In-FEEP Cluster for LISA Pathfinderelectricrocket.org/IEPC/224.pdf · ion beam divergence. This paper shows the design of this new In-FEEP 4x4 Cluster, and experimental results on

In-FEEP Cluster for LISA Pathfinder

I E P C - 2 0 0 5 - 2 2 4

Presented at the 29 th International Electric Propulsion Conference, Princeton University, October 3 1 - November 4, 2005

N. Buldrini,* A. Genovese, t and M. Tajmal ~ ARC Seibersdorf research, Seibersdorf A-2444, Austria

In the GOCE Micro-thruster Program, ARC Seibersdorf research (ARC-sr) proposed a cluster of 12 In-FEEP ion emitters. Each emitter was commanded by an individual high voltage power supply, thus increasing the complexity and the costs of ihe thruster system. ARC-sr is now developing a new In-FEEP Cluster prototype which can operate with a single power supply. This cluster has been designed to fulfil the challenging requirements of the future ESA mission LISA Pathfinder (LPF). It consists of up to 16 In-FEEP ion emitters, which are connected in parallel to a single high voltage power supply through a pre-resistor of 50 MOhm; this pre-resistor is used to increase the emitter electrical impedance, allowing a more homogenous operation of the cluster (lower scatter of emitter currents). The cluster breadboard includes a focusing electrode, in order to fulfil the LPF requirements in terms of ion beam divergence. This paper shows the design of this new In-FEEP 4x4 Cluster, and experimental results on a smaller 2x2 cluster, which is fully derived from the 4x4 breadboard. This experimental characterization shows that the new In-FEEP Cluster concept can fulfill all LISA Pathfinder requirements.

I. I n t r o d u c t i o n

D uring the GOCE Micro-thruster Program and follow-up endurance tests, 1'2'3 ARCS has collected more than 3800 hours at a mean thrust of 10 gN with one micro-thruster, and 18000 hours of cumulative operation with several thrusters. The lessons learned during this intensive test campaign allowed to greatly improve the performance of the indium FEEP emitters: higher mass efficiency, lower erosion, higher lifetime. These emitters have a maximum continuous thrust level of 10-12 gN. In older to fulfill the thrust requirements of missions like LISA Pathfinder (100 gN), it is necessary to cluster a certain number of emitters. In the GOCE Micro-thruster Program, ARCS proposed a cluster of 12 In-FEEP ion emitters. Each emitter was commanded by an independent high voltage power supply, thus greatly increasing the complexity and the costs of such a cluster.

ARCS is now developing an In-FEEP Cluster which can operate with a single power supply. Two cluster breadboards have been designed and manufactured, a 4x4 cluster with 16 In-FEEP emitters and a smaller 2x2 cluster with 4 emitters, which has been recently tested and characterized.

II. 4x4 I n - F E E P C l u s t e r

ARCS has designed and manufactured a 4x4 In-FEEP Cluster Breadboard, which will be used as a test-bed for the LISA Pathfinder Program; hence, the design is based on the requirements of the LPF mission. The cluster consists of two separable parts: the main module and an electronic box screwed at the bottom of the module (Fig.l). Its main characteristics are as follows:

Staff Member, Space Propulsion, email: [email protected] t Senior Staff Member, Space Propulsion, email: [email protected]

Department Head, Space Propulsion, email: [email protected] 1

The 29 th International Electric Propulsion Conference, Princeton University, October 3 1 - November 4, 2005

• 16 In-FEEP ion emitters, each one with an indium reservoir of 15 g • Maximum thrust = 200 laN

• Integrated 50 MOhm pre-resistors, which allow cluster operation with single power supply • Ion beam focusing system; beam divergence < 30 ° (half-angle), as required by the LPF mission. 4 • 8 mm extractor holes, with no need of extractor heaters for indium evaporation • Bias-able cover-plate electrode, in order to shield the emitters from secondary electrons

• Multi-layer design, for an easy mounting and replacement of components • Total impulse of 8000 Ns, assuming a low value lbr the mean mass efficiency (40%, in a recent 5000h

endurance test a mean mass efficiency of 70% has been demonstrated)

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Figure 1. The 4x4 In-FEEP Cluster

A. Multi-layer Design The 4x4 Cluster construction is based on a multi-layer design. Each layer is inserted from atop forming a stack.

