The project team has a large complement of diagnostic equipment for measuring plasma properties in the laboratory experiment contributed by UT, UH, Rice, ORNL, and LANL. These diagnostics are tailored for measuring plasma performance and validating modeling tools. The diagnostic instruments include a triple probe, a Mach probe [Hutchinson, 1987], a bolometer, a television monitor, an H-a photometer, a spectrometer, neutral gas pressure and flow measurements, several retarding potential analyzers (RPA), including a directional, steerable RPA, and a large multi-anode RPA [Bering, 2002], a surface recombination probe system, an emission probe, a microwave interferometer and other diagnostics. Reciprocating Langmuir and Mach probes are the primary plasma diagnostics. The Langmuir probe measures electron density and temperature profiles while the Mach probe measures flow profiles. Together this gives total plasma particle flux. An array of thermocouples provides a temperature map of the system. Improvements to the diagnostic complement will be studied in the early stages of the research effort. LANL is producing an RF-compensated Langmuir probe for insertion in the helicon discharge based on past experience with this type of work [Light, 1995].
As noted elsewhere, development of the TRL demonstration experiment will require an on-going series of studies with the existing VX-10 apparatus. These experiments are pointless unless the properties of the exhaust and the apparent thrust output are measured and understood. Thus, we will require continued maintenance and operation of these instruments. Incremental improvements in the diagnostics are needed in certain areas to clarify ambiguities and to develop the capability of verifying plasma detachment. The analysis and interpretation of the data obtained from each step in the development process will entail the time and effort of trained personnel.
Three significant increments in the available diagnostic and support equipment will be required to verify plasma detachment using the VX-10. First the high capacity vacuum pumps that are now sitting in crates need to be installed on the system. Second, we will need an additional set of higher sensitivity instruments to diagnose plasma parameters at large axial distances in the exhaust chamber (see Table 1). Third, a mechanism for in situ axial transport of diagnostics as big as the multi-anode RPA head is required to follow the development of the exhaust plume from regions of high to low magnetic field and to make the measurements required to validate model predictions.
Table 1. VX-10 plasma detachment demonstration instrumentation
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4. Diagnostics |
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4.1. Detachment Diagnostics |
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4.1.1. Probes |
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4.1.1.1. DC magnetometer, 3 axis |
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4.1.1.2. Triple probe |
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4.1.1.3. Mach Probe |
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4.1.1.4. High gain RPA |
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4.1.2. Axial Transport |
4.1.1 Probes - devices inserted into the plasma to measure electrical and magnetic properties. With the exception of magnetometers, probes are generally not commercially available. Typically, each probe is designed by a scientist to meet experimental requirements.
4.1.1.1 3 axis DC magnetometer – indicates the strength and direction of the local magnetic field. The magnetic field in the exhaust plume will change direction from that of the vacuum field where plasma detachment occurs.
4.1.1.2 Langmuir triple probes – measure density and temperature of plasma electrons. These are two of the parameters that determine when conditions conducive to detachment have been attained.
4.1.1.3 Mach probe – measures plasma flow velocity.
4.1.1.4 High gain RPA – measures ion energy spectrum in the low-density plasma of the downstream exhaust plume where detachment should occur. The ion energy distribution is another plasma characteristic affecting detachment.
4.1.2 Axial transport – the diagnostic devices of section 4.1.1 need to be moved throughout the volume of the downstream exhaust plume. This may be achieved by a system of rails with motorized translators to mount and move the probes in the exhaust plume so that plasma conditions upstream and downstream of the Alfvenic transition (where detachment occurs) can be measured.
Thrust testing of a VASIMR prototype in other facilities will require a significant array of diagnostic instruments. Three basic diagnostic tasks must be addressed in instrument selection. These tasks are characterizing the plasma that is produced within the engine, verifying and quantifying the details of the plasma detachment process, and measuring the momentum flux in the exhaust plume for comparison with thrust stand results.
Instrumentation will be obtained to answer these questions: 1) Is a plasma discharge being produced? 2) What is the ionization efficiency? 3) What are the electron and ion temperatures? 4) What densities, flow rates, and momentum fluxes are being achieved? 5) Is plasma detachment occurring?
