Temperature StabilizedCharacterization of High Voltage

Power Supplies

Department of Physics and Astronomy, Aarhus UniversityOle Krarup∗

August 23, 2017

Abstract

High precision measurements of the masses of nuclear ionsin the ISOLTRAP experiment relies on an MR-ToF. A majorsource of noise and drift is the instability of the high voltage powersupplies employed. Electrical noise and temperature changes canbroaden peaks in time-of-flight spectra and shift the position ofpeaks between runs. In this report we investigate how the noise anddrift of high-voltage power supplies can be characterized. Resultsindicate that analog power supplies generally have better relativestability than digitally controlled ones, and that the high temper-ature coefficients of all power supplies merit efforts to stabilizethem.

1 Motivation

The ISOLTRAP experiment provides accurate mass spectrometry ofnuclear ions with a resolving power, m/δm ∼ 3 · 105. This is achieved inan MR-ToF by reflecting an ensemble of different nuclei between two highvoltage electrostatic potential mirrors. Each of these mirrors consistsof 5 cylindrical electrodes which can be positioned to achieve a desiredpotential distribution. Due to the different charge to mass ratios of theions, light ones with high charge will be reflected earlier shortening theirflight path compared to heavier ones with less charge. Upon ejectionthe ions will be sorted according to this ratio and (all other parametersbeing known) their time of flight provides accurate information abouttheir mass.

A major source of uncertainty and drift in the MR-ToF is the thepower supplies, which set up the electrostatic mirrors (see Figure 1).Since many reflections are involved, random fluctuations can broaden

∗e-mail:ole.krarup.dk@gmail.com

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Figure 1:Top: Sketch of the axial potential distribution created by the high voltageelectrodes in an MR-ToF. The size of the electrodes is indicated by theorange squares in the bottom.Bottom: Effect of deviation of power supplies in ppm on the measuredToF specturm. Electrode 5 is at the turning point of the ions where theyspend the most time. It therefore has the biggest impact on their ToF.

the ToF spectrum of the ions, while drifts in average delivered voltagecan shift the measured spectrum between runs. To characterize powersupplies for the experiment a system is needed which can handle thehigh voltages and remain unaffected by changes in ambient temperaturesthroughout the day. This project will describe the design, constructionand implementation of such a system.

2 Equipment

This chapter contains an overview of the different electronic devicesmaking up the high voltage measurement system.

2.1 KEITHLEY, 2002 Multimerer

The 2002 multimeter from KEITHLEY can measure voltages down to 9digits, making it ideal for characterizing the fluctuations of the powersupply. To characterize its performance it was set to high-accuracy mode,no averaging and 8.5 digit accuracy. The + and - ports were connected

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Figure 2: Intrinsic voltage measured by KEITHLEY 2002 after activation.Colour indicates time after the experiment was initiated, where blue datapoints are close to the start and yellow ones are close to the end.

directly to each other and the device was read out over the course of21 hours. The result is seen in figure Figure 2. The multimeter reachesstability exponentially with a time constant of 50 minutes. Upon settlingthe intrinsic voltage is 0.70(4) µV. The noticeable fluctuations in Figure 2after the 10 hour mark are due to temperature changes in the laboratory.This is not a problem for the accuracy of the ultimate reading since thepower supplies will output in the range of 5 kV to 30 kV and a devicecalled a "voltage divider" (see next section) will scale this down by a factor1.000 so the multimeter can read it. In other words one volt measured bythe multimeter corresponds to 1.000 V in the power supply. Thereforethe intrinsic 0.70(4) µV on the multimeter will change the measurementof the power supply by 0.7(4) mV. As input voltages are on the scale of1 kV this will cause a change of around 1 ppm. According to Figure 1such an effect will be negligible.

2.2 OHM-LABS, KV-30A High Voltage Divider

A circuit diagram of a voltage divider can be seen in Figure 3. Thevoltage is scaled down according to

Vout = 11 + R1

R2

Vin,

where Vin is the high input voltage, R1 and R2 are resistors, and Vout isthe voltage read by the multimeter. To reduce the voltage by a factor

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Figure 3: KV-30A and voltage divider circuit. The power supply isconnected to the Vin node and ground. The multimeter is connected tothe Vout node after R2.