The electrical contacts are assured by spring connectors, thus eliminating the need for screwing and welding. This philosophy allows an easy and safe mounting and dismounting of the assembly, essential in the experimental phase. Fig. 2 depicts a sectional view of the cluster in which is possible to distinguish each layer. Following is a short description of the layers and their functions:

Base Plate Made out of high temperature plastic (Peek), it supports the central screws (thus giving rigidity to the assembly) and the spring connectors of the extractor and of the cover plate.

Radiation Shield 1 mm thick aluminium plate mirrored on the top side: reflect the radiated heat coming from the heating plate, reducing power losses towards the bottom, moreover protecting the electronics.

Heating Plate 2 mm thick aluminium sheet. It distributes the heating power generated by two Minco Thermofoil TM mica heaters to the 16 emitters. Two PT100 sensors are installed on the plate to measure the temperature near the centre and on the edge.

Emitter Lock Plate It press down the emitters to the heating plate, keeping them on place. It hosts also the high voltage feeding system that connects the emitters to the focusing electrodes. The material is high temperature plastic Sintimid Pur HT (polyimide).

2 The 29 th International Electric Propulsion Conference, Princeton University,

October 3 1 - November 4, 2005

Extractor Layer One piece aluminium electrode. Each extractor hole is shaped in a special way to form a labyrinth with the previous layer in order to prevent cross-contamination between the emitters. This electrode is connected at ground potential by means of a spring connector screwed on the base plate.

Focusing Layer 16 aluminium focusing electrodes are screwed in a holding plate made out of Sintimid Pur HT (polyimide). Each electrode is electrically connected to the respective emitter through a spring contact.

Cover Plate Layer One piece aluminium electrode. This electrode is connected to - lkV by means of a spring connector screwed on the base plate.

Figure 2. Sectional view of the 4x4 In-FEEP Cluster

B. Ion Beam Focusing System

Due to the principle by which a liquid metal ion source (LMIS) creates ions and accelerates them (space charge), the resulting ion beam diverges depending on the emission current. While currents of 10~A are spread over a half-cone angle of 30 °, currents of 100 ~A are emitted in a 60 ° half angle, as shown in Fig. 3 for an emitter current of 120 ~A. In order to meet the LPF maximum required beam half-angle divergence of 35 °, a focusing electrode was designed, which is on the same potential as the emitter. For this purpose, a SIMION simulation was developed, following the ions from its initial beam divergence close to the tip up to the exit of the thruster. Simulating ion beams at currents ranging from 0 to 300 ~A and observing the effect of the designed focus electrode on the ion beam divergence gives a performance envelope from which the beam divergence during operation at any current can be deducted. Fig. 4 shows such a performance envelope of a focus which is capable of safely guiding currents up to 300~A. This indicates that, up to this critical current, no charged particles come in contact with any surface and that there is no danger of sputtering caused by ion impacts.

In the 4x4 In-FEEP Cluster, there is a focusing electrode for each ion emitter, biased at the same high voltage of the corresponding emitter]



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C. The H e a t i n g S y s t e m

Fig. 5 shows the assembly of the heating plate. Two Minco Thermofoil TM mica heaters (HM6807-R3.9) provide the needed heating power. Each heater is rated 200W max and can withstands up to 600°C, s o they will operate well below their limits (180°C are needed). The heat will flow to the emitter through a specially shaped aluminium oxide shell. The aluminium oxide has the highly desirable property to be a pretty good heat conductor (- 20 W/m K @ 300°C), and being at the same time an optimal electrical isolator (22 kV/mm). The multiple grooves on the shell increase the surface path from the emitter (and from the emitter HV line) to the heating plate, which is at ground potential, limiting the danger of insulation losses.



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The right picture in Fig. 5 shows the bottom part of the actual heating plate: the two heating elements and the two mountings for the PT100 temperature sensors are visible. Each mica heater is pressed to the plate by one rigid metal backing disc, isolated from the heater by a ceramic paper sheet.

