8.1 Introduction

The RPWS Langmuir Probe (LP) instrument for Cassini consists of one spherical probe on a stiff boom, a boom root preamplifier, interface circuitry for the cylindrical plus and minus antennas, and an electronics board located in the RPWS electronics box. The LP can be used to estimate a number of fundamental plasma parameters, among which the electron density is usually the most requested parameter. The LP instrument is designed to measure the dc and ac variations of the electrical current to a dedicated spherical probe, but also in a limited time sharing mode make use off the minus and plus RPWS cylindrical antenna for additional interferometer measurements. From these measured currents it is possible to derive the following physical parameters.

Electron density (Ne)

Electron temperature (Te)

Density variations up to 10 kHz ( n/n)

Effective ion temperature over mean ion mass (Ti/mi)

UV intensity (photoelectron flux) mostly from Ly-

Spacecraft potential (Usc)

Dust impacts on spacecraft

The accuracies of these parameters have been estimated to about 10% for the electron density and about 20% for the other parameters from the in-flight commissioning as well as during the inter-instrument calibrations made during the Earth flyby in 1998. The instrument will be operative during a significant part of the Saturn tour in a wide range of physical environments. The accuracy of each parameter under various conditions may therefore vary somewhat. Measurements are expected to be possible down to as low plasma densities as 1 cm-3, depending somewhat on solar activity conditions.

A bias voltage sweep of up to ±32V referred to the satellite ground can be made on the dedicated spherical probe to obtain most of the physical parameters defined above. In ac mode the bias voltage will be set manually or automatically to a level determined by the local plasma parameters obtained by a sweep or pre-estimated from mission parameters. The cylindrical antenna bias is limited to ±12V in order to fulfill the satellite potential drift requirement.

The scientific capabilities of the instrument proposal and the design parameters are given in Table 1. The comparison show that the instrument performs as original proposed in most ways. The design drivers for the instrument have been the capability to cover up to five decades of electron and ion density ranges, and to provide ac density variations from all probes to the RPWS wave analyzers WBR and WFR.

Table 1. LP scientific instrument performance (proposal vs. calibration results)

Instrument proposal	Calibration results
Quantity	Measurement range.	Frequency range	Measurement range	Frequency range
Sphere DC	±5 dec (1nA-
100uA)	0-10Hz	±6 dec (±6.5 at 20C)	0-6Hz
Sphere AC	50-0.1% mod	0-10kHz	100-0.05% min	0-6kHz
Sphere Sweep	±50 V	1kHz	±32 V	1kHz
±5 Cylp DC	±5 dec (1nA-
100uA)	2.5kHz at 1uA	±5 dec (1nA-
100uA)	2.7kHz at 1uA
Cylp Bias	±10 V	---	±12 V	---

From calibration point of view the instrument consists of several modules with unique transfer functions. The contact surface between plasma and sensor, the preamplifier or cylinder probe electronics interface, the analogue signal processing, the buffers for ac signals distribution to RPWS wave analyzers, multiplexes, bias circuitry, filter and the ADC, and the controller unit are all modules which will be addressed in this document. For detailed information on the interconnections between modules see the block diagram in Figure 1.

Figure 1. Langmuir Probe block diagram.

Extensive calibration has been carried out on the LP assembly as well as the LP integrated into the complete RPWS. The main reference calibration was performed on the stand-alone LP assembly in Uppsala before delivery to RPWS. On satellite level only a few tests have been carried out to verify correct operation of the instrument. Details are shown in the block diagram of main input (encircled) and output (in a box) quantities used to establish proper transfer functions.

The Calibration fulfill the following purposes:

To verify instrument performance in relation to scientific requirements.

To obtain the transfer functions for the instrument, and adequate information for conversion of TM to physical units.

To determine drift of parameters due to temperature changes.

8.2 Dedicated spherical probe mechanical drawings

Figure 2. Dedicated Langmuir Probe assembly mechanical configuration before and after deployment.

Figure 3. Dedicated Langmuir Probe mechanical labeling.

8.3 Spherical probe

The spherical probe sensor is situated on top of a stiff boom 1.2 m from the spacecraft body when deployed. For calibration purposes a cable and probe simulator was used to replace the boom assembly.

Figure 4. Dedicated Langmuir Probe sensor.

8.3.1 Spherical probe low frequency calibration.

In Figure 5 a generic test set-up for dc calibrations is shown. The Keithley 220 current generator feed current to the preamplifier via a boom cable. The resulting voltage on the input of the ADC and the digital ADC output is collected by the controller and stored in the calibration file. The controller also operates the current generator to form a closed loop calibration sequence. Heavy filtering of the 220 mains supply is needed to ensure a low noise environment required for low current measurements.

Figure 5. The dc test set-up.

8.3.1.2 Converting telemetry values to physical units.

The proper physical units for the spherical probe measurements will be obtained using a physical parameters model applied to the in-flight measured probe current vs. bias (not given here). The dc probe current to TM unit conversion is given by calibration curves in 9.3.1.3.

8.3.1.3 Spherical probe dc calibration.

The calibration output data are stored in files with a unique file name describing the calibration type and at which temperature it was made.