1.000 the following equations must be fulfilled.

Vout

Vin= 1

1.000 = 11 + R1

R2

⇒

R1 = 999R2.

ModificationsWith the goal of characterizing different power supplies it was necessaryto ensure that the voltage divider would not affect the measured voltage.As it is well known that the resistance of a component depends on itstemperature and since a voltage divider consists mostly of resistors, acustom made cooling system was installed in the voltage divider. Essen-tially it consists of a so-called Peltier element sandwiched between andinternal, and an external heatsink with accompanying fans (see Figure 5).The Peltier effect works by applying a voltage across two materials withdifferent Peltier coefficients, ΠA and ΠB , measured in energy per charge,as shown in the insert of Figure 5. The dissipated heat, Q̇, is given by

Q̇ = (ΠA −ΠB)I,

where I is the applied current. Detailed to-scale 3D models of eachcomponent were made in Inventor before assembly to verify that thedesign was feasible. A sketch of the final setup with all employed devicesand their connections can be seen in Figure 4

3 Results

3.1 Temperature characterization of KV-30A

According to its specification sheet the KV-30A is "designed to minimizechanges from temperature", though no numerical values are stated. Since

4

PC w. LabVIEW& PID loop

PT-104PT100 Data Logger

KV-30AVolt. Div.

1 32

+-

High Volt.Power Supply

Inside Temp.

Air Temp.

PS. Temp.

HV - out

DAQ

Low Volt.Power Supply

Peltier

Multimeter

0.21054321

Figure 4: Sketch of the final setup including all wires and devices. Arrowsindicate input and output. Wires for the fans of the Peltier element’sheat sinks are not drawn as they are only connected to the wall plug -not other devices.

its performance is crucial to ultimately getting a valid reading, this had tobe quantified. To ensure a stable temperature, a thermometer connectedto a PT-104 box from pico Technology, and the power supply for thePeltier element was connected to a Data Acquisition Device (DAQ). Thesewere connected to a computer running a Proportional-integral-differential(PID) program in labVIEW. The PID program allows one to specify adesired temperature, whereupon the current to the Peltier is increased ordecreased based on the measured internal temperature. The current isproportional to the momentary difference from the desired temperature,the integral of previous differences, and the momentary rate of change intemperature. Mathematically the following expression is evaluated forevery time step,

I(t) = K

(e(t) + 1

Ti

∫ t

0e(τ) dτ + Td

d

dte(t)

),

where I is the current to the Peltier element, K is an overall proportion-ality constant, e(t) is the distance from the set temperature, Ti (looselyspeaking) the time it should take the program to correct for previous

5

Peltier element

Fan

Heat sink

Fan

Heat sink

Thermometer

Resistor

Insulation tubeCooled Surface

NP

Dissipated Heat

Figure 5:Top: Sketch of the voltage divider and the modifications applied to it.Insert: Diagram of the Peltier effect. A current running through twomaterials with different Peltier coefficients causes charges to remove heatfrom one side of the element and deposit it on the other.Bottom: 3D model of the KV-30A voltage divider made in Inventor.All modifications were measured with a caliper, modeled to scale andassembled in the program to ensure that they would fit together.

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Figure 6: PID stabilized temperature inside the KV-30A over the courseof 2 hours. The average oscillation amplitude can be taken to be equalto the standard deviation of the data, σ ≈ 0.15 ◦C

errors, and Td is the amount of time into the future the temperaturechange should be extrapolated. The importance of each of the termscan be tuned by changing the coefficients. Using the in-built autotunefunction and some trial-and-error, it was found that optimal stabilitywas reached if K was set to −1000 and the other parameters to zero. Inpractice this sets the current to the max value if the temperature is toohigh and zero if it is too low. An example of the resulting stability overthe course of 2 hours can be seen in Figure 6. It was found that otherchoices of parameters (such as the ones in the canonical Ziegler-Nicholsmethod) did not allow the temperature to be centered around the settemperature. Centering was deemed more important than low oscillationamplitude as the manufacturers put the resistors of the KV-30A insidean plastic tube to insulate them electrically (and thus unintentionallyinsulated them thermally). Their thermalization time should thereforebe so long that they are only sensitive to the average temperature insidethe box.