D. Integrated Electronics The sequence in Fig. 6 shows the circuitry contained in the electronic box of the 4x4 Module. The electric schematics is shown in Fig. 7. The In-FEEP Cluster is operated with a single high voltage power supply (HV IN). Each ion emitter is connected to the power supply through a pre-resistor of 50 MOhm (R1 - R16), in order to increase the electrical impedance and to reduce the scatter in individual thrusts.

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Figure 6. Top, middle, and bottom view of the 4x4 In-FEEP integrated electronics

The HV dividers monitor the FEEP emitter operating voltages and create a proportional low voltage signal, which can be fed into the data acquisition system. They consist of high voltage, high temperature, high precision, low temperature drift 5GOhm resistors (R17 to R32), and high precision, low temperature drift, metal-oxide 3.3MOhm resistors (R33 to R48), with a total conversion factor of 1:1516 (10000V to 6.6V, 15161V to 10V). The supply voltage (OUT_COMM) is also monitored (R49, R50), and knowing the pre-resistor value and the leak current through the dividers, the individual emitter current can be calculated. From the individual emitter voltage and current values, the actual thrust of each emitter can be finally calculated. Capacitors of 22 nF (C1 - C17) are used to smoothen the output voltage and to reduce noise, and fast 16V Zener diodes (D 1-D 17) are used to eliminate over- voltages in order to protect the data acquisition card inputs.



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E. Mass and Volume In Fig. 8, the dimensions of the 4x4 Cluster are reported. The base, where the mechanical interfaces are, has the following dimensions: 200x168 mm. The cover-plate, that represents the upper part of the module, has a square shape with a 172 mm side. The height measured from the rim of the plume shields and the bottom of the housing is 80mm. With the electronic box the total height becomes 122mm. The dimensions of the electronics box are 146x146x40 mm (excluding the mounting flange). This box is only used on ground to monitor the individual electrical parameters; it will not be included in a In-FEEP Flight Model. The total mass of a 4x4 module, without electronic box, is about 1.9 kg. The weight is so distributed:

• Main housing without electrodes: 770 g • Weight of 16 (indium filled) emitters:-~512 g • Emitter lock plate: 130g • Extractor electrode: 110g

• Focusing electrode: 146g • Cover-plate: 231 g

Figure 8. Dimens ions of the 4x4 In -FEEP Cluster



III . 2x2 I n - F E E P C l u s t e r

In order to verify the LPF requirements, ARC-sr has recently manufactured and tested a 2x2 In-FEEP Cluster, based on the same design of the larger 4x4 Cluster (see Fig. 9). In particular, its main features are:

• 4 In-FEEP ion emitters, each one with an indium reservoir of 15 g • Maximum thrust = 50 ~N • Ion beam focusing system; beam divergence < 30 ° (LPF requirement < 35 °) • Total impulse of 2000 Ns, assuming a low value for the mean mass efficiency (40%, in a recent 5000h

endurance test a mean mass efficiency of 70% has been demonstrated)

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Figure 9. The 2x2 In-FEEP Cluster

F. Electrical Set-up

The 2x2 In-FEEP Assembly Breadboard has been tested in the LIFET2 vacuum chamber. This chamber is equipped with an Ion Beam Diagnostic System for beam divergence and thrust vector measurements.

Fig. 10 shows the electrical set-up of the 2x2 In-FEEP Characterization Test. As shown in the previous chapters, The 2x2 In-FEEP Assembly is operated with a single 0-12 kV power supply. Each ion emitter is connected to the power supply through a pre-resistor of 50 MOhm, in order to increase the electrical impedance. The individual emitter voltages are monitored by frequency compensated voltage dividers; this allows to calculate the actual thrust provided by each emitter; the sum of the individual thrusts gives the actual total thrust.

An aluminum collector (70 cm diameter, 1 m deep) is placed in front of the thruster to measure the actual ion current, which is the sum of the four emitter currents minus losses in the extractor, focusing and cover-plate electrodes.

The individual emitter voltages, collector current and voltage and current outputs of the single HV power supply are fed to a commercial data acquisition system (NI - DAQ, 6036E) and stored with 16-bit accuracy.

