Merk, B.

Voss, K.-O.

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High-Precision Heating Stage for the Bio Endstation of the MicroprobeK.-O. Voss∗1 and B. Merk1,21GSI Helmholtzzentrum fu¨r Schwerionenforschung GSI; 2Technische Universita¨t DarmstadtIntroductionFor single-particle irradiation with sub-micron accuracytemperature control of the entire microprobe facility is es-sential. With relevant lengths of the most sensitive partsof the setup ranging around tens of cm and a thermal ex-pansion coefficient of steel or aluminum around 15-25 µmper meter and Kelvin one can expect a temperature changeof 100 mK to introduce a shift of a few hundred nanome-ters. With the current absolute targeting accuracy of oursystem being 700 nm such a shift is not negligible and weare thus continuously striving for improvements of this sit-uation. For this reason, sources of heat have either beeneliminated whenever possible (e.g. use of LED illumina-tion as soon as commercially available [1]) or thoroughlyseparated from the most critical components. At the sametime, these components have been kept at minimum lengthsand are allowed to thermally stabilize before precision irra-diations are conducted.In conflict with this struggle, the vast majority of exper-iments depend on the introduction of various heat sources.Here, we report on a heat source with up to a whopping25 Watts being flanged onto the thermally sensitive micro-scope stage of the bio endstation. This heat source allowsfor accurate irradiation of cell cultures at 37 ◦C insteadof the constant but unregulated 30 ◦C with the unheatedstage. The temperature is deemed to be a contributing pa-rameter for absolute measurements of protein kinetics [2],where temperature is expected to strongly modify micro-’viscosity’.Lazy Man’s Approach – On/Off Thermostat-+NTC heaterNTCheater-+-+-+-+ΔU-ΔU-∫UdtΔU+∫UdtFigure 1: Schematic of a simple on/off-type thermostat(left) and a slightly more elaborate PI regulator (right).A simple circuit that reads from a temperature sensorand drives current through heating resistors whenever themeasured temperature is smaller than 37 ◦C is shown infig. 1 (left). Here, a negative-temperature-coefficient re-sistor (NTC) mounted as close to the sample position aspossible is part of a voltage devider. The thermo-voltageis compared to a reference voltage, with the full compara-tor sweep switching the current through the heating resistor∗K.-O.Voss@gsi.deon or off when the sensor reading is too low or too high,respectively. Using this regulator, we found a temperaturehysteresis at the sample position of 0.4 K resulting in a ther-mal drift measured to be 1.2 µm. The hysteresis dependson the distance of heating and sensing elements as well asheat capacity and thermal conductivity of the stage and is assuch difficult to improve while sticking with the thermostatapproach.OK, That Didn’t Work – PI RegulationTo cater for the thermal ’inertia’ of the stage, the circuitin fig. 1 (right) introduces an integrating branch yielding asignal that represents the past difference between set-pointand measured temperature. This slow integral (I) part issummed up with a voltage proportional (P) to the currenttemperature deviation from the set-point to speed up re-covery from temperature excursions. The sum then adjuststhe collector-emitter resistance of a power transistor, effec-tively regulating the current flowing through the heater.   0 1 2 3 4 5 6 7 837,53636,537Time / minTemperature / °CFigure 2: Recovery of the set-point temperature after aforced temperature excursion (arrow at 30 s). For this 1 Kexcursion, temperature returns to set point within roughly100 s and remains stable to less than 1/20 K. Arrow at 5minutes marks small forced excursion to higher temper-atures. Temperature measured at sample site, fully inte-grated into microbeam setup.Figure 2 shows the response of the well admixed propor-tional and integral branches to an excursion one might ex-pect from a sample exchange (1 K) with a very small over-shoot on the way back to 37 ◦C. When locked to 37 ◦C, nomovement of the stage is detectable.References[1] B. Merk, B.E. Fischer, R. Neumann, K.-O. Voss, GSI Sci-entific Report 2008, 361[2] B. Merk, K.-O. Voss, I. Mu¨ller, B.E. Fischer, B. Jakob, G.Taucher-Scholz, C. Trautmann, M. Durante, Biophysics sec-tion of this GSI Scientific ReportGSI SCIENTIFIC REPORT 2012 PNI-INHOUSE-EXP-52421  PNI-INHOUSE-EXP-52 GSI SCIENTIFIC REPORT 2012422

High-Precision Heating Stage for the Bio Endstation of the Microprobe

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High-Precision Heating Stage for the Bio Endstation of the Microprobe

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