The Low Energy Neutral Atom (LENA) Imager - Hardware Development

Our scrapbook detailing our experience
assembling an instrument
that can do what's never been done...

...all experience is an arch wherethrough
Gleams that untraveled world whose margin fades
Forever and forever when I move.

---A. Tennyson

LENA Institutions

NASA/Goddard Space Flight Center

Lockheed Martin Advanced Technology Center

University of Bern, Switzerland

University of Maryland, College Park

University of New Hampshire

University of Denver

List of Topics

For the subheadings and information that follows, click on the highlighted numbers to view the pictures.

Conversion Surface Electrode

Affectionately referred to with monikers such as the flying saucer and bed pan, the conversion surface electrode cradles the four tungsten facets and holds them at -20 kilovolts potential. As is apparent from the pictures, this piece was difficult to machine.

plastic model -1-

editor's note on picture 1: On the next version of LENA, we intend to plate the plastic model.

unplated electrode -1- -2-

plated electrode in clean room -1- -2- -3- -4-

Trapezoidal Facets

There is some concern that UV exposure due to the sun may degrade the performance of the tungsten facets. Consequently, the UV hardiness of barium zirconate will be explored.

barium zirconate facet -1-

The Extraction Lens

After converting neutrals to negative ions through near-specular reflection off the tungsten surface, an extraction lens fabricated by Lockheed-Martin accelerates the ions to about 20 kV and images them spatially according to ion energy. Thus, position sensing of the start secondary electrons in the time-of-flight unit provides energy information on the incoming neutral beam.

extraction lens -1- -2- -3- -4- -5-

editor's note on picture 6: Please wear gloves when handling flight hardware. Thank you.

picture and progress report from Steve Fuselier (1 September 1998)

The extraction lens assembly in our clean room -1- -2- -3-

Electrometer

It is anticipated that as the IMAGE spacecraft rotates, allowing the sun to shine on the facets, the ultraviolet component of the light will liberate electrons from the tugsten surface. To monitor this effect and to evaluate possible surface deterioration due to cleansing, an electrometer, which measures electron current from the facets, is included inside the LENA instrument.

electrometer -1- -2- -3- -4-

Lower Electrode

The lower electrode was originally the cesium dispenser mount before the LENA instrument design was re-evaluated, eliminating the "need" for dispensing cesium and heating, for purge purposes, the tungsten surface. It is anticipated that these modifications did not adversely affect instrument performance to any large degree (perhaps lowering conversion efficiency by a factor of two).

lower electrode -1- -2- -3-

Start Assembly Testing

Once the initially neutral population has been separated from the ion component, converted to negative ions by interacting with the tungsten surface, and accelerated by a twenty kilovolt potential, the ions pass through a carbon foil producing secondary electrons. These secondary electrons excite the position sensitive start anode and create a fast start pulse (rise time less than three nanoseconds).

Initial start testing, shown here, was performed using a 5 keV ion beam.

start assembly test set-up -1- -2- -3- -4-

analogue position signals directly from the A111 preamplifiers -1- -2- -3- -4- -5- -6- -7-

editor's note on picture 7: The A111 is clipping a small fraction of the signals here. See the flat-topped traces near the top.

fast timing start signals directly from the anode -1- -2-

TOF/Position Sensing Testing

Within the time-of-flight unit, an electrostatic mirror deflects the secondary electrons emitted from the 2 microgram per centimeter squared carbon foil towards the start anode.

Floating carbon foils is extremely tricky business. It requires patience and a steady hand. These figures show our laboratory set-up designed to separate the cabon foil from the slides to which they are attached when received from the manufacturer (Arizona Carbon Foil Company). -1- -2-

electrostatic mirror assembly from the engineering model -1- -2- -3-

Time-Of-Flight (TOF) Electronics

The University of Maryland Space Physics Group is responsible for the LENA time-of-flight electronics. Initially, only an almost flight configuration start anode was available. Because of the position sensing capability of the start anode, the quality of the fast timing (start) signal from the start anode was anticipated to be inferior to that of the stop anode. Consequently, for testing the tof electronics as a worst possible case, the start signal was divided into two and used for both the start pulse and, after being routed through a delay cable, the stop signal as well. Because the tof electronics was located exterior to the vacuum chamber, it was also anticipated that ringing and distortions due to cable length and transmitting the signal through the feed-thru's would make the test a worst possible case scenario. Nevertheless, the elctronics performed admirably.

fast start pulse and responding tof electronics logic pulse -1- -2-

energy (vertical or wedge) direction test data from facet one -1-

Four Facet Start Anode Tests

For this round of testing, all four facets were populated with unscrubbed channel plates to examine the important issue of what transitions between the facets will look like in the data.

populated start assembly set up -1- -2- -3- -4-

editor's note on picture 1: This view shows the underside of the start test assembly in the mirror on which the assembly sits.

populated start assembly set up in vacuum chamber with power supply -1- -2- -3- -4- -5- -6-

angular scan of the four assembled facets looking at start rate -1-

angular position-sensing scan across the strips of all four assembled facets -1-

Stop Assembly Fabrication

The stop assembly does not have position sensing capability so the stop anode does not have the intricate wedge and strip geometry of the start anode. These pictures show various components of the stop assembly prior to fabrication.