In this test, the 2x2 Breadboard is operated in thrust stabilization mode (closed loop), controlling the common current and voltage only (voltage and current of the single HV power supply, sensed by a frequency compensated voltage divider). The "thrust" calculated from these two parameters is not the actual thrust, which corresponds to the sum of the four individual thrusts. However, due to costs, complexity and safety reasons, in flight the only operating parameters available will be the common current and voltage; hence, only this "pseudo-thrust" will be known. This test can show how big the difference between pseudo-thrust and actual thrust, and its evolution with time is.

As a consequence, the 2x2 In-FEEP Assembly Breadboard is operated in the pseudo-thrust stabilization mode, i.e. the following "pseudo-thrust" Tp will be commanded and stabilized by digital feedback to the programmed high voltage supply:



Tp - k . I a /U o - I R / N f (t~) k = 1.5427xl 0 -3 (for indium) d)

Where I [/,tA] is the common current, Uo [V] the common voltage, R is the pre-resistor value (50 MOhm), N is the number of emitters in the assembly (4), plus a correction factor f(a) < 1 due to the ion beam divergence. The value in the square root can be considered as a mean emitter voltage. Of course, pseudo-thrust and actual thrust only coincide if the emitters in the assembly have the same current/voltage characteristic.

The Data Acquisition and Control System (DACS) computes this pseudo-thrust function (at a 10 I-Iz rate), compares it with a set value and feeds the amplified difference as a 16 - bit analog signal to the current - or voltage control input of the HV power supply (at a 1 Hz rate). DACS and stabilization tasks are performed by the same software package (NI Lab-View 7.1).

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2 0 0 0

0 -

757 .7

26 /09 /2005

! ! i i i i

. . . . . . . . . . . . . I - . . . . . . . . . . . - I . . . . . . . . . . . .

i i i i i i ! ~ ! i i i i i i ! ! ! !

............. i- ........... q ............. ~- . . . . . . 4 . . . . . . . i- ......... ~- ............ i ............ 4 ............ i ............ 4 ........... i i I I I I I i I I i i i i i i ! ! ! ! i i i i i i ! ! ! !

. . . . . . . . . . . . . F- . . . . . . . . . . . q . . . . . . . . . . . . . F, . . . . . . 4 - . . . . . . i - . . . . . . . . . i - . . . . . . . . . . . . i . . . . . . . . . . . . 4 . . . . . . . . . . . . i . . . . . . . . . . . . 4 . . . . . . . . . . . i i i i i i ! ! ! !

! ! ! ! ! ! ! i I I I I I I i i ~ C o m m o n v o l t a g e

. . . . . . . . . . . . . F . . . . . . . . . . . . 4 . . . . . . . . . . . . . ~- . . . . . . + . . . . . . ~- . . . . . . . . . + . . . . . . . . . . . . i . . . . . . . . . . . . 4 . . . . . E m i t t e r 1 ! ! ! ! ! ! ! i

~ ~ ~ ~ ~ ~ ~ .-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-. E m i t t e r 2 ! ! ! ! ! ! ! i

. . . . . . . . . . . . . i . . . . . . . . . . . . . - i . . . . . . . . . . . . . ~- . . . . . . 4 . . . . . . . ~ - . . . . . . . . . ~. . . . . . . . . . . . . i . . . . . . . . . . . . . i . ............................................. E m i t t e r 3 i i i i i i i i

' ' ' ' ~ ~ ' ' ~ E m i t t e r 4

7 5 7 . 8 7 5 7 . 9 758 .0 758 .1 758 .2 758 .3 758 .4 758 .5 758 .6 7 5 8 . 7 758 .8

T i m e [h ]

Figure 13. Individual voltages and common voltage

26 /09 /2005 LPF 2x2 t es t 11234 750h 120 - -

i i i i i i i i i i

- - - " ; : . . . a . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . L . . . . . . . . . . . . * ...................... ; . . . . : ............ ; ~ ....................... , ................... :.,.,.,..~ | ~ ..........................................................................................................................

~= 7o • , . , 6 0 e -

~. 50

0 4 0

2 0

" ! ! ! ! ! ! ! ! ! 90 . . . . . . . , 4 . . . . . . . i - . . . . . - i . . . . . . . . . . . . i - . . . . . . . . . . . -~ . . . . . . . . . . . . . i - . . . . . . . . . . . .4 . . . . . . . . . . . . . i - . . . . . . . . . . . - i . . . . . . . . . . ~ . . . . . . .

