Fabrication of a Transimpedance Amplifier For Use Under Cryogenic Temperatures

In a superconducting radio-frequency particle accelerator, such as that being developed at Fermilab, it is often important to measure the amount of ionizing radiation emitted by particles due to Bremsstrahlung and other effects, in order to protect the accelerator from damage. Excessive exposure of machine components to this type of radiation can result in a variety of failures. Cryogenic loss monitors strategically located inside the machine cryostat help in the diagnosis. We designed and tested a key component, a transimpedance amplifier, operational at cryogenic temperatures as part of the loss monitor diagnostic. Our study focuses on optimizing critical characteristics of this single supply circuit at liquid nitrogen temperature such as frequency response, signal-to-noise ratio and bandwidth. We have optimized a transimpedance amplifier at liquid nitrogen temperatures with a commercial 1.4MHz OPamp (ICL7611), 6.6pF capacitor, 10MΩ resistor, and 2.5 kHz bandwidth. Introduction Electronic circuits that function in harsh environments can simplify design, increase efficiency, thermal conductivity, and resistance, and mitigate thermal electrical noise1. Thus the optimization of the critical passive parts at cryogenic temperatures concerns a plethora of scientists, including those of NASA concerned with reliable deep-space exploration equipment. A study by NASA examined COTS resistors and capacitors at extreme temperature ranges. Their results demonstrated that the metal oxide 10 kΩ resistors had some temperature dependency (frequency at 1 kHz and 100 kHz) as resistance increased by 14% from 295 k to 83 k2. They also showed that the non-polarized (NPO) ceramic capacitors had only minute changes in capacitance (from 0.010μF to 0.095μF, respectively) at 83 k with a small dissipation factor2. Another study confirmed these results by measuring a 0.35% parameter change in NPO ceramic capacitors from 295 k to 77 k3. Even Teyssandier & Prele4 determined a 0.5% capacitance fluctuation from 300 k to 77 k for a NPO 100 pF capacitor. Patterson, Hammoud, & Gerber5, on behalf of NASA, also conducted a different study on the dielectric properties of film, mica, solid titanium, and electric double layer capacitors at low temperatures. Their experiments showed that a 0.1uF Mica capacitor showed the best results at a broad temperature span with 500 VDC and +-10% capacitance tolerance and very little aging over time5. Interestingly, the solid titanium capacitors displayed a slight decrease in capacitance, a significant increase in the dissipation factor, and an increase of dielectric loss at liquid nitrogen (-196 degrees Celsius)5. Mica, polypropylene, Fabrication of a Transimpedance Amplifier For Use Under Cryogenic Temperatures Robert Schurz1, Renè Padilla2*, and Arden Warner2 Student: Illinois Math and Science Academy, 1500 Sullivan Rd., Aurora, IL Mentor2: Fermi National Accelerator Laboratory, Batavia, IL *Corresponding author: [email protected] INTERNSHIP ARTICLE and polycarbonate capacitors showed excellent stability at these temperatures5. The critical commercial components of our single supply I-V circuit were also modified to a final 10 MΩ metal oxide resistor and a 6.6 pF ceramic capacitor, after testing for a linear DC response and minimal frequency response overshoot (horizontal line) in the desired range (2.5 kHz). These characteristics of the single supply circuit showed a lower percent error (0.2%) for DC response at 77 k than a dual supply with 8% error, frequency overshoot, and more wiring. Typically, loss monitors and other cryogenic measurement devices require long wires to external equipments causing a coupling capacitance that degrades signal quality. A poor signal to noise (S/N) ratio can dramatically affect a small signal and yield unacceptable measurements6. In order to mitigate these effects, a similar study also focused on operating a cold preamplifier at cryogenic temperatures for atomic force microscopes6. Using a COTS OPamp ICL7611, they constructed an I-V converter with a confirmed transimpedance gain of 106 V/A and a bandwidth of 200 kHz at 299.9 k and 77.8 k6. The improved S/N ratio suggests that the I-V converter has great application to other small current measurements6. This is also applicable to loss monitors, as their S/N ratio can be drastically improved by installing the I-V converter inside the cryostat. Since the single supply circuit still had a lot of noise when built, cables were wound through a ferrite core to minimize the Eddy current noise. In addition, a low pass filter could be added, if needed, to further mitigate the degradation of the signal due to noise. It is apparent that various scientists have conducted research on electronics in cryogenic temperatures. Evidently, it appears that the passive parts of these circuits are key to the success of this experiment. We needed to find a resistor and capacitor that works best at liquid nitrogen and helium temperatures with a small