stop housing -1- -2-

components assembled within the stop housing -1- -2- -3- -4-

Stop Assembly Testing

Prior to testing an integrated start/stop/foil/engineering model mirror assembly, the stop anode was tested separately to validate its performance.

stop assembly mounted for testing -1-

results with lab power supplies -1-

The Nosepiece

Various views of the LENA nosepiece which will serve to filter charged particles out of the beam so that LENA will be responding only to neutrals.

naked nosepiece -1- -2- -3- -4- -5- -6-

Housing

These figures show an initial attempt at machining a housing. This housing could potentially be used, but does have some minor problems.

housing -1- -2- -3-

housing blackened -1- -2- -3-

Shipping

These figures show the LENA shipping crate.

crate -1- -2- -3-

The LENA Litter

LENA was assembled in the clean room on the first floor of Building 2 at GSFC. High voltage/thermal vacuum testing required transporting the instrument downstairs to the ground floor where the thermal vacuum system Tycho VonRosenvinge of Code 661 generously let us use resides. Thus, we needed to build a "litter" that would allow LENA to be transported around Building 2 without becoming dirty.

building the litter and transporting LENA -1- -2- -3- -4-

getting set-up to perform thermal vacuum/high voltage tests -1- -2- -3- -4- -5-

For Eric Hertzberg, Lockheed-Martin:

These pictures show the gold black problems that John Keller spoke with you about.

gold black (large files-potentially long load times) -1- -2- -3-

The Denver University Atomic Oxygen Facility

description and picture of facility

Descoped Heater Assembly

Originally, LENA included cesium dispensers to coat and heaters to purge the tungsten surfaces. These were eventually removed from the final version of the instrument because the expense and difficulty in producing these systems did not justify the modest increase in efficiency they were expected to effect. Here we document some of the heater-related investigations.

some prototype heaters assembled -1- -2-

some heater prototypes disassembled -1- -2- -3-

Calibration Information and Data

There have been, to date (30 August 1998), two calibration runs done out at the Denver facility which will aid our understanding of LENA instrument operation.

3-12 June 1998 Denver Calibration

Goddard Neutral Beam Creation Experiments

For preliminary LENA testing prior to relocating to the Denver neutral beam facility, we wanted to assure that we could create a neutral beam in our facility here at Goddard. This was possible because some fraction of the ion beam in our accelerator is neutral. So, we first filtered out the positive ions by applying a potential. Then, we did what LENA does and scattered the remaining neutrals off a tungsten surface, converting some fraction to negative ions. This fraction could be eliminated from the larger fraction which remained neutral by applying another potential. By making a few assumptions, we were able to estimate the conversion efficiency of neutral to negative ions by the tungsten surface as being on the order of 1%.

summary of GSFC neutral beam creation results

Poster Figures

-1- -2-

Denouement

Finally, it is all coming together as the complete LENA instrument materializes in our clean room.

The (almost) complete LENA flight unit -1- -2- -3- -4- -5- -6- -7- -8- -9-

The (slightly more) complete LENA flight unit -1- -2-

EMI/EMC tests

After lugging the instrument over to Building 7's basement, we were able to begin (over the weekend) EMI/EMC tests in the facility.

LENA in the cave and the GSE on the opposite side of a shielding wall -1- -2-

Flight Time-of-Flight Unit Testing

Prior to calibration at Denver, a comprehensive evaluation of the LENA tof unit, the most complicated subsystem on the instrument, was performed.

TOF assembly pictures -1- -2- -3- -4- -5- -6- -7- -8-

Stop and start oscilloscope traces -1- -2- -3- -4- -5-

Time-of-flight spectrum for H₂ on the lab computer -1-

editor's note: some of the pictures are contrast-adjusted

TOF Unit Noise Characteristics

Noise characteristics are shown here as a function of channel plate voltage for both the University of Maryland and an external Bertan supply.

University of Maryland supply -1- -2- -3- -4-

Bertan supply -1- -2- -3- -4- -5-

GSFC Pre-Calibration Calibration

Some preliminary runs were performed in house on the LENA instrument prior to shipping to Denver to assure everything was in working order.

Imaging data scanned across start anode -1- -2- -3- -4- -5- -6- -7- -8- -9- -10- -11- -12- -13- -14- -15-

Off to Denver...

Finally, after some minor glitches here and there, we load LENA into John Laudadio's Windstar to see it off to BWI and ultimately Denver.

Saying goodbye, at least for now -1- -2- -3- -4- -5- -6-

At Denver, the first time.

It took a little bit of work once we got to Denver to get LENA situated and in the chamber.

Getting ready to pump down (click for pictures)

Waiting for pump down (click for pictures)

Sample calibration data (Mr. Snappy's) -1- -2- -3- -4-

Atomic oxygen efficiencies -1-

Efficiency variation with optics level -1-

Time-of-flight spectrum -1- -2- -3-

We lost two preamplifiers.

This is a bummer; it kills our position sensing. Fixing the problem and determining exactly what went wrong requires tearing the instrument apart one more time.

The tof unit -1- -2- -3- -4- -5- -6- -7- -8-

The instrument -1- -2-