SUPERCONDUCTING MAGNET SPECTROMETER

The objective of the flight was to fly a system using a superconducting magnetic spectrometer to measure cosmic ray rigidity spectra up to 350 GV. The spectrometer was a development of the NASA-Manned Spacecraft Center with the collaboration of Lockheed Electronics Co. and the Lawrence Radiation Laboratory, Berkeley and was the was the first superconducting magnet experiment to have been flown successfully by balloon. Flights of the apparatus were intended to be used for general cosmic ray surveys, including identification and analysis of all components of the charged particle flux.

In the image at left can be seen a basic scheme of the instrument (click for more details). It's major components were the emulsion spectrometer for particle identification and rigidity measurement, the superconducting magnet, a temperature controller for maintaining dimensional stability of the emulsion spectrometer, and the telemetry transceiver for controlling the experiment and verifying the conditions under which the emulsions were exposed.

The spectrometer consisted of five glass plates coated on both sides with a 4" x 6" area of nuclear emulsion (Kodak NTB-3). The first and last plates were coated with 200 µm emulsion. The three center plates were coated with 100 µm emulsion. Each plate was approximately 1400 µm thick. The plates were spaced 11/2" apart in a light-tight aluminum box. One-half inch below the lowest glass plate, 20 sheets of 4" x 3" x 300 µm emulsion (Ilford-G5) were arranged in a horizontal plane to form the central stack. These sheets were shifted with respect to the spectrometer when the experiment reached float altitude and again when the magnet was discharged. Beneath the shifted stack there was a 6" x 4" x 6" target block of 600 µm Ilford-G5 emulsion stacked vertically.

The superconducting magnet provided an average 10 kGauss field in the region of the emulsion spectrometer. The 12" inner diameter coil -which produced a magnetomotive force of 0.53 x 10^6 ampturns- was formed of 4431 turns of copper clad single strand niobium-titanium wire insulated by a copper oxide layer. The coil winding formed one end of a 25 1iter liquid helium cryostat. The cryostat was a triple-walled system using a liquid nitrogen dewar as an intermediate heat shield. Both the inner and outer portion of the vacuum space were insulated with 15 layers of aluminized mylar insulation. The dewar's capacity allowed 50 hours of magnet operation without replenishing the helium. To prevent excessive helium boil-off during the flight, the helium dewar pressure was maintained above 8 psi by an automatic regulator. The magnet was launched in the charged condition and discharged in flight by ratio command 1 1/2 hours before flight termination. To discharge the magnet, a diode was switched into the magnet current loop. The diode's forward-bias voltage-drop discharged the magnet in approximately 19 minutes. The magnetic field was monitored in flight by using a magneto-resistive probe within the dewar.

In order to maintain dimensional stability, the spectrometer was temperature controlled using electric heaters. The absolute temperature was regulated to ~ 0.5 °C and temperature gradients were maintained to less than 0.01 °C/cm. Temperatures and temperature gradients were monitored throughout the flight. The onboard telemetry system provided verification of the emulsion exposure conditions (i.e., magnetic field, temperature, etc.) and provided a means of controlling the experiment environment. Commands were available to move the emulsion shifter plates, to turn the magnet off, and to override the temperature control system.

The gondola orientation was of interest because the data was used to study East-West effects. Due to the large magnetic momentum of the magnet, the gondola was coupled to the earth's magnetic field with up to 2 ft.-lbs, of torque. A swivel was provided in the flight train to allow the gondola to point North. In order to verify the gondola orientation, a magnetometer was located at a point about 4 ft. from the magnet at a location where the magnet's field was roughly vertical.

Balloon launched on: 6/4/1969 at 20:16 cdt
Launch site: Columbia Scientific Balloon Facility, Palestine, Texas, US  
Balloon launched by: National Center for Atmospheric Research (NCAR)
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen 10.600.000 cuft (0.5/0.7 mIL.) Stratofilm)
Balloon serial number: SF 305.86-050-NSC Serial Nº 302
Flight identification number: 478P
End of flight (L for landing time, W for last contact, otherwise termination time): 6/5/1969
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): F 9 h 50 m
Landing site: Van Horn, Texas, US
Payload weight: 899 lbs.

The balloon was launched by dynamic method from the NCAR Balloon Launch Station of Palestine, Texas on June 4, 1969 at 20:16 CDT. Following launch, the experiment reached a float altitude of 136.000 feet after Which the superconducting magnet remained energized for an 8.4 hour period. The magnet was then discharged. The remainder of the flight time was: used to accumulate straight tracks for determining the plate alignment. The central emulsion Stack was shifted 1 mm upon reaching 125.000 feet, then shifted another 1 mm when the magnet discharge was initiated.

The recovery of the experiment was successful but unfortunately a heater circuit malfunction occurred during the landing impact. This resulted in the loss (by melting) of the central stack and the lower block of emulsions. However, the five glass plates survived the heating and could be scanned and measured.

• A simple magnetic spectrometer for cosmic ray studies to 300 GV Nuclear Instruments and Methods, Volume 113, Issue 3, 1 December 1973, Pag. 349
• Ballooning with Superconducting Magnets 12th International Conference on Cosmic Rays, Tasmania, Australia, 16-25 August, 1971. Volume 4., p.1571
• Measurements of cosmic ray rigidity spectra using a superconducting magnet 11th International Conference on Cosmic Rays, 1969
• NCAR Scientific Balloon Facility Annual Report, 1969 National Center for Atmospheric Research, January 1970
• Techniques of Magnet-Emulsion Spectrometry 12th International Conference on Cosmic Rays, Tasmania, Australia, 16-25 August, 1971. Volume 4., p.1577