Temperature fluctuations are an important physical phenomena to take into account for LISA and for LPF. In §1.5.1 we have presented the different mechanisms whereby temperature fluctuations cause forces in a macroscopic body and thus, challenge the free fall of the body. For this reason in LISA and LPF temperature fluctuations must be kept within certain limits. The required temper- ature stability is based on calculations (and on ground experiments) assuming certain feedthrough factors14.
In view of this, the role of the thermal diagnostic subsystem in the LTP on-board LPF is twofold: on the one hand it is used to excite thermally different subsystems of the LTP (mainly, the GRS and the OMS) to estimate the transfer functions between temperature and TMs acceleration and OMS performance. On the other hand, it must monitor the temperature stability in the LCA which is ensured by a suitable thermal shield that keeps the fluctuations at the level of ∼10−4K Hz−1/2 in the milli-Hertz range —see§1.5.1. With this information the purpose of the thermal diagnostic subsystem is to provide information to identify the fraction of noise in the test masses motion caused by thermal effects, with the goal of diagnosing the LTP performance, and guiding the search for the final sensitivity leap from Eq. (1.8) to (1.7). This is the reason why the LTP diagnostics in general is such an important subsystem: it would surely be less relevant should LPF be the ultimate mission, i.e., with no further projection into LISA.
The following chapters focus on aspects of the thermal diagnostics on-board LISA Pathfinder, specially, on the temperature measurement subsystem (TMS). They are organised as follows:
In chapter 2 we describe the TMS designed for the LTP. The system must reach a noise equiv- alent temperature of 10−5K Hz−1/2 in the LTP measurement bandwidth. Aspects related to the positioning and connections of the items forming the LTP TMS are also discussed. Chapter 3 focuses on potential problems of placing temperature sensors in the GRS. First, we
analyse the compatibility of the sensors with the required magnetic cleanliness in the GRS. Temperature sensors in the LTP are thermistors which exhibit a tiny ferromagnetic behaviour. Second, we take into account the capacitive coupling between the cables of the thermistors and the high frequency signals present in the capacitive sensor since they can degrade the TMS performance.
In chapter 4 we describe the test bed designed in order to validate the LTP TMS. The validation requires to place the temperature sensors in an environment where fluctuations are kept below 10−5K Hz−1/2 for f ≥ 1 mHz. Another test bed to validate the noise performance of the sensors in the LISA band (f ≥ 0.1 mHz) is also described.
Chapter 5 presents the results of the different test campaigns for the validation of the TMS. The results belong to the prototype validation campaign, to the engineering model (EM) cam- paign and to the flight model (FM) campaign. In addition, results of the noise investigations of the TMS in the LISA band are presented.
14Feedthrough factors stand for the coefficients relating thermal effects with TM motion (radiation pressure,
Chapter 6 focuses on the improvement of the LTP TMS in view of the LISA mission which is more demanding in terms of bandwidth and sensitivity. Effects related to the analog-to- digital converters that limit the sensitivity at low frequency are discussed. The reduction of the floor noise of the TMS is also analysed.
Chapter 7 deals with the issues related to the thermal experiments to be performed in the GRS in order to estimate the feedthrough factors relating temperature and forces induced in the test masses. An analysis of the suitable signals to extract the maximum information of the thermal experiments is presented altogether with simulations.
Finally, in chapter 8 we present the conclusion of this work. A few appendices are added to expand technical details.
The LTP temperature
measurement subsystem
In this chapter we describe in detail the LTP temperature measurement subsystem (TMS). Once the noise requirement of the measurement has been defined the design of the system can be addressed. The TMS must comply with the noise requirement given in Eq. (1.27) —cf.§1.5. We write it down again
ST , req1/2 (ω) ≤ 10−5K Hz−1/2, 1 mHz ≤ ω/2π ≤ 30 mHz (2.1) in the temperature range of 10 to 30 . This means a root-mean-square (rms) noise of ∼ 0.3 µK in a bandwidth of 1 mHz.
The chapter is organised as follows: first, we review the state of the art in low noise temperature measurements. Second, the LTP TMS is described: in§2.2 the temperature sensor itself and in §2.3 the analog and the digital signal processing chains [119]. The last section of the chapter deals with the positioning and the connections of the thermal diagnostic devices in the LTP.
2.1
State of the art
A variety of techniques are available to detect temperature variations [28]. Our interest lies in the detection of temperature fluctuations of small amplitude (< 10−4K Hz−1/2) at the frequency of the milli-Hertz and at room temperature. Different techniques, configurations and temperature sensors have been designed for this purpose [28, 111, 159, 54, 161, 60]. Considerable efforts have been done in the development of bolometers for applications such as infrared astronomy and microcalometry. Basically, all these designs are based on an ac bridge and the subsequent signal demodulation at the bias frequency. This configuration allows measurements of temperature fluctuations of ∼ µK Hz−1/2 at low frequencies (tens of milli-Hertz). However, most of these designs have been used at low temperature. Table 2.1 summarises the main results reported in the literature.
Low noise temperature measurements are also required in microdegree temperature controllers which are important devices in many areas of research and applications (investigations in pure fluids, laser heterodyne research [45], study of weak teleseismic signals [40], microgravity experiments on fluids, etc.) [144]. Most of the designs described in the literature use ac techniques altogether with lock-in amplifiers and thermistors as the sensing element. Attempts with dc techniques are also done, however, the temperature stability achieved is worse due to problems related to offset drifts, thermally induced voltages, 1/f electronic noise, etc. The results reported of controllers at room temperature are of tens of micro-Kelvin during a few hours1[144, 40, 45].
Ref. T [K] NET [K Hz−1/2] f [Hz] Technique
[29] 2.17 . 10−10 &0.01 SQUID
[35] 2.17 . 5 × 10−11 (not specified) SQUID
[53] 2.17 . 5 × 10−10 (not specified) SQUID
[158] 3.31 . 10−9 &0.1 SQUID (for low temperature
experiment in Earth orbit)
[159] 0.3 6 · 10−8 0.01 NTC (Ge:Ga)+ac bridge
[37] 0.3 10−6 0.035 NTC (Ge:Ga)+ac bridge
[110] 87.4 8 · 10−6 10 YBCO thermometers
[54] 0.1 — 0.1 thermistor+capacitive load in ac bridge
[60] 90 10−4 0.001 YBCO+ac bridge
[161] 300 6 · 10−7 10 LSMO thermometers
Table 2.1: Summary of high-sensitivity temperature measurement system designs re- ported. The ones in the first rows use SQUIDs (superconducting quantum interference device) in conjunction with materials that change their magnetisation with temperature.
All in all temperature fluctuations measurements at the micro-Kelvin level and at the milli-Hertz range at room temperature have been hardly done. Results for uncooled (at room temperature) bolometric detectors have been reported in [161], where noise levels of 0.6 µK Hz−1/2 at 10 Hz and dissipating 10 mW in the sensor are shown. Another design is reported for an active cavity radiometer where platinum resistance thermometers and yttrium barium cuprate (YBCO) super- conducting thermometers were tested at room temperature exhibiting noise levels of 10 µK Hz−1/2 at 0.1 Hz [60]. In view of this, dedicated investigations in the design of a system able to meet the requirement set in Eq. (2.1) are mandatory. Furthermore, the design of the system is subject to space missions constraints such as availability of space qualified components, power constraints or room.