temperature coefficient and error. From the studies presented it seems as though the wirewound resistor and ceramic capacitor fit perfectly for our purpose of building a current to voltage converter. Furthermore, it seems that other scientist stumbled on similar limitations of the fact that not many industries make these specific parts so they tested specific COTS products. Several particle accelerator facilities, such as Fermilab (FNAL), have long employed loss monitors in superconducting particle accelerators to detect levels of radiation that can damage equipment. The Superconducting Radio Frequency (SRF) Test facility currently under construction at Fermilab will employ several loss monitors within the cryogenic system itself. Superconductors operate at low temperatures to minimize electrical resistance; as a consequence the cryogenic accelerating cavities are confined to Robert Schurz, Renè Padilla, and Arden Warner Page 2 of 4 Experiments and Results In order to implement a current to voltage (I-V) circuit in cold sections of the accelerator at Fermilab, we designed a series of five test circuits using a commercially available 1.4 MHz operational amplifier (model number ICL7611), two 0.1 μF capacitors, BNC connectors, Teflon wiring, and G10 FR4 metal clad circuit board shown in. For our first DC and frequency response tests at room temperature we employed a 1 MΩ feedback resistor Rf and a 3.3 pF ceramic phase compensation capacitor Cf for the reference and five test circuits. In order to determine the DC response of the amplifier, we measured the output voltage (Vo) versus the input current (V=I*Ri where Ri=1MΩ and V=0-7 Volts). The bandwidth at 3db point was determined with a sine wave generator (1Vp-p) by plotting the output Vp-p wave versus the input frequency analyzed with the oscilloscope. We observed an overshoot phenomena called a gain peak for the frequency response. After conducting the first series of experiments we eliminated gain peak and simplified the circuit by removing the two 0.1 μF capacitors and changing the capacitance of Cf to 4.7pF. However, further tests had to be conducted since the bypass capacitors help keep the operational amplifier from self-oscillating and improve its transient response. Tests were then conducted at two different temperatures: 273 k and 76 k. To protect the components from the cooling liquid, we encapsulated the circuit. The first circuit was cleaned and encapsulated with a gray, cryogenic withstanding paste RTV157 to be first measured at room temperatures. Results from the two tests indicated that this paste did not successfully seal the circuit and so we encapsulated the next circuit in a box with an epoxy (Emerson & Cuming Stycast 2651mm black epoxy and catalyst 23LV). Eventually the epoxy was applied in several individual layers to make this method work. After successfully testing the circuit at room temperature the cryostats7. Such an environment is not well suited for components of conventional loss monitors other than the sensor. Particles moving in the accelerator experience Bremsstrahlung, emitting radiation in forms of x-rays that can be detected by a sensor: in our case a helium gas chamber outputs a current when hit by x-rays. Typically, long wires must connect this output signal to external equipment, such as a current-to-voltage (transimpedance) amplifier, outside of the cryostat. Due to long wiring, noise is picked up from the environment causing the signal quality to decrease6. Implementing a transimpedance amplifier within the cryostat can mitigate noise coupling. Thus our study focuses on constructing a Commercial-Off-The-Shelf (COTS) single signal input amplifier operational at cryogenic temperatures with a low signal-to-noise ratio and investigating its characteristics. Figure 1. Modified single supply transimpedance amplifier with 10Ω Resistor and 6.6 pF Capacitance. This figure illustrates the schematics employed during optimization of circuit characteristics. Parts are labeled accordingly. Figure 2. Frequency response before (A) and after (B) encapsulation in epoxy for the final single-supply circuit with a 6.6pF feedback capacitor and 10MΩ feedback resistor at room temperature. The additional resistance from the epoxy (when encapsulated) mitigated the gain peak and stabilized the frequency response. The effects on bandwidth for the unipolar circuit were minimal. A B Robert Schurz, Renè Padilla, and Arden Warner Page 3 of 4 entire box was immersed in liquid nitrogen and a response was measured. This measurement was repeated at room temperature to make sure that the circuit properties had not changed. The data was used to do a percent error analysis and predict the characteristics at helium temperatures. The final circuit was modified from a dual (±8V) to a single supply (-15V) with lower noise in order to be able to implement the circuit into the cavity. After examining a series of DC and frequency response tests the feedback network was again changed to a 6.6 pF capacitor and 10 MΩ resistor as shown in Fig 1. Figure 2 