With the temperature stabilization ready a test was conducted toinvestigate how the resistance of the KV-30A depends on temperature.It was set to reduce the input voltage by a factor 1.000 and the leastnoisy high voltage power supply available was connected to it. Once bothdevices were thermalized the voltage could then be measured as a functionof the set temperature inside the voltage divider. The result is seen infigure Figure 7. Over a 1 degree range the measured voltage changesby less than two ppm. As cooling can be stable down to an oscillationof ±0.15 ◦C with a period of 3 minutes(arguably much shorter than itsthermalization time), the KV-30A can effectively be made completely

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Figure 7: Temperature dependence of KV-30A. When the temperature ischanged by one degree the mean measured voltage at a given temperaturechanges by about 2 ppm. Because the voltage divider can be stabilized toat least ±0.3 ◦C (see Figure 6) its temperature drift is negligible. Graphcolor indicates when a particular data point was taken, where the firstone is blue and last one is yellow.

stable with respect to temperature.

3.2 Characterization of power supplies

Two parameters are of particular importance for the characterizationof the power supplies. One is the temperature coefficient measured in[ppm/K], while the other is the relative stability in [ppm], which isthe standard deviation of the voltage divided by the average voltage.To determine the relative stability the air-conditioning in the lab wasset to 24 ◦C and the voltage divider was stabilized to 23.5 ◦C. Thisshould minimize the drift in the power supply and the multimeter due totemperature changes.

The voltage of the power supply is gradually increased to its maximumvalue. For the relative stability is determined by dividing the standarddeviation of the voltage in a short interval with the mean voltage in theinterval. For the voltage with the best relative stability the temperatureof the power supply is gradually changed by placing a bag over it andallowing it to heat itself. Thus the temperature coefficient is extracted.The final results for 3 different power supplies are seen in Table 1. Onenotable conclusion is that power supplies are highly sensitive to tempera-ture. Given the notable air temperature drifts in the ISOLDE hall (seeFigure 8) it would be worth implementing a stabilization system similarto the one for the voltage divider. Another conclusion is that analog

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Power supply Iseg DPR 60 Thorn alpha IIIOperation Digital Analog AnalogVmax [kV] 6 2 30

Rel. Stab. [ppm] 15(2) 2.2(5) 5(1)Apr. Temp. coeff. [ppm/K] 100 10 300

1/e settling time [min] 3 3 20Apr. stab. time [min] 5 60 10

Comments Stabilize Temp. Occational spikes Stabilize Temp.

Table 1: Table of characterized high voltage power supplies. The temper-ature coefficient is listed as approximate as the surface temperature ofthe devices was used. The stability time is the time over which a powersupply drifts by 10 ppm.

power supplies (though harder to control remotely) are less noisy thantheir digital counterparts.

The long term stability of the power supplies is on the scale of a fewminutes to an hour at best. A typical measurement of their voltage inppm of the average over a long timescale can be seen in Figure 9. Thedigitally controlled iseg shows sinusoidal drifts with a frequency of aboutone second, while the analog ones have a much more stochastic output.From the top graph of Figure 9 it can be seen that the alpha III, whilehaving less relative noise than the iseg, drifts much more over time. Thisis consistent with it having the largest temperature coefficient.

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1 7 0 5 0 9 2 3 : 0 0

1 7 0 5 1 0 2 3 : 0 0

1 7 0 5 1 1 2 3 : 0 0

1 7 0 5 1 2 2 3 : 0 0

1 7 0 5 1 3 2 3 : 0 0

1 7 0 5 1 4 2 3 : 0 01 7 . 2

1 7 . 4

1 7 . 6

1 7 . 8

1 8 . 0

1 8 . 2

1 8 . 4

Temp

eratur

e in th

e ISO

LDE h

all / °

C

Figure 8: Air temperature drift in the ISOLDE hall over the course of 5days. According to Table 1 a drift of one degree in a power supply couldcause a change in output voltage of ∼ 100 ppm. This in turn may causea change in the ToF spectrum of 50 ppm according to Figure 1.

