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Detector for the measurement of the kinetic energy of microparticles in space

Categories:
  • Fundamentals of Metrology
06.10.2008

In cooperation with partners from industry and from universities, a detector has been developed for the in-situ measurement of the kinetic energy of impacting, fast microparticles. An array composed of sensible microcalorimeters measures the heating of the absorber caused by the particle impact. The novel measuring principle offers - compared to the detectors so far used - considerably smaller measurement uncertainties and a higher reliability.

To an ever increasing extent, space debris endangers the unlimited usability of near-Earth space. With more than 4000 rocket launches worldwide, especially a very great number of microparticles has - in addition to the actual payloads (satellites) - been brought into space as, e.g. residues from rocket motors or fragments produced in explosions. Due to the usually high relative velocity in the case of a collision - typically 10 km/s - even particles in the micrometer range have a damaging effect. To estimate and minimize the danger caused by small particles, these particles are statistically assessed with the aid of models. The models require authentic particle measurement data as data basis, whereby the measurands detection moment, particle size, velocity and direction furnish the most important information.

As none of the existing detectors presently deliver these data with satisfying quality, the development of a two-stage detector assembly called AIDA (Advanced Impact Detector Assembly) was initiated in a research cooperation with eta_max Space GmbH, Braunschweig [1]. Compared to existing measurement procedures, the selected two-stage measurement concept offers clearly smaller measurement uncertainties and a higher reliability. The first measurement stage shall determine - by non-contact measurement - the vectorial velocity of an impacting particle via laser light curtains, and the second measuring stage determines its kinetic energy from the material heating while the particle hits a detector surface.

In the past few years, a breadboard model of the calorimetric energy detector AIDA-cal has been successfully developed and tested within the scope of a study promoted by ESA/ESTEC. The measuring set-up is based on a two-dimensional thermopile array of the IPHT Jena with 16x16 elements each, which is fitted with a correspondingly structured array made of small metallic absorber plates [2, 3]. The heating of the hit absorber (area: 3.6 x 3.6 mm², and thickness: some micrometers) resulting from a hyper-velocity impact (HVI) is measured by the thermopile below it which is thermally connected with the absorber via a glue dot. HVI tests with accelerated iron dust particles performed with the breadboard model at the MPI for nuclear physics in Heidelberg have shown that the calorimetric procedure is very well suited for the measurement of the kinetic energy of fast microparticles. Figure 1 shows a characteristics recorded with gold absorbers 2.8 µm in thickness. The diagram visualises the measuring signals of 131 particle impacts of different energies. The efficiency of the conversion of kinetic energy of the impacting particle into heat energy absorbed by the absorber has been determined for the first time and amounts to approx. 40 %. In the range from 2 km/s to 9 km/s, no dependence on the impact velocity was observed.

Characteristics of the calorimetric energy detector measured in HVI tests: breadboard model, Au absorber 2.8 µm in thickness.

Figure 1: Characteristics of the calorimetric energy detector measured in HVI tests: breadboard model, Au absorber 2.8 µm in thickness.

Now, an advanced measuring set-up of the calorimetric energy detector has been developed in a follow-up project promoted by the DLR in cooperation with additional partners, the institutes IDA, IFS and IWF of Braunschweig Technical University. With this so-called development model, proof was furnished that the technologies required for a future flight model are mastered. Complete absorber arrays with 16x16 elements manufactured at the Department 5.5 were cut out of a gold foil by means of laser ablation (see Figure 2). After reproducibly proportioned glue drops had been applied, the absorber and thermopile arrays were assembled in a semi-automatic process.

Microphoto of the absorber array: development model, small gold absorber plates 50 µm in thickness connected via narrow bars at the edges.

Figure 2: Microphoto of the absorber array: development model, small gold absorber plates 50 µm in thickness connected via narrow bars at the edges.

Figure 3 shows a finished microcalorimeter array bonded to a sensor board, fitted with gold absorbers 50 µm in thickness. The detector housing in modular construction (dimensions: 25 x 24 x 6 cm³) can accommodate a total of 9 sensor boards, including the measuring electronics (see Figure 4). The energy measuring range of the calorimetric AIDA-cal detector can easily be adapted to the mission-specific needs by the selection of the absorber thickness [4]. The further development towards a flight version will be started within a short time.

Sensor board of the development model

Figure 3: Sensor board of the development model

Development model of the calorimetric detector AIDA-cal:  The detector housing is fitted here with two (of nine) sensor boards.

Figure 4: Development model of the calorimetric detector AIDA-cal: The detector housing is fitted here with two (of nine) sensor boards.

[1] K.-D. Bunte; M. Kobusch; J. Hollandt; J. Illemann; F. Jäger; M. Gläser; S. Sarge: AIDA - An Advanced Impact Detector Assembly, International Astronautical Congress 2003, Sept. 29-Oct. 3, Bremen, Germany, CD-ROM, IAC-03-IAA.5.P02.

[2] M. Kobusch; F. Jäger; K.-D. Bunte; T. Fichna: Breadboard Model of a Calorimetric Impact Detector, Proceedings of the 4th European Congress on Space Debris, April 18-20, 2005, ESOC, Darmstadt, pp. 189-194.

[3] M. Kobusch; F. Jäger; K.-D. Bunte; T. Fichna; E. Kessler: Calorimetric Energy Detector for Space Debris, 57th International Astronautical Congress, October 2-6, 2006, Valencia, Spain, DVD-ROM, IAC-06-B6.3.9.

[4] M. Kobusch; S. Sarge; K.D. Bunte; T. Fichna; D. Hagedorn; F. Jäger; R. Meeß: Microcalorimeter array for the measurement of kinetic energies of small particles in space, 2008, zur Veröffentlichung eingereicht in Thermochimica Acta.

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M. Kobusch, Department 1.3, WG 1.34, michael.kobusch@ptb.de

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