I ! i i i i i I 1

80 1 1 L . . . . . . . . . . . L . . . . . . . . . . . J . . . . . . . . . . . . . L . . . . . . . . . . . . . . . . . . . . . | ii

. . . . . . . 1- . . . . . . r . . . . LPF 2x2 Cluster; thrust stabilisation ......... ! i ....................................... E m i t t e r 2

. . . . . . . + . . . . . . ~- . . . . I n d i v i d u a l c u r r e n t s + i i ....................................... E m i t t e r 3 i i e x t r a c t o r a n d c o v e r - p l a t e c u r r e n t s i i ~ E m i t t e r 4 I I ! ! l F . . . . . . . . . . . -I . . . . . . . . . . . . . F . . . . . . . . . . . . . . . . . . . . . . E x t r a c t o r c u r r e n t

30 . . . . . . . 4 - . . . . . . i - . . . . . 4 - . . . . . . . . . . . i-- . . . . . . . . . . . --i . . . . . . . . . . . . . F . . . . . . . . . . . - i ............................................... C o v e r - p l a t e c u r r e n t ! ! ! ! ! !

. . . . . . . 4 . . . . . . . ~ - . . . . . 4 . . . . . . . . . . . . ! - . . . . . . . . . . . J . . . . . . . . . . . . . I - . . . . . . . . . . . - ! . . . . . . . .

. . . . . . . i i I 0 jl .,~I ~I i i i ..... ~:: ! ~ ..... i::

0 ................. ," ............... i ................ ; ............. I ............. ; ............... i ................ ; ................ i ................ ; ............... i ................ ; ................ i ............... ; . . . . . . . . . . . . . . . i ................ ; ................ i ................ ; ............... i ................. ; . . . . . . . . . . . . . . . . . . . . . . , ....................

757 ,7 757 ,8 757 ,9 758 ,0 758 ,1 758 ,2 758 ,3 758 ,4 758 ,5 758 ,6 758 ,7 758 ,8

T i m e [h ]

Figure 14. Individual currents, extractor and cover-plate currents.

Fig. 14 shows that the individual currents differ not more than 10 I~A from each other; the current losses on the common extractor and cover-plate electrodes are very small, < 5 I~A. This is confirmed by the collector current in Fig. 15 (ion current collected by the aluminum collector in front of the 2x2 Cluster, i.e. the real current emitted), which differs from the common current of less than 10 I~A (~ 2%). This is a good indication of the correct functioning of the four focusing electrodes; the four ion beams are well focused, only a small percentage of the current is lost in the electrodes.

10


4 4 0

4 2 0

• ~ 4 0 0 c-

O 3 8 0

3 6 0

7 5 7 , 7

2 6 / 0 9 / 2 0 0 5 L P F 2 x 2 t e s t I H z + l c o l 7 5 0 h _

I . . . . . . . . . . . . . . . . . . . . . . . . ~._J L P F 2 x 2 C l u s t e r ; t h r u s t s t a b i l i s a t i o n | ~ . . . . . . . . . . . .~ [

I l / Co o...,oo,,oo,oro°rro.,. I I I I i i i i i i i i i

. . . . . . . . . i . . . . . . . . . . . . i . . . . . . . . . . . i- . . . . . . . . i . . . . . . . . . . + . . . . . . . . . . . . i- . . . . . . . . i . . . . . . . . . 4 . . . . . . . . . . . + . . . . . . . . . i . . . . - : - - - -

~ , , w , ~ P , , . m - e - . . . . . . . . ~ . . . . . = . . . . . ~ ~ " ~ - = t -,= . . . . . ~ - . . ~ - ~

. . . . . . . . . I . . . . . . . . . . . * . . . . . . . . . . . l . . . . . . . . I . . . . . . . . . . t . . . . . . . . . . . . ~ - - - - - F - . . . . . . . . I . . . . . . . . . . . t . . . . . . . . . I . . . . . . . . . i i i i i i i i i i I ! ! i ! ! i i i i i I i I i i i i i i i i I ! ! i ! ! i i i i i /

. . . . . . . . . I . . . . . . . . . . . 1" . . . . . . . . . . . I- . . . . . . . . I . . . . . . . . . . "1 = . . . . . . . . . . . . I" . . . . . . . . I . . . . . . . . . / - - - I i i i i i i i C o m m o n c u r r e n t I I ! ! i ! ! i i I I / ! ! ! ! ! ! ! C o l l e c t o r c u r r e n t I / , , , , , , , - - .