These questions will be addressed by a portable package of instruments external to the engine core. The UT, UH and Rice team members will develop a final list of candidate instruments during the initial phase of this project. Subsequent work will include procuring or building the selected list. Perturbations inside the core will be minimized by use of photometry or microwave interferometry to ascertain plasma conditions inside the thruster. Outside of the thruster core, these instruments can be in situ probes, which are typically lighter and less expensive than remote sensing devices. Langmuir probes, RPA’s and plasma wave receivers will be required. The Langmuir probes will measure plume density and electron temperature and the RPA will measure ion temperature and flow rate. The plasma wave receivers will be used to study instabilities and generation of plasma emissions. Locations to be considered include reciprocating mounts located near the VASIMR, instruments on Wilson seal booms and a mobile package that can be used to probe the farther reaches of the exhaust plume.
Table 2. VX-50 performance diagnostic instrumentation
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4.2. Portable Diagnostic Instruments |
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4.2.1. Probes |
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4.2.1.1. DC magnetometers, high field, 3 axis |
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4.2.1.2. Langmuir triple probes |
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4.2.1.3. Langmuir Mach probes |
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4.2.1.4. Emissive Langmuir probe |
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4.2.1.5. Induction magnetometers (B-dot) |
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4.2.1.6. Wave e-field |
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4.2.1.7. RPA |
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4.2.2. Remote Sensing |
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4.2.2.1. Photometers |
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4.2.2.2. Microwave interferometer |
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4.2.2.3. Fabry Perot Interferometer |
4.2.1 Probes – devices inserted into the plasma to measure electrical and magnetic properties. With the exception of magnetometers, probes are generally not commercially available. Typically, each probe is designed by a scientist to meet experimental requirements.
4.2.1.1 as in 4.1.1
4.2.1.2 as in 4.1.1, but designed to include operation in high-density plasma conditions.
4.2.1.3 as in 4.1.1
4.2.1.4 Emissive Langmuir probe – used to determine electric potential in the plasma. Allows RPA ion energy data to be converted to flow velocity.
4.2.1.5 B-dot probes – measure the magnetic field strength and direction, and variation with time. These probes verify that the antennas are generating the desired wave magnetic fields.
4.2.1.6 Wave E-field – measure the electric field strength and direction, and variation with time. These probes verify that the antennas are generating the desired wave electric fields.
4.2.1.7 RPA – measures ion and electron energy spectra; design for high-density conditions near the discharge is somewhat different from that for the downstream exhaust plume, but operating principles are the same.
4.2.2 Remote sensing – these devices are not inserted into the plasma but infer its properties by observing electromagnetic radiation emitted by the plasma or effects on radiation transmitted through the plasma. Because they do not enter the plasma, their use does not involve the issue of plasma perturbation, unlike the electric probes.
4.2.2.1 Photometers – measure intensity of emitted radiation in a narrow wavelength band. Ratios of intensities at different wavelengths yield information about discharge qualities such as density and temperature. Components are commercially available and assembled by the experimenter.
4.2.2.2 Microwave interferometer – most accurate method of measuring plasma density. Microwave radiation launched from a horn on one side of the plasma is received by a second horn. Effects such as rotation of the direction of polarization of the radiation yield plasma density. Sophisticated variants yield information about plasma fluctuations. Commercially available.
4.2.2.3 Fabry-Perot interferometer – examines the detailed structure of atomic emission lines to obtain velocity information (Doppler effect). This diagnostic can quantify the ion cyclotron heating mechanism by direct observation, and can also measure plasma flow velocity. Commercially available components are assembled into an instrument whose design is tailored to the experimental conditions.
UH shares responsibility for measurement of the ion velocity phase space distribution function with Dr. Tim Glover. At present UH has three operation ion energy analyzers or retarding potential analyzers (RPA’s). Two of these are side-looking cylindrical probes that can make 180° angle scans at a single axial location. However, the angular precision of the scans is rather coarse. UH also has an end aperture cylindrical probe with precise angle scan capability. However, since this scan is accomplished by means of a hinge at the rear of the probe body, scanning through large angles produces some translation in z. The axially mounted Rice RPA does not have this angle scan capability. Present models of the effect of the ICRH predict that the heating produced will appear as a perpendicular component of the ion velocity. The effect of this may be to create an ion distribution that can only be observed by a scanning or widely collimated RPA instead of the fixed, narrowly collimated RPA. Therefore, a major ongoing responsibility of the UH group will be to take angle scan and wide aperture data during ICRH and other critical plasma experiments. To do this UH will operate the side mounted retarding potential analyzers (RPA) as required to diagnose large bore helicon and ICRH operations, and test and operate precision angular scan end mount RPA to look for evidence of perpendicular heating.
