GSFC Calibration Plan

GSFC Calibration Plan

By Ed Sittler

March 30, 2002

1. Ion Beam Calibration
2. Ion Beam Hardware

The ion beam hardware is composed of three major components. We first have an ion source which is composed of gas feed into ionization chamber using electron impact ionization technique. The gas flow is aligned with the beam axis and electron beam is orthogonal to the gas flow. One can adjust the filament current for electron emission and biased to adjust electron impact energy on neutral gas. We then have an extraction lens which pulls ions out of chamber and then accelerated and focused with einzel lens section. Steering electrodes are then used to adjust alignment of beam along beam axis. The second component is a wien filter with mass resolution of 33. This wien filter is a flight spare of the ISEE-3/ICE design. The wien filter is used to select the ion mass of the ions produced by the ion source. During assembly a laser beam is used to mechanically align the ion source and wien filter. Both of these components are floated at the beam exit energy in volts. A voltage for the wien filter is adjusted to select the ion mass (to go to light mass ions such as H2+ may have to change power supply from 250 volts to 500 volts. This voltage will be a function of the ion energy. The ion source is usually operated at a few hundred volts for exit ion beam energy. Extending across the high voltage break is a large einzel lens, the third component of the beam hardware, which is designed to withstand 50 kV and used to accelerate/de-accelerate the ion beam to the desired energy and focus to a desired cross-section area. A quantar imaging micro-channel plate is used to measure the cross-sectional beam profile.

1.2Ion Selection

The ions produced by the ion source can be controlled by changing the neutral gas leaked into the ion source, adjusting the electron energy and adjusting the neutral gas flow rate to adjust the gas pressure in the ionization chamber of the ion source (10-5 torr < P < 10-4 torr). We can install a turbo-pump for differential pumping of the ion source vacuum housing to keep the pressure low in the main chamber. The goal is to have a vacuum pressure less than 10-7 torr in the main chamber. The wien filter is then used to select the desired ion from the ion source.

1.3Ion Beam Calibration

To calibrate the ion beam we will use various atomic ion species to cover a broad range of ion mass from 1 amu to over 100 amu. The ions to be used are H+, He+, N+, and noble gases Ne+, Ar+, Kr+ and Xe+. For each species we will vary the ion energy from 50 volts to 20 kvolts. It may be difficult to get low energy ions through wien filter. Here we would de-accelerate ions with main einzel lens. From our understanding this should be possible. The energies used will be a subset of the logarithmically spaced voltages used by the flight IMS. We will record the ion source voltages and wien filter voltage settings for each ion mass and ion energy. These results will be recorded in a file on the control and data acquisition computer and stored on optical disk. This data will be used to allow fairly rapid control of the ion beam when calibrating the prototype of the IMS and allow a fairly comprehensive calibration of the IMS over a period of about 3-4 months. This calibration of the ion beam is estimated to take 1-2 weeks.

2.0 Ion Selection for Calibration of Prototype IMS

2.1 The ion selection is based on what is expected to be observed during the 4 year tour of the Saturn System. This listed will contain some species which can be toxic and will be avoided since our present facility has the roughing pumps venting into the calibration facility room. We propose to perform a calibration of the more toxic gases in our building 21 facility which will have the capability to vent our roughing pumps through a ventilation system that vents the gases outside the building on the roof with exhaust fan. At present we are in building 2 and the move to building 21 will occur one year from now.