. . . . . . . . . ! . . . . . . . . . . . 4 . . . . . . . . . . . F . . . . . . . . ! . . . . . . . . . . + . . . . . . . . . . . . F . . . . . . . . ! . . . . . . . . . ~ . . . . . . . . . . . . + . . . . . . . . . ~ . . . . . . • " - ' - - - I ! i ! ! ! ! ! ! ! ! / I I I I I I I I I I /

. i . . I . i . i . I . I . I . I . I . /

7 5 7 , 8 7 5 7 , 9 7 5 8 , 0 7 5 8 , 1 7 5 8 , 2 7 5 8 , 3 7 5 8 , 4 7 5 8 , 5 7 5 8 , 6 7 5 8 , 7 7 5 8 , 8

T i m e [ h ]

F i g u r e 15. C o m m o n c u r r e n t a n d co l l ec to r c u r r e n t .

Fig. 16 shows the thrust range of the 2x2 In-FEEP Cluster; with a maximum thrust of 50 pN, it is shown that the LPF requirement of 100 pN can be fulfilled with a cluster of 8-9 In-FEEP emitters.

Fig. 17 shows that the 2x2 In -Feep Clus te r can p rov ide a m i n i m u m thrust lower than 0.1 p N wi th a commerc i a l

H V p o w e r supply , that is l ower than the L P F target r equ i r emen t (0.3 pN, 0.1 p N target) . No t i ce that, at this thrust level, there is no s ignif icant d i f ference b e tween the actual thrust and pseudo-thrust . Fig. 17 also shows that thrust

steps o f 0.1 p N can be easi ly resolved; hence, the thrust reso lu t ion at ve ry low thrust levels is c lear ly bet ter than the L P F requi rement (< 0.3 pN, < 0.1 p N target).

Fig. 18 sh o ws that it is poss ib le to r e so lve th rus t s teps o f 0.2 p N at the m a x i m u m thrus t o f 45 pN. This is

compl ian t wi th the reso lu t ion requi red b y L P F (< 0.3 pN) . It is expec ted that, wi th an i m p r o v e d thrust s tabi l izer , it

wi l l be poss ib le to have a r e so lu t ion < 0.1 p N even at m a x i m u m thrust .

5O

4O

~ = ~ o

I - 20

10

0

2 6 1 0 9 1 2 0 0 5 L P F 27,2 t es t M a x T h 7 5 0 h

i i I L P F 2 } ( 2 C l u s t e r ; t h r u s t s t a b i l i s a t i o n il i ; - 7 ! ! ' ' I ! j ! m m i i i i m ~ ! I I I I I I

. . . . . T ~ " ! . . . . . . . ,& . . . . . . T . . . . . . . . . . "~.'-! . . . . . . . . . . . . . " r ~ . . . . . . . . . . -T'." . . ,,.

. . . . . ~-- ,L .... 1 ... . ~_._~. ... . J. . . . . . . . . . $._.~! . . . . . . . . . . . . . J L - ~ . . . . . . . ~1_ . . . . . . . . . . . . . J. . . . . . . . . . . . . . 1_ . . . . . . . . . . . _1__. l ~, ! # & ! $ 1 ~ & ! ! ! !

/1 ' i f &[' [ A "i' I [' , • P s e u d o - t h r u s t - - y t - - - ~ . . . . . - I - / - . . . . . ~ ; - t . . . . . - t . . . . . . i ~ - . . . . . . . . l~ . . . . ~: . . . . . ~ - . . . . . . . . . . . . . I - A c t u a l t h r u s t ~ - - "

"~-T . . . . . . E ' - -~ 'E . . . . . . . . . . "~T .... T . . . . . . . ! -"~ . . . . . . g - ~ . . . . . . T--- ' -F . . . . . . . . . . . . . ~ " r . . . . . . . . . . . ~--"

'- .. . . -~ . . . . . . . -~--~ . . . . . . . . . . . . . . ~-.-g- . . . . . . . . i---&-.-.~ .... 4- . . . . . . . . . ~--~ . . . . . . . . . . . . . . 4 - ~- . . . . . . . . . . . 4 - - . ! ~ g ! !~,~" ! ~,k' ! & ! ! ! !