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Figure 9:Top: Drift of the three analyzed power supplies over several hours.

Middle: Drift over the course of 30 seconds. The iseg shows si-nusoidal oscillations possibly due to some internal stabilizationmechanism.

Bottom: FFT of voltage in iseg power supply. The bump at15 Hz is due to the sampling rate of 30 Hz. The bump at 1 Hzcorresponds to the sinusoidal peaks of the blue graph in the middle.

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Side project: Fine tuning RFQ capacitors

While waiting for components to be delivered or lengthy measurements tobe taken, work was put into designing a system for tuning the impedanceof the matchbox connected to the RFQ cooler and buncher remotely. AnAC signal with a certain frequency is amplified and sent to a numberof rods parallel to the beam, but since the rods have different sizes theamplitude can only be optimized for one at a time. Therefore certaindevices ("matchboxes") containing tuneable capacitors are inserted beforethe rods. Tuning the capacitors can ensure that all rods receive the sameRF signal, but this must currently be done manually with a screwdriverwhen the beam is off. Moreover an oscilloscope must be connected toverify the quality of the RF signal, but this introduces an extra source ofnoise. A remotely controlled system would allow the RFQ to be tunedon the fly using the properties of the beam as a measure of quality (seeFigure 10 for a sketch).

The following test-setup was constructed. A screwdriver head wasmounted on the rod of a stepper motor. This was linked to an Arduinoboard with an Adafruit "shield" (extra mountable board) designed forcontrolling stepper motors. After uploading an appropriate program tothe board it could be controlled with a LabVIEW interface. An externalpower supply can be plugged into the Adafruit allowing the motor todeliver more torque. A procedure for obtaining and installing the relevantsoftware is show below.

• 1) Download "AdafruitV2MotorShieldStepperDriver.zip" from http://forums.ni.com/t5/LabVIEW-Interface-for-Arduino/AdafruitV2MotorshieldStepperDriver/ta-p/3519569.

• 2) Install the Arduino IDE software from https://www.arduino.cc/en/Main/Software. This is the program that allows scripts tobe uploaded to the Arduino.

• 3) The .zip file includes two folders, "Adafruit_ MotorShield" and"TextFinder", which must be moved to the "libraries" folder for theArduino. Just paste them into [C:/Program Files (x86)/Arduino/libraries/].

• 4) Launch the Arduino IDE software and go to "Sketch" in thetoolbar. Select "Show Sketch Folder". Extract the folder "Adafruit-StepperDriverVersion2" from the .zip file to the desktop. Fromthere copy it into the sketch folder. It must be done in this (ratherconvoluted) way because the sketch folder is at C:/Users/localUser/AppData/Local/Temp/untitled880709186.tmp/, and "AppData"can’t be accessed using the pathfinder for some reason.

• 5) With "AdafruitStepperDriverVersion2" in the sketch folder, openit and double click on the IDE program called "AdafruitStepper-DriverVersion2" (same name as the folder). Go to Sketch and pressVerify/Compile. Go to "Tools" and check that "Board:Arduino/GenuinoUno" and "COM4" (or whichever port it’s plugged into) are selected.Go to "Sketch" and press "Upload".

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Figure 10: Sketch of set-up, which allows the capacitors controlling theoscillating magnetic field of the RFQ to be tuned remotely.

• 6) The Arduino now has the software required for LabVIEW tocommunicate with it, and which tells it how to operate the steppermotor. Extract all the VI’s from the .zip file to some folder andrun "AdafruitMotorShieldStepperDriver.VI". Set "Serial Port" to"COM4" (or whichever is being used) and choose a value of "X StepsSetpoint". 1 step should be 1.8 degrees meaning that 1 revolutionis 200 steps.

• 7) Press the "Send" button and watch it turn. The motor heats upquite a bit, but this is to be expected. To reset the position to 0,read the number in "X Step Position", multiply by -1 and enter itinto "X Steps Setpoint".

At the time of writing the stepper motors can be controlled usingLabVIEW but are not strong enough to turn a capacitor screw. Astronger power supply may be needed.

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