2.2 By consulting with Hasso Niemann’s Group and members of the astro-chemistry branch we have compiled a list of ions to be used in calibrating the IMS. The list includes the following: H+, H2+, He+, (C++), (N++), C+, N+, O+, (OH+), (H2O+), (H3O+), CH4+, CH3+, CH2+, CH+, [NH3+], NH2+, NH+, Ne+, CH5+, C2H2+, C2H3+, C2H4+, C2H5+, C2H6+, C3H2+, C3H3+, C3H4+, C3H5+, C3H7+, C3H8+, C4H2+, C4H3+, C4H4+, C4H5+, C4H7+, C5H4+, C5H5+, C5H7+, C5H9+, [C6H6+] [H2CN+], [HCN+], [C2N+], [C3H2N+], N2+, [CO+], CO2+, O2+, [O3+], [H2O2+], C2+, Ar+, Kr+, [Na+], [K+], (S+), [SO2+], (Si+), (Fe+), (FeO2+), (MgO+), (SiO2+) and (Ca+). Hasso’s group and the astro-chemistry lab has given us a list of suppliers where we can get the appropriate gases etc. Sulfates are also a possibility. The ones in parenthesis () need to use special techniques to produce and thus difficult to do. Those in brackets [] are considered toxic and will not be done now. Hydrocarbons of the same number of carbon ions can be produced by a single gas such as C2H6 and then breakup the molecules by adjusting the electron energy and selecting the appropriate ion with the wien filter. For more polymer ions wien filter selection will be more difficult. For the multiple charge states we will use the plasma source. This will be done last. The more difficult ions will also be reserved during the latter part of the calibration period. For Na and K one can purchase filaments with Na and K coatings on them. Here we will need to modify the design of the ion source ionization chamber. By mixing N2 and CH4 gases with electron ionization one can produce many of the HCN compounds etc. in the ion source. Some of the metallic and silicates etc. will need to be generated using pyrolysis techniques and using He as a carrier gas (Here the astro-chemistry group would be able to help us). Ozone will also be tricky in making but by adjusting the electron energy it may be possible. For gases more difficult to eliminate from ion source we will wrap the ion source ionization chamber with heater wire and bake out. The water group ions will be produced by pumping on a small cup with water to extract water vapor which is then fed into ion source gas inlet; again He as a carrier gas may be used.

2.3 Notes from Regina Cody (Astro-chemistry Branch)

2.3.1 Chemicals which are not safe to use without a pump exhaust connection to outside of building.

H2O2, O3, F2, CO, H2CN (a reaction product), SO2, Na, Mg, Cl2, K, Ni, NH3, PH3, H2S, HCN, HCl, HC3N (cyanoacetylene), NO (nitrogen oxide), CN (cyanide radical, would have to be made in situ), C2H3CN (cyanoethylene), N2H4 (hydrazine), N2H3 (radical), GeH4 (germane), AsH3 (arsine), C6H6 (Benzene), C2N+ or C3H2N+ (if they contain cyanide radical). Source of information on how to handle various gases etc. is MSDS = Material Safety Data Sheet.

2.3.2 Other Safety Considerations

H2O2 (Hydrogen Peroxide). You would need to buy it in solution concentration > 30%. At these higher concentrations, the solutions can be unstable and require special handling. The handling aspect should be thoroughly investigated before purchase.

O3. Ozone would need to be generated in the lab. Preferably in situ. O3 is very unstable. Storage requires special care and conditions.

F. Fluorine atom would have to be made from F2, preferably a low % mixture of F2 in inert gas. Very corrosive to skin and respiratory system. You need to purchase a gas regulator specifically made for F2 and make sure gas lines are tight. I would recommend a regulator with a purge connection.

CO. Carbon monoxide is deadly because it has no smell. Purchase a CO moniter from the hardware store and mount near equipment.

CN. Any CN containing compound needs a pump exhaust.

Na and K metal react violently with water. But should be OK with special heater units you can install in ion source.

Cl. Would have to be generated from Cl2. A corrosive gas; not as bad as F2.

Ni. Nickel is toxic if breathed. Would have to be hot enough to be in gaseous phase.

PH3 (phosphine), GeH4 (germane), and AsH3 (arsine) flame when exposed to air as purchased from supply companies. Also very toxic so need to handle carefully.

HCl and NO are corrosive to respiratory tract.

N2H4 Very unstable compound. Needs special handling.

