup-shaped of our sample, they have to be flat surface, open on a 2pi() steradian angle, because of the instrumentation that monitor what is going on in the sample.For the diagram, I will have to spend some time on my favorite electrotech software to make it. But I was originally counting on your suggestion on how to differentiate the current produced by the source and the one produced by the ion beam. I don't think this would be a problem, considering the resources I have posted in my original text.If you have any additional question, please let me know.Thank you!

author: dmills
author_rank: Super Contributor
timestamp: November 18, 2021, 06:25:00 pm
content: No I meant cascode (Common gate).https://en.wikipedia.org/wiki/CascodeYou connect your current sink to the source, the gate to a voltage that puts the source within the compliance range of the current sink, and the load goes on the drain, Id = Is so the mosfet does not hurt the precision, but the source is only a few volts below the gate, so the current source does not have to stand off high voltage (that is the mosfets job).Regards, Dan.

author: penfold
author_rank: Frequent Contributor
timestamp: November 18, 2021, 08:51:19 pm
content: Quote from: matemathieu on November 18, 2021, 03:59:34 pmAll, well, that's why I was querying after a diagram of the setup, partly just to have something to refer to voltages and currents against and make sure I'm not getting lost with the polarities, what you can/can't do etc, not worth going to too much trouble over,I was wondering about the voltage because controlling that may make the current easier to balance, voltage being the integral of current on the target's capacitance. I'm working on the principle that you could have a system to sweep the target's voltage whilst monitoring the current, and there'd two signposts to look for 1) the current beginning to decrease and 2) the current changing very little wrt voltage. But that method has the fatal flaw in that you wouldn't be able to determine quite which voltage is the point where electrons are not even exiting the surface of the target, only when they all drift back in. You would also need to be able to continuously perturb the voltage to check you're still optimal. So probably a no-go.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 24, 2021, 04:52:35 pm
content: So there I was pondering the many ways current sources had wrecked my life- I wiped away the tears and then I thought about how to improve on common design.The good old precision current mirror.I simulated it and got >10Meg of output impedance <1ppm over temperature (-20 to +50) and compliance of 300V. I doubt its real world performance is as good.Ignoring ther notional isolation scheme,it would cost <$20 to give it a whirl.The crucial components are the three precison resistors, the zero drifthigh voltageopamps and the 10V reference. I carefully selected the mosfets. The ZVP0545A cascode will handle 450V. If you fully isolate it then you can use it to source or sink.You do need squeaky clean power supplies and a buffered reference. The isolation is the expensive bit.I feel free to be mistaken about anything and everything in all my posts

author: Fronberry
author_rank: Contributor
timestamp: November 24, 2021, 10:55:09 pm
content: Is the attached diagram accurate to what you are proposing? If not, can you explain further?Sounds like an interesting problem.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 25, 2021, 01:56:08 pm
content: The 10 voltage reference must be able to source at least the current drawn by the load, opamps and associated components. The stability of this reference crucial as are the three precision resistors. The first opamp and associated components form a precision current sink. The output current is set by voltage at the non-inverting input and R4. Io=Vin/R4. This current flows in R3 generating a fixed voltage below the 10v reference. In my example 1mA is sunk by this current pulling the source of X1 to 9.0V. ie -1V per mAThe second opamp and associated components form a precision current source. Accordingly its set voltage for X2s noninverting input must set negative WRT to 10V ref. The current set resistor has to sink current from the 10V ref to achieve this. It is common to see this arrangement with all the opamp positive supply and the current set resistors tied to a common rail. That ok if the rail is very stable and the opamps common mode range includes the positive rail. I elected for separate ref to create a stable rail and some comfortable head room for the CMV input range. At 15V the CMV range is about 12V or 3V shy of the positive rail; 5V of headroom keeps us in the comfort zone.X5 is a cascode follower. It isolates the current source from changes in voltage across the load thus improving the output impedance. It also permits the source to supply curents to loads negative of ground. The maximum permitted Vds for the ZVP0545 is 450V.As this circuit is designed with isolation in mind the use of a negative voltage as the current return is not problematic. The set current input can be derived from the 10V ref via a DAC or precision voltage divider. Note that all ground symbols are internal to the isolated circuit.All power supplies and control i/o need to be galvanically isolated. It would make good economic sense to put a microcontroller within the isolated circuitry and use digital isolators. Good practice suggests mutually isolating the analog control inputs or digital i/o, current output and power supplies. AKA three port isolation. Have a look at an AD210 functional diagram.Note that the RHS current source is an example of a precision current mirror. seehttps://wiki.analog.com/university/courses/electronics/text/chapter-4A general remark. Mostly when people mention a current source they invariably go on to illustrate a current sink.Errata, Opamps are LTC1051.

