Michael Tartz

Characterization of a modular broad beam ion source

We characterize the performance of a modular broad beam ion source by energy resolved mass spectrometry and beam profile measurements. Using the same source housing and grid system, we performed our experiments powering the source with a hot filament or an ECR excitation, respectively. In the ion energy distribution we detect various peak structures reflecting the potential profile across the ion source as well as different charge exchange and dissociation processes occurring in the beam. The position of these peaks on the energy scale allows conclusions to be formed about the original charge state of the ion, when produced by charge exchange, or on the original molecule, when produced by dissociation. The most important contribution to the ion beam is caused by process gas ions with important impurities from source materials appearing with reactive process gases. Using electronegative gases, a large amount of negative ions generated in the beam is observed. The performance of both excitation types due to the beam parameters is very similar. A comparable power supplied to the source plasma delivers similar beam currents and profiles. The resulting beam current is determined by the density and the potential structure in the source plasma. The axial potential gradient in the plasma, and with it the resulting beam current, is strongly affected by the total voltage applied between the grids and can be derived from the shape of the main peak in the ion energy distribution. The detected radial beam profile is determined simultaneously by the ion optical parameters of the grid system and the space charge potential in the beam. For a fixed grid geometry the shape of the plasma sheath at the screen grid is an important factor controlling the source performance. The shape of the plasma sheath is simultaneously controlled by the voltage difference between the grids and the Debye length. Changes in the shape can be derived from the ion energy distribution with the experimental results being in good agreement with numerical beam simulations. The only important difference between both excitation types is that using the Kaufman type excitation the potential of the source plasma is exactly fixed by the beam potential, whereas with the ECR source it deviates by several tens of volts depending on microwave power and process gas.

Inductive coupled radio frequency plasma bridge neutralizer

A 13.56 MHz radio frequency plasma bridge neutralizer (rf-PBN) for ion thruster applications as well as ion beam surface processing of insulating materials is presented. The energy for the plasma excitation is inductively coupled into the plasma chamber. Because no components are located inside the plasma, the lifetime of the rf-PBN is expected to be very long. A compact tuning system adapts the input power to the plasma impedance. The electron current may be controlled over a wide range by the rf input power. An electron current of up to 1.6 A has been extracted.

MODELING OF BROAD BEAM ION SOURCES

Our approach to ion beam modeling breaks down the complex interplay of competing elementary processes into a series of independent consecutive steps. Thus, both in methodological development and computational modeling, the main effort can be directed towards the processes which are of greatest relevance in the context given. Moreover, employment of the special software available allows for comprehensive and efficient optimization of multigrid ion sources for low ion beam divergence and long lifetime, even in a personal computer environment. Here, for our Kaufman-type two-grid ion source RR-ISQ 76 some ion beam characteristics in their dependence on geometrical two-grid configuration are calculated and discussed, first of all in terms of shape and area of the plasma sheath. These dependencies are used to optimize this particular ion source for a low beam divergence.Our approach to ion beam modeling breaks down the complex interplay of competing elementary processes into a series of independent consecutive steps. Thus, both in methodological development and computational modeling, the main effort can be directed towards the processes which are of greatest relevance in the context given. Moreover, employment of the special software available allows for comprehensive and efficient optimization of multigrid ion sources for low ion beam divergence and long lifetime, even in a personal computer environment. Here, for our Kaufman-type two-grid ion source RR-ISQ 76 some ion beam characteristics in their dependence on geometrical two-grid configuration are calculated and discussed, first of all in terms of shape and area of the plasma sheath. These dependencies are used to optimize this particular ion source for a low beam divergence.

Sputter investigation of ion thrusters grid materials

We performed angle and energy dependent sputter yield measurements of various ion thruster grid materials under xenon ion incidence in the energy range 200..1400eV and at normal and oblique incidence up to 70°. Materials investigated are metals as molybdenum and titanium and carbon materials as high-dense graphite of various grain sizes, CarbonCarbon material and pyrolytic graphite. No significant difference between the various carbon materials w.r.t. sputter yield was found. The reduction of the sputter yield of the metals by nitriding was proven but it seems to be not useful as material for ion thruster grid.

Validated simulation of the ion extraction grid lifetimea)

This article describes the validation of our extraction grid erosion modeling for gridded ion thrusters. Experimental data from three life tests of different thruster sizes and operation conditions was available. For all test cases, an excellent agreement was achieved between experimental and simulated data, which increased the confidence in the lifetime predictions of the code for space missions.

RIT-µX - High Precision Micro Ion Propulsion System based on RF-Technology

RIT-µX is a radio frequency ion engine for micro propulsion applications. The ionization of the propellant by electro magnetic fields offers inherently highest thrust control, -stability and resolution. The function principle of the engine and the specific advantages are explained and the layout of a propulsion system is presented. Potential missions are described and selected test results are given.

RIT-22 Ion Propulsion System: 5,000h Endurance Test Results and Life Prediction

§** †† ‡‡ §§ Electric Propulsion based on gridded ion thruster technology is the key for several upcoming missions. Namely the European Space Agency ESAs corner stone mission BepiColombo to the Suns nearest planet mercury is a representative for these type of missions. Beside harsh thermal and radiation environment the required total impulse and engine lifetime is a challenge for the electric propulsion system. Astrium GmbH BL Equipment and Propulsion proposes its new ion thruster RIT-22 based on cathodless radiofrequency technology for this mission. In 2006 a successful 3,000h test was extended for another 2,000h. The 5,000h Endurance test demonstrates the compliance between egine performance and mission requirements. This paper describes the ion engine, the test setup and the test facility. One section is devoted to the control software which bases on the flight proven architecture for ARTEMIS. Test results are given and the life time prediction is presented.

Note: An advanced in situ diagnostic system for characterization of electric propulsion thrusters and ion beam sources

We present an advanced diagnostic system for in situ characterization of electric propulsion thrusters and ion beam sources. The system uses a high-precision five-axis positioning system with a modular setup and the following diagnostic tools: a telemicroscopy head for optical imaging, a triangular laser head for surface profile scanning, a pyrometer for temperature scanning, a Faraday probe for current density mapping, and an energy-selective mass spectrometer for beam characterization (energy and mass distribution, composition). The capabilities of our diagnostic system are demonstrated with a Hall effect thruster SPT-100D EM1.

Effects of aperture displacement in broad-beam ion extraction systems

Low-energy broad-beam ion sources cover a wide range of applications. Geometrical perturbations of the axial-symmetric extraction channels by off-axis shifts of the accelerator grid holes give rise to an ion beamlet tilt which under certain conditions is dependent on the beamlet current. In this study, the characteristics of the beamlet deflection are presented which were obtained by a three-dimensional trajectory simulation.

In Situ Thermal Characterization of the Accelerator Grid of an Ion Thruster

hole diameter, distance between holes, or grid shape. These parameters are also measured in situ with a telemicroscope for high-resolution optical imaging and a triangular laser head for surface profile scanning. The distance between grid surface and pyrometer optics are precisely monitored with the support of the triangular laser head, for which the position is fixed with respect to the pyrometer. The distance measurement allows for correcting the measurement spot size of the pyrometer. The temperature profiles at three different beam power levels (1250, 2250,and4000W),andwarm-upandcool-downphases demonstrate thecapabilities ofthe complexequipment.Itis found that thermal steady state is reached after 4 h of thruster firing. Furthermore, it is shown that the accelerator grid surface temperature increases almost linearly with increasing beam current.