Before you use any of the compounds on the “needs pump exhaust list”, you should get the MSDS for the compound and understand its particular safety concerns.

2.3.3 Possible Radicals from Decomposition of Stable Molecules (per photolysis).

CH4 CH3, CH2

C2H2 C2H, C2 ?

C2H6 (ethane)  C2H5, CH3, C2H4 (ethelyne)

C3H8 (propane)  C3H7, CH3, C2H5, C2H4

C2H4 (ethylene)  C2H3

C3H4 (methyl acetylene) H3C – C = CH  CH3, C2H This form detected in planets (double bond is really triple bond). 2nd form is H2C=C=CH2 (allene); symmetric form probably is reason not seen (doesn’t radiate)

C4H2 (diacetylene)  C2H

NH3 NH2, NH, N?

PH3 PH2, PH, P?

HC3N (H-C=C-C=N)  C=C-CN, H-C=C, CN (cyanoacetylene) (double bonds are really triple bonds).

2.3.4 Species which are radicals and atoms and need to be generated in situ.

F, H2CN (a reaction product), C2H5, S2? (probably), Si, Cl, O++, N++, C++, C2+, NH2+, CN+, N2H3+, CH+, CH2+, CH3+, NH+, C2H+, CN, C2H3+, C3H2+, C3H3+, C4H3+, C4H4+ (ethylene? Acetylene?), C5H5+, N2H+, C2N+, C3H2N+, CH5+, C3H5+, C3H7+, C4H5+, C4H7+, C5H7+, C5H9+.

3.0 Initial Set Up and Operation of IMS

3.1 In preparation for Beth Nordholt we will have new flanges fabricated with the required high voltage feed throughs, while flanges for interfacing the power lines and digital lines are already available. We will also order the appropriate gas bottles and filament sources we will need to carryout our calibration. Also, while in contact with Beth we will design and fabricate the mounting bracket that will be needed to mount the IMS to the turn table in the vacuum chamber. For the Na and K sources we will design and fabricate a new ionization chamber for our ion source. We will then be ready for Beth’s arrival. When Beth arrives we will mount the IMS to the turntable and connect all the high voltage, power and signal lines from the IMS to the appropriate flange connectors. We will then perform a low voltage checkout and a complete check of the system using a checklist. We will then close the chamber and pump to low pressure. We will use heater wire wrapped around the chamber to bake it out to achieve a vacuum pressure less than 10-7 torr and use thermocouple to monitor temperature. Once this is done we will gradually bring the MCP voltages up to their normal operating voltages. We will then scan the ESA voltages for checkout purposes. Finally, we will gradually raise the high voltage across the TOF section to  14.4 kV. Then using the quantar we will tune the beam to about 1 kV and use N+ ions. Once this is satisfactory we will move the quantar away and align the IMS to its nominal beam acceptance setting (ESA, 0, 0) where E/Q  6.25*ESA, 0 is the polar angle and 0 is the azimuthal angle and IMS collimator aligned with the beam in x and y (i.e., two translational axis of turn table). These parameters will be adjusted to provide maximum singles and ion count rates; while doing this we will make sure the count rates do not exceed safe levels of operation for the MCPs. We will do this for several ions and beam energies. We estimate that Beth’s required stay will be one week.

4.0 IMS Calibration

4.1 The Prototype IMS comes with resistor divider that provides high voltage across TOF section and 1 kV across floating MCP. The front-end electronics then interfaces to an EM version of the TDC. The TDC will be outside the chamber and interfaces with a PC that comes with it. Beth will help us get this part of the set-up operational. We will use our PC to orient the IMS at desired azimuth, polar angles etc, obtain beam, take Quantar snapshot, record relevant details in LOG book (e.g., beam energy, size, pressure, species, Wien filter settings, ion source settings, date/time stamp so we can cross-reference it to the data accumulated with EM TDC PC. In the beginning we will select a subset of the original ions used to calibrate the flight IMS and the beam energies used for both the SwRI and LANL calibrations. We estimate this will take about two weeks. While this is going on we will be comparing our data with the SwRI and LANL data to look for offsets in TOF or other systematic differences between the two data sets. We will also make comparisons with the Earth flyby data with regard to H+ and O+.