I II ! I I i I i I I , , I , I , I , I

7 5 4 . 8 0 7 5 4 . 8 5 7 5 4 . 9 0 7 5 4 . 9 5 7 5 5 . 0 0 7 5 5 . 0 5 7 5 5 . 1 0 7 5 5 . 1 5 7 5 5 . 2 0

T i m e [ h ]

F i g u r e 16. T h r u s t r a n g e of t h e 2 x 2 I n - F E E P C l u s t e r

11


0 2 / 0 6 / 2 0 0 5 0 . 4 0 , ,

Cluster test MinTh 2 3 0 0 h

0 . 3 5

0.30

, - - , 0 . 2 5

Z .=,. . ~ 0 . 2 0 (/)

L . = 0 . 1 5 I - -

0 . 1 0

0 . 0 5

0.00

i i 2 x 2 I n - F E E P C l u s t e r " t h r u s t s t a b i l i s a t i o n II _ . ' . . . . i --~ ~-.--- ' I1--- ~cl:ual /nrusl: I- . . . . .

[ i M i n i m u m thrust < 0.1 pN II Pseudo thrust I . ._[ L _ _ LPF requirement = 0.3 pN (target = 0.1 pN) I1--- - I1_~

i i Thrust resolut ion

~i +~:~+"1 !;ii ................ ~ !+ i+ i+ i+ i+ i+ i+ i+ i+ i+ i+ i+ i+ i+ +i ++ i+ i+ i+ i+ i+ i+ i+ i+ i+ i i +i i+ i+ i+ i+ i+ i+ i+ i+ i+ i+ ++ ++ ++ i+ i+ i+ i+ i+ i+ i+ i+ i+ i+ ++ i+ i+ i+ i+ i+ i+ i+ i+ i+ i+ i i ++ i+ i+ i+ i+ i+ i+ i+ i+ i+ i+ ii ++ i+ i! ~ .......... iii ....... ++ ~:+"~ : : i ......... +!+iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii++iiiiiiiiiiiiiiiiiiiiiiiiiii+iiiiiiiiiii+!! +' ......... iii ..... +

..... ! ........ :~::::i::Mii i ........... . . . . . . ~+~++:~+~:~!~ ii .i '+i:1 ~ ~ ~::~ .... :~:: ~ i.~:.~'~: i:: :~:i~i: : ....................... :;i+ :;;L;iLI:I::LI ................ :;:i

~ii~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~iiiii~.~.~ .................................................. . - - . - . .~ ............... .......

21/06/2005 150mm_150pA_s tab_500h I | I i | i I I ! | ! ! | ! / I • i i ! i ! ! i !

/ I V15"RN10 e m i t t e r + + + + + B ! B 1 . 2 J . . . . . . ~ 4--..---,---&.--~.-.-~.--.,--J.--.4-.--

/ 1 1 5 0 pA emitter current ~ i ~ m m + m ! m I I . . . . . . . . i + i ~ ~ + ! B ! l 11 ou m m T r o m e m m e r tn i i i ! i ! ! i ! " 1 I , - ~L i ~ i ! i ! ! i ! / I ' ~n - - /U I I ! R ! i ! ! i ! i

/ I ~ o A n k ! ! ! i ! ! i ! i 1 1 - L - I . - ~ v , , .._,__.,,__,___,.__,_._,__.,__,__,.__

• I OA t~L . i ~ i ! i ! ! i ! ' ~ I ................. ;:::~ ............... OL ' I 'U I i | ~ | i | | § ! §

i . . . . . i ~ i i i , i i i i I - - r - - ~vvun + ~ . J l l ~ + ~ ~ + B D B : : : : ; . ! ~ ~ ! ! ! ! ! ! !