Proper interpretation of the RPA data requires knowledge of the plasma potential. The method of choice for determining plasma potential is the emissive Langmuir probe. ASPL presently has two of these, one constructed by Rice and one by UH. We are in agreement with the Rice investigator that having multiple instruments with the same nominal capability and different designs is an extremely desirable situation. Comparison of data will ensure accuracy and availability of two probes enables mapping and easy calibration of the performance of RPA’s and other instruments at multiple locations. To do this UH will install and test a side mounted emission probe and operate as required in conjunction with RPA data taking.
An essential part of all research and development is the analysis, interpretation and publication of results. To this end, UH will participate in analysis and interpretation of all diagnostic data, write VASIMR scientific papers, and present two conference papers on VASIMR at the U.S. National URSI and the IUGG meetings, in June and July respectively.
As the VX-10 makes the transition to VX-50 and improvements in power level and pump capacity are installed, it will become much more important to measure the behavior of the plume in the “cone” and large exhaust chamber regions, which we will collectively term the far plume. At present, there is limited capability to observe in this region. This problem needs to be rectified. During the coming summer, UH will begin to develop designs and specifications for detachment region or far plume instrumentation suite. This task will include definition and selection of instruments. Detailed design efforts will focus on a dc magnetometer and one selected probe, which we anticipate will turn out to be a Mach probe.
During the second fiscal year of the project, most of the tasks will be continuations of the year 1 tasks. Thus, UH will continue and /or complete all year 1 activities as required and appropriate.
Once the task of selecting and designing the far plume instruments has been completed, UH will proceed to construct and install one of these instruments. One instrument that we can already identify will be a 3-axis, high field dc magnetometer. Therefore, UH will procure, mount and interface such a magnetometer. UH will also fabricate, mount and interface one additional far plume probe from the list that has been selected. At this time the most likely choice appears to be a Mach probe.
UH will continue to operate all of the RPA’s as needed to support ongoing operations. UH will also continue data analysis, interpretation and publication activities. One full time UH physics graduate student, Michael Brukardt, will conduct student thesis research.
The spreading of the beam with axial distance downstream is directly related to nozzle efficiency and plume detachment. Measuring nozzle efficiency and demonstrating plume detachment will require the availability of an instrument that can make simultaneous measurements of the ion velocity phase space distribution function as a function of position across the plume. With support from a previous grant, UH has built a multi-anode RPA that we can use for this purpose. At the present time, initial fabrication is complete. The RPA awaits final assembly and testing. UH will assemble and test this large multi-anode RPA as a back burner, time available task. The multi-anode RPA will produce 32 simultaneous data streams; each comprised of 16 RPA energy or voltage sweeps per shot. The existing RPA software will have some trouble coping with this kind of a flood of data. We also have no software for displaying maps of the plane transverse to the plume. Writing software for analysis and presentation of the data from the multi-anode RPA will be major year 2 tasks.
Year 1:
During year 1 of the project, the UH principal investigator and students will undertake the following:
Begin to develop designs and specifications for detachment region or far plume instrumentation suite. This task will include definition and selection of instruments.
Operate side mounted retarding potential analyzer (RPA) as required to diagnose large bore helicon and ICRH operations.
Test and operate precision angular scan end mount RPA to look for evidence of perpendicular heating.
Install and test side mounted emission probe. Operate as required in conjunction with RPA data taking.
Participate in analysis and interpretation of all diagnostic data.
Write VASIMR scientific papers.
Present two conference papers on VASIMR at U.S. National URSI and IUGG meetings.
Year 2:
During year 2 of the project, the UH principal investigator and students will undertake the following:
Continue and /or complete all year 1 activities as required and appropriate.
Procure, mount and interface high field dc magnetometer.
Fabricate, mount and interface far plume Mach probe or probe TBD.
Continue publication activities.
Conduct full time student thesis research (Brukardt).
Finish and test large multi-anode RPA.
Develop software for analysis and presentation of data from multi-anode RPA