Langmuir probe experiment - LINDA

IRF-U 1998.12.10

Bengt Holback


The aim with the Langmuir probe experiment, LINDA, is:

* to study fine structures in the plasma density and, by using a double probe set up, resolve temporal and spatial effects of the measured signals.

* to measure the overall distribution of plasma density in especially the auroral zone.

In order to achieve this the output of the instrument is as follows: * The probe current to each of the two probes. The current is close to proportional to the plasma density (sampling frequency 16 samples/sec).

* Occasionally the probe current resulting from a sweeping bias voltage. This will give the characteristics of the plasma like electron temperature and absolute density.

* The fine structures of the probe currents amplified so that they can be compared and analysed in detail at high resolution in both amplitude and time (dn/n) (sample rate 8 ksamples/sec).

Experiment layout and signal handling.

The instrument comprises two spherically shaped probes mounted on two booms of about 0.67 m length. The booms are mounted at 15 degrees angle to the spin plane on the outer edges of two of the solar panels giving a probe to probe distance of about 2.9 meters. The probes and booms are designed and qualified by the IRF-U.

The electronics is built by IRF-U and integrated into the box of the Electric field experiment supplied by the Royal Institute of Technology in Stockholm (KTH). The probe signals will be fed to the electronics via tri-axial cables.

A common processor, EMMA system unit, for control of LINDA, the Electric Field Experiment, the Magnetometer and the Star camera as well as the interface to the Astrid System Unit (ASU) is designed by the Royal Institute of Technology (KTH). Also the power supply converter (DC/DC) is designed by KTH.

All signals will be filtered by low-pass filters to avoid aliasing effects.


The electronics is allocated on two printed circuit boards and contains preamplifiers for each of the probes, low pass filters for bandwidth limitations, analogue to digital conversion, a XILINX circuit used for calculation of the dn/n ratio.

A mass memory of 12 Mbytes, common for all experiments controlled by the EMMA system unit, is also allocated in the electronics box and can be used for storage of data sampled during times when the satellite is not visible from any of the two ground stations.

Booms and probes.

There are two identical deployable booms for LINDA, one for each of the probes. The booms are made of two sections with a hinge and the material is aluminium tubes of 6 mm outer diameter. The hinges and the retaining mechanisms are made of dural-aluminium and Titanium. The Titanium parts have a surface treatment of Titanium Nitride (TiN) in order to have a non-stick surface finish. In stowed position the booms are folded along the outer rim of the outermost solar panels. The retaining mechanisms contains two holders for each of the booms and the releas mechanisms are activated by the release of the solar panels.

Figure: "Top" view of S/C with solar panels and LINDA booms deployed (Wire booms excluded).

The probes are spherically shaped with 10 mm diameter. They are made of Titanium with a surface treatment of Titanium-Nitride (TiN) which has proven to have excellent properties for a good contact to the plasma. Between the probes and the outer end of the booms there are thin Stubs of 5 cm length on which the potential is regulated to be the same as the probe bias voltage. This is to minimise the influence of photoelectrons on the measured probe current. The rest of the booms are connected to the satellite ground.

Biasing of the probes.

The probes can only be operated in "Langmuir" mode, i.e. with low input impedance. There are two main biasing methods: fixed bias and swept bias. Fixed bias is used normally when the dn/n quantity is measured. Every 1 minute in Normal & Burst mode the bias is swept with a duration of about 2 seconds, in Survey mode, every 2 minutes. This is to obtain the bias-current curve which gives information on the performance of the probe and also the absolute density and electron temperature.

Fixed bias is used for measurements of the dn/n quantity. The probe current is direct proportional to the plasma density in this mode (assumed a non-varying electron temperature). The bias voltage value can be set to any number between -10 to +10 Volts with respect to the satellite ground, but for normal operations a value around +8 Volts will be used.

Sweeping the bias means that the bias voltage is changed in steps, up and down, according to a scheme that contains the following parameters and that can be set by command.

- Step length in seconds.

- Step height in volts.

- Number of steps.

- A complete sweep sequence contains one down and one up sweep.

The dn/n quantity

The only on board processing performed on the LINDA data is the calculation of the dn/n quantity. By doing this the amount of data is reduced significantly and dependent on resolution chosen, up to a factor of 2 is saved. The resolution can be set to 8, 10 or 12 bits in the dn/n value. Normally 8 bits is considered good enough as the value is a relative value (given in per cent). The calculation is made digitally in a Xilinx processor.

Modes of operation

The instrument can be operated in different modes which can be controlled by ground commands and/or by the time tagged commands stored onboard in the ASU. However, all measured signals, with the exception of the dn/n quantity, are transmitted to ground untreated. Experience from earlier projects show that bringing down the measured signals as time series to ground for final and detail analysis, is by far the best method to proceed in order to get the best physics out of the measurements. The limited telemetry capacity, on the average about 16 kbits/sec for the LINDA experiment, (out of 128 kbits/sec total ) does not allow continuous measurements of all the high bandwidth signals. In stead snapshots of various duration's will be taken, i.e. sampling at high rates (8 ksamples/sec) but only for short periods of time. The repetition rate will be dependent on the length of the sampling intervals and the memory allocation but the duty cycle will be of the order of a few percent only. By combining the lengths of the sampling intervals and the use of the onboard mass memory the measurementa can be optimised. Some signals, which are sampled at low rate, will though be transferred to ground continuously (probe DC signals sampled at 16 samples/sec).

The following scheme shows the main content of the different modes:

Normal & Burst mode

- The DC signals (n) from both probes continuously

- Bias sweeps every 1 minute

- The AC signals (dn/n) from both probes in snapshots

- Status information

Survey mode (used for survey of whole orbits)

- The DC signal (n) from one probe continuously

- Bias sweeps every 2 minutes

- The AC signal (dn/n) from one probe in snapshots

- Status information

Sweep mode

- Bias swept continuously. No dn/n data

- Status information

Specifications summary:
Electronics boards (2) Size  177.5 X 122.1 mm
Booms (2) Weight incl. probe and cable 80 g (for 1 boom)
  Retaining mech. Weight 20 g (for 1 boom)
Spherical probes Size  10 mm diameter
  Material  Titanium
  Surface treatment  TiN
Density (n) signal  Sampling frequency  16 Hz
  Resolution  16 bits, 2 ranges
  Probe current limits  10(-10) to 5 x 10(-5) A
  Density limits  10(7) to 5 x 10(12) m-3
  Low pass filter  8 Hz
dn/n signal  Sampling frequency  8 kHz
  Resolution 8, 10 or 12 bits
  Amplitude  Corresp to 0.01 - 50% of n
  Low pass filter 4 kHz
  High pass filter 10 Hz
Telemetry Average bit allocation to LINDA 16 kbits/s (out of 128)
  For 2 DC signals  0.5 kbits/s
  For 2 AC signals  15 kbit/s
  For status and others  0.5 kbits/s
  Buffer memory  <2 Mbytes