summarizes the results for the final single supply circuit with a 6.6 pF feedback capacitor and 10 MΩ feedback resistor before and after encapsulation at room temperature. Note the elimination of the gain peak after adding the epoxy. At liquid nitrogen temperatures the bandwidth of the circuit was decreased from 3 to 2.5 kHz and the offset for DC response was minimized as represented in Fig. 3. A final plot of the DC response at various temperatures including the predicted liquid helium temperatures shows more stability throughout the temperature ranges as indicated in Fig. 4, which includes low slope uncertainties. The model predicts that colder temperatures will have Figure 3. DC (A) and Frequency (B) response for the final encapsulated single-supply circuit with a 6.6pF feedback capacitor and 10MΩ feedback resistor at liquid nitrogen temperature. The DC response did not change significantly at the cryogenic temperature and experienced a decrease in offset. However, the bandwidth decreased to 2500Hz, which is still in the desired working range, under these conditions. A short signal to noise ratio test suggest an addition of a low pass filter to mitigate noise. The results indicate that this unipolar circuit is suitable for implementation in the superconductor. A B Figure 4. Overview of DC response for a single supply circuit(6.6 pF feedback capacitor and 10MΩ feedback resistor) atroom (blue diamond obscured), liquid nitrogen (red square),and predicted liquid helium (green triangle) temperatures.The uncertainties on the slopes of room, liquid nitrogen and heliumtemperatures are +/0.00028, +/0.00018, and +/0.00015,respectively.Table 1. Percent error calculation forDC response at room(RT), liquidnitrogen(LnT), and predicted liquidhelium temperatures(HeT) for the finalencapsulated single-supply circuit witha 6.6pF feedback capacitor and 10MΩfeedback resistor. AcknowledgementsThe study was made possible by the collaboration of IMSA andFermilab efforts. The author would like to thank all SIR staffmembers, that have made the investigation possible.References1. Soyars, W., Bossert, R., Darve, B., Degraff, B., Klebnar, A.Martinez, A., Pei, L., & Theilacker, J. (2008). SuperconductingRadio-Frequency Modules Test Facility Operating Experience.AIP Conference Proceedings, 985, 127-134. 2. Hayashi, K., Saitoh, K., Shibayama, J., & Shirahama, K. (2009).A Current to Voltage Converter for Cyrogenics Using a CMOSOperational Amplifier. Journal of Physics: Conference Series, 150(1),1-4. 3. Balestra, F., & Ghibaudo, G. (2001). Device and CircuitCryogenic Operation for Low Temperature Electronics. Boston:Kluwer Academic Publishers. 4. Patterson, R., Hammoud, A., & Dones, K. R. (2009). Evaluationof Advanced Cots Passive Devices for Extreme TemperatureOperation. Springfield, Va: NASA. 5. Lui, T., Gong, D., Hou, S., Liu, C., Su, D.-S., Teng, P.-K.,Xiang, A.C., & Ye, J. (2012). Cryogenic Digital Data Links for theLiquid Argon Time Projection Chamber. IOP Science: Journal ofInstrumentation, 7 (1), 1-14. 6. Teyssandier, F., & Prele, D. (2011). Commercially AvailableCapacitors at Cryogenic Temperatures. Ninth InternationalWorkshop on Low Temperature Electronics, 1, 97-103. 7. Patterson, R. L., Hammoud, A., & Gerber, S. S. (1998).Evaluation of Capacitors at Cryogenic Temperatures for SpaceApplications. Springfield, Va: NASA.DiscussionIt is apparent that various scientists have conducted research on electronics in cryogenic temperatures. Evidently, it appears that thepassive parts of these circuits are key to the success of this experiment. We needed to find a resistor and capacitor that works best atliquid nitrogen and helium temperatures with a small temperature coefficient and error. From the studies presented it seems as though thewirewound resistor and ceramic capacitor fit perfectly for our purpose of building a current to voltage converter. Furthermore, it seemsthat other scientist stumbled on similar limitations of the fact that not many industries make these specific parts so they tested specificCOTS products.We have successfully constructed a single supply transimpedance amplifier using only COTS components, such as a 6.6 pF capacitor,a 10 MΩ resistor, and a 1.4 MHz OPamp ICL7611 and confirmed its operation at liquid nitrogen temperatures with a bandwidth of 2.5kHz. This I-V circuit, with minimal noise, is now ready for implementation in cryogenic loss monitors throughout the superconductingaccelerator to protect machinery from radiation. In addition, this circuit is useful for measuring small currents in various cryogenicenvironments.no significant effects on the linear relationship accuracy for the unipolar circuit. The percent error for the single supply circuit is lower thanthat of the dual supply as shown in Table 1. The percent error for this unipolar circuit was minimal compared to earlier calculations forthe dual supply circuit. These results indicate that such a unipolar circuit is suitable for implementation in the superconducting accelerator.Robert Schurz, Renè Padilla, and Arden WarnerPage 4 of 4