1 i V

I I i I i ~ : : i : ~ ~ I i i I i i i ! ~ ! ~ : ' ~ ! i i ' ~ ~ ! ! i !

o.~ . . . . 4-.-+-+--i---~+--4--+--i---~-~+~--F-+--4--+.--

i i ! ~ ~... '.. ': '~ ~ i ! i ! ~ ! ! i ! i i ! i ~ '~ ~ i i ! i ~ ! ~ i I I i U " ~ I I i I i ~ l i i I i ! ! i ~ ! ! i ! i . ! i

R ~ ! i ~ ~ i ~ ~ i . . . . I ' I ' I " I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I '

-64 -56 -48 -40 -32 -24 -16 -8 0 8 16 24 32 40 48 56 64

D i v e r g e n c e h a l f - a n g l e [o ]

Figure 19. Ion beam divergence of the 2x2 In-FEEP Cluster. 7

IV. Conc lus ion

In the framework of a development program for an In-FEEP cluster suitable for the LI SA Pathfinder mission, ARC-sr has designed and manufactured a 4x4 In-FEEP Cluster, which will be used as a test-bed for the LISA Pathfinder Program. This cluster can provide a maximum thrust of 200 pN, and includes a focusing system that reduces the ion beam divergence at maximum thrust from 55 ° to less than 30 °, as required by LISA Pathfinder.

ARC-sr has also manufactured and tested a smaller 2x2 In-FEEP Cluster, based on the same design of the 4x4 Cluster. The characterization test performed has shown that the new In-FEEP Cluster can fulfill all LISA Pathfinder requirements. In particular, the 2x2 Cluster has demonstrated a maximum thrust of 50 pN, a minimum thrust of 0.1 pN, and a resolution lower than 0.1 pN. The focusing system has been tested for 500 hours, the results showing the fulfillment of the design goals (beam divergence smaller than 30 ° at maximum thrust).

Acknowledgments

The authors would like to thank the European Space Agency and the Austrian Space Agency for their financial support, in particular J. Gonzalez and H. Posch. This work could not have been performed without the unlimited help of the ARC-st technicians K. Andres, F. Plesescu, and M. Baca, and the valuable support of our colleagues E. Tamas, I. Vasiljevich, K. Marhold, C. Scharlemann and W. Steiger.

References

1Tajmar, M., Genovese, A., and Steiger, W., "Indium FEEP Microthruster Fxperimental Characterization", Journal of Propulsion and Power, Vol. 20, No. 2, March-April 2004.

2Genovese, A., Tajmar, M., Buldrini, N., and Steiger, W., "2000h Endurance Test of an Indium FEEP Microthruster Cluster", Journal of Propulsion andPower, Vol. 20, No. 2, March-April 2004

3Genovese, A., Tajmar, M., Buldrini, N., Scheerer, M., Semerad, E., and Steiger, W., "Indium FEEP Multiemitter Development and Test Results", Proceedings of the 40 th Joint Propulsion Conference, Florida, 2004

4Hunter, J. C., "LISA Pathfinder Requirement Specification - FEEP Micropropulsion System", EADS Astrium Ltd, S2.ASU.RS.2002, Issue 3, 2005

5Tajmar, M., Andres, K., Plescscu, F., Genovese, A., and Semerad, E., "Clusterf'~ihige Ionenquelle", Austrian patent A1233, 2004

6Rtidenauer, F., and Genovese, A., "Statistical Evaluation of In-FEEP Thruster Types with Respect to Clusterability", ARCS internal report, Oct. 2004

7Vasiljevich, I., Tajmar, M., Buldrini, N., Genovese, A., and Marhold, K., "Development of a Focus Electrode for an Indium FEEP Thruster", Proceedings of the 41 st Joint Propulsion Conference, Arizona, 2005

8Tajmar, M., Marhold, K., and Kropatschek, S., "Three-Dimensional In-FEEP Plasmadiagnostics", Proceedings of the 284 International Electric Propulsion Conference, Toulouse, IEPC-03-161, 2003

9Genovese, A., Buldrini, N., Tajmar, M., Tamas, E., Vasiljevich, I., Marhold, K., and Ruedenauer, F., "Indium FEEP Cluster Development", Proceedings of the 41 th Joint Propulsion Conference, AIAA 2005-4385, Arizona, July 2005



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In-FEEP Cluster for LISA Pathfinderelectricrocket.org/IEPC/224.pdf · ion beam divergence. This paper shows the design of this new In-FEEP 4x4 Cluster, and experimental results on