author: matemathieu
author_rank: Contributor
timestamp: November 25, 2021, 01:59:29 pm
content: Whoooa! I am so grateful of your answers. You guys rock!@FronberryYou are correct, this schematic is the best description that can be made of our issue.Just one question, what do you represent by using R1 connected between the Ampmeter and the ground? Is it the subject of our discussion?@Terry BitesThis is amazing! It looks very well though from a non-electronic engineer point of view.I really feel really like a newbee when I see these acronyms like RNCF...So after looking for it, it seems you are advising to use a surface mount resistor, and it seems important that it is a surface mount type. This would make my task more complex, because I am planning on soldering it on my own. This would be a first for me!Does an MELF package would be fine as well?What do you imply by the Port_#. I assume these are connectors, right?I thought a full analog circuit would make the job. What is the purpose of the DAC and ADC (via Port_1), assuming it has to do with analog to digital and vice-versa.I feel I lack some fundamental concepts, because you are routing the -350V via Port_3 instead just connecting it. Does Port_3 is the 350V source?For the 15V source, should we use a AC/AC coil/toroïdal transformer?But then we have to find a AC/DC transformer that has the same isolation.Would this do the job?https://www.mornsun-power.com/html/products/2/on-board.htmlI wonder that kind of current we would be able to resolve. Our classical beam current is between 10 to 100μA. Do you think this is is fine?I assumed from a 1mA sink, resolving 2 to 3 orders of magnitude was fine.Thank you so much for the attention to details you are showing, I am glad you have taken the time to submit a solution.It looks very attainable for a one guy operation in making this up. If anybody has proficiency to make a comment on it, I would be very interested.@penfoldThe beam is a positively charged up to 20keV and the current is up to 100μA. If you look at the excellent schematic from Fronberry, it would maybe clear your doubts.Making the voltage vary is not a promary objective, a fixed 300V voltage bias is very nice to prevent the emission of secondary electrons.@dmillsIt really look like the principle I was thinking about. Thanks for the link.Again, you guys are helping a lot, thank you so much!

author: Fronberry
author_rank: Contributor
timestamp: November 26, 2021, 05:41:50 am
content: Quote from: matemathieu on November 25, 2021, 01:59:29 pmYes, the R1 is a bit of a placeholder and the subject of our discussion. What you essentially are looking for, I think, is a way to hold the target at a significant positive charge while allowing a current drain due to the effects of the proton beam.I tend to agree with penfold's assessments of using controlled voltage on the target and the source such that the small current would be easier to manage. (Such as holding the current drain to be similarly but slightly higher voltage than the target and introducing a reverse bias diode barrier, or the aforementioned cascode setup that dmills explained.)This may be naive, but would introducing a electric field within the chamber using, for example, a solenoid around the beam chamber not be feasible? If the field strength could be tuned such that the effect on the massive protons is negligible and the desired effect on the ejected electrons is achieved.

author: penfold
author_rank: Frequent Contributor
timestamp: November 26, 2021, 09:11:42 am
content: Quote from: matemathieu on November 25, 2021, 01:59:29 pmThe point I’m trying to make is that, there being an equivalence between voltage and current sources, controlling the current in any way will affect the voltage. If you have an a static set current sink, with a slight error, the voltage will just happily continue to ramp to whatever voltage it likes. But in ‘open-loop’ without feedback, that voltage it reaches may or may not be sufficient or known to keep the electrons within the target. So how do you know if it’s sufficient? A small error between the proton current and current-sink would allow for a large voltage range on the target, is that a problem?Now, if a 300V bias on the target is ok, why does there need to be a current source?300V is quite an easy ‘high’ voltage to generate, not cheap in module form, I think the ones from Advanced Energy were 100GBP, however, if you have a known minimum current and a known desired voltage, resistors are quite good at establishing that condition.

author: matemathieu
author_rank: Contributor
timestamp: November 26, 2021, 09:58:19 am
content: Dear Terry,Thank you for these additional explanations, they are very useful for a rookie like me.It looks to me like you have taken the "classical" Cascode design and added an extra step of current management goodness.It looks very proper as suggested by Terry, in my situation and I am glad you have brought your expertise and, as it seems, many years of experience in the design of these current sources/sinks.The extra-room in voltage Bias is very welcome and we are used to use high voltage DC sources in our nuclear instrumentation for Bias/stimulation potentials.I have noted most of your remarks in my following note, and listed the parts that would be necessary to source.CMV: common mode voltage?QuoteIs it X3 rather than X2?New note in markdown:211125 Solution of the current swallowerby Terry bites from forum.eevblog.com- Precision resistor should be selected with great care.- X1 is the current sink part- Output current is set by I~0~=V~in~/R~4~- Voltage drop accross R3 sets the gain of the current sink- 1v/mA in his example- Second opamp X3 is precision current source- All power supplies and control i/o need to be galvanically isolatedBill of materials:- 1x LTC1051- 1x ZVP0545A- 2x BS250P- 1x 10V, 10mA reference (suggestions are welcome)- 2x 1kR precision resistor 2.5ppm or better- 1x 100R precision resistor 2.5ppm or better- 2x 1kR- 2x 100R- 2x 100pF- 1x Iso DAC- 1x Iso ADC- 1x 230V AC to 15VDC converter with >1kV isoloation- 1x 350V DC power supply (we have those for scintillator bias voltages, e.g. NaI of HPGe detectors)- 1x Microcontroller or a AD210

author: matemathieu
author_rank: Contributor
timestamp: November 26, 2021, 10:01:08 am
content: QuoteThe landscape is very crowded, there are stuff everywhere, and this is part of the reason of this additional electronics, because we don't need to add stuff around.Plus the chamber is in μmetal, to collect very low energy electrons with Si detectors.