4.2 Measure Geometric Factor and Cross-Talk: To measure the geometric factor we will use a subset of atomic and molecular ions. These ions will be H+, He+, O+, CH4+ , Ne+, N2+, Ar+ and CO2+. We will also select a range of beam energies E/Q  6.25*ESA. The beam energies will be 50 volts, 100 volts, 500 volts, 1000 volts, 5000 volts, 10000 volts, 15000 volts and 20000 volts. For a specific ion and beam energy we will, for example, scan the ESA voltage around the nominal beam acceptance voltage of the ESA for a specific polar, 0 , and azimuthal angle, 0 for which the counts from the ESA are maximized. We will then perform a 2D scan of the angles  and  around (0 , 0) with an ESA voltage scan for each angle pair of the 2D scan, while recording the output counts of the ESA. For each setting (ESA, , ) we will record starts, stops, and coincident ion counts. Before and after a set of (ESA, , ) scans we will record the 2D count rate from the quantar. From this information we can get a measure of the quasi-absolute determination of the instruments geometric factor.

This information can also be used to determine the instruments efficiency by comparing singles data and coincident counts. We will also be able to measure the background spectrum as a function of beam intensity. Since we will also be recording the starts and coincident ion counts as a function of angular sector we will also get a measure of cross-talk as a function of (ESA, , ) for a particular incident ion, beam energy and beam intensity.

4.3 Calibration Survey Mode: In this mode we will select a specific ion of those listed above for calibration. Since we will not be running 24 hours a day, we will bring the voltages down for the IMS at the end of each day and then at the beginning of each day bring the voltages up. Our plan is to do a group of similar ions (i.e., water group ions) in a week. Therefore, in 3-4 months we will be able to do about 14 ion groups which should cover all the ions listed above. For each ion we will tune the IMS (ESA,, ) for maximum ion counts. We will then record an accumulated time of flight spectrum for all 2048 channels and 8 angular sectors. Above 10 kV beam energy we will sweep the beam energy across the ESA pass band to get a true measure of the TOF line profiles widths. In order to have complete traceability we will record time of the measurement, ion beam settings, IMS settings, vacuum chamber pressures and relative humidity of lab. This data will be stored electronically and stored on a CD for permanent storage. This data will be made available to the whole team once a CDF common format is determined for the calibration data.

4.4 GSFC Ion Beam Capabilities

Following is Summary of features at available space plasma calibration facility at GSFC

Vacuum Chamber 33” dia. Cylinder, by 40” length.

Automated total dry pumping system:-

Two 8” cryo pumps, base pressure ~ 1*10e-7 torr, (no instrument in chamber)

HH coils null Earth’s field.

SRS 200 Residual Gas Analyzer, monitor bkgnd, leaks etc.

Computer controlled Motion Table (Expt. load 50 lbs, )

4 axes of table motion, 2 linear, 2 rotational.

Ion Beam Energy 50eV to 20,000eV

Dimensions, 5 to 50 mm Dia,

Intensity 100pA,

Ion Beam Monitor 40mm dia. Quantar imaging detector

Source electron impact ionization, Species H2, He, Ne, N2, O2, CO2, Ar, Xe

Alternative plasma source for multiple charge states of Nitrogen,

N+, N2+, N3+ and N4+, (ready for use January, 02)

Turbo Pump for Differential Pumping of Ion Source

Electron Beam, 50eV to 1000eV

Solar intensity UV source

Automated data acquisition, display, plot and storage, (written in HTBasic).

Create ASCII files for further analysis using IDL.

Camac System for TDC, ADC etc using Sparrow software pkg.