FLP (Fast Langmuir Probe)


A main objective of this instrument is to derive the electron density and electron temperature from the current - voltage relationship of the cylindrical Langmuir probe. The estimated density and temperature variation will be compared with the neutral temperature from NTV observation and the neutral wind from the ground-based Fabry-Perot Interferometer observation for a study of the dynamics and energetics in the lower thermosphere.

On the sounding rocket "S-310-35", the payload zone will be separated into two stage; upper stage (called PI part) and lower stage (called SUB-PI part), to avoid a possible influence of positive charging due to electron beam emission of the NTV instrument. Passive instruments such as FLP and NEI (impedance probe) are installed on the lower stage while the NTV being on the upper stage. In this occasion, the electron beam emission is thought to have no influence on the potential of the lower stage of the payload zone, where the FLP is installed.

Instrument specification

In order to avoid an undesirable influence of the surface contamination on the probe measurement, the stainless probe is baked at the temperature of 200°C for about 24 hours to get outgas from the surface, and subsequently the probe is glass-sealed. The glass will be cut and removed during the rocket flight (approximately at the altitude of 70 km) so that the probe can start measuring with a clean surface.

The probe is a cylindrical shape with a length of 14 cm and a diameter of 3 mm, and is installed on the payload zone. During the rocket flight, the probe will be deployed in the direction vertical to the rocket spin axis to measure the electron/ion current outside of the rocket sheath. The probe is directly biased by a triangular voltage with amplitude of 3 V with respect to the rocket potential in order to provide the incident current versus voltage relationship. The incident probe current is amplified and subsequently transferred to the telemeter. Three levels of the current gain; low, middle, high, are prepared so that the probe can measure in a wide range of the electron density.

In order to measure the ion current as well as the electron current, the amplifier has an offset voltage of +1 V; the voltage higher than 1V means the electron current while the lower one does the ion current. Every 30 seconds the calibration signal is obtained by switching the electric input from the probe to the resistance.

A timer sequence for the FLP will be executed as follows:

Time from the launch (sec)Item
63FLP-Window Open
65FLP-Glass Cut
66FLP-Probe Extension
120FLP-Gain Down
259FLP-Gain Up

Detailed Specification of the FLP instrument is as follows:

Sweep voltageTriangular, 3 Vp-p
Sweep period250 msec
Sampling period2.5 msec
Weight1.5kg (FLP-S)
1.5kg (FLP-E)
0.25kg (FLP-PRE)
Size255 x 65 x 92.2H mm (FLP-S)
211 x 100 x 60H mm (FLP-E)
126 x 29 x 83H mm (FLP-PRE)
Power250mA (+18V), 150mA (-18V)
Telemeter outputFLP-L NA04(W4)
FLP-M NA05(W5)
FLP-H NA06(W6)


T. Abe (ISAS/JAXA), K.-I. Oyama (ISAS/JAXA), AD Co., Ltd.