Table 2. File name identification:

Character position	Description
1	Spherical probe
2	P = positive input current N = negative input current
3	Current
4,5	LG = low preamplifier gain HG = high preamplifier gain
6,7	LF = 6Hz filter HF = No filter
8-12	Temperature in deg. Celsius.

Table 3. Data file format:

Column	Description
1	Input current to probe (Amp)
2	Digitized data from LP 12bit ADC (0-4095 -5 to 5 V)
3	Input voltage to ADC (Voltage –5 to 5 V)

Spherical probe dc current calibration plots:

Figure 6. The dc transfer function from Sph to ADC dig. (See block diagram)

Preamplifier at high gain, pos. and neg. input current, 6 Hz filter, and 22C.

Figure 7. The dc transfer function from Sph to ADC dig. (See block diagram)

Preamplifier at high gain, pos. and neg. input current, without filter, and 22C.

Figure 8. The dc transfer function from Sph to ADC dig. (See block diagram)

Preamplifier at low gain, pos. and neg. input current, 6Hz filter, and 22C.

Figure 9. The dc transfer function from Sph to ADC dig. (See block diagram)

Preamplifier at low gain, pos. and neg. input current, without filter, and 22C.

Table 4. List of files for the dc spherical probe calibration:

Temperature	Fig No.	Positive Current	Negative Current
22C	1	SPCHGLF_22.CAS	SNCHGLF_22.CAS
2	SPCHGHF_22.CAS	SNCHGHF_22.CAS
3	SPCLGLF_22.CAS	SNCLGLF_22.CAS
4	SPCLGHF_22.CAS	SNCLGHF_22.CAS
-20C	SPCHGLF_-20.CAS	SNCHGLF_-20.CAS
SPCHGHF_-20.CAS	SNCHGHF_-20.CAS
SPCLGLF_-20.CAS	SNCLGLF_-20.CAS
SPCLGHF_-20.CAS	SNCLGHF_-20.CAS
0C	SPCHGLF_0.CAS	SNCHGLF_0.CAS
SPCHGHF_0.CAS	SNCHGHF_0.CAS
SPCLGLF_0.CAS	SNCLGLF_0.CAS
SPCLGHF_0.CAS	SNCLGHF_0.CAS
40C

ADC input

voltage only	SPCHGLF_40.CAS	SNCHGLF_-20.CAS
-	-
SPCLGLF_-20.CAS	SNCLGLF_-20.CAS
-	-

8.3.1.4 ADC filter characteristic.

Figure 10. ADC input aliasing filter response.

8.3.2 Spherical probe high frequency calibration

Figure 11. The ac test set-up.

Figure 11 show the test set-up used for the ac calibration. The transfer function from Sph to Sps ac is determined in this calibration (see block diagram).

Adding a constant current from the current generator to the ac current from the Network Analyzer generates an ac modulated current on the input of the probe. It should be noted that the input impedance for the preamplifier is in the order of a few Ohm at frequencies up to 10kHz. That is why the voltage output from the Network Analyzer is converted, via the resistor, to a defined the ac current at the input of the preamplifier. Half the ac p-p current divided by the dc current define the modulation (m), which is used as the main input parameter for this calibration. The modulation is defined as:

m = (Iac,p-p/2)/Idc

8.3.2.1 Converting telemetry values to physical units

The conversion from modulation (m) to mean output voltage (Vrms) is covered by this calibration. The estimate of signal power from modulation values requires a scientific model, which is not included here.

8.3.2.2 HF calibration tests / results for the WFR and WBR LP output.

Output voltage rms. vs. input m and the noise floor for a constant dc current is given by the MX plots. Those quantities can also be extracted from the frequency response calibration (S) plots and the dc Diff calibration. As the logarithmic factor varies with the input dc current the Diff calibration must be used if accuracy better than ±1.5 dB is required.

In the Modulation calibration (MX plots) the frequency is 660 Hz and m is varied from 50% to 0.05% in seven steps. The resulting instrument response is stored on paper plots (see figure 12).

Figure 12. M1 modulation plot. Idc = +10 nAmp

The output voltage can be obtained from the following equation

log(Vout) = k + log(m)

where k = -0.625 for an average of M1 to M15 levels.

Table 5. List of M plots:

Plot No.
File name
Input current
Preamp. Gain
Noise floor (<m)
1
m01.tif
+10nA
high
0.01%
2
m01.tif
+1nA
high
0.1%
3
m02.tif
-1nA
high
0.1%
4
m03.tif
-10nA
high
0.01%
5
m03.tif
+100nA
high
0.005%
6
m04.tif
+100nA
low
0.005%
7
m04.tif
+10nA
low
0.01%
8
m05.tif
+1uA
low
0.001%

The preamplifier is the dominating pole in the path from probe to LP output to WFR and WBR, and the frequency response is given by its gain. The 3dB point is 6kHz for high gain and 8kHz for low gain and independent on input current. Figure 13 show the frequency response for high gain and Figure 14 show the response for low gain.

Figure 13. Frequency response for the Spherical probe output to the RPWS wave receivers. High gain, m = 10% and IDC = 100nA.

Figure 14. Frequency response for the spherical probe output to the RPWS wave receivers. Low gain, m = 10% and Idc = 10nA.

Table 6. List of S plots:

Plot No.
File name
m %
Input DC current
Preamp. gain
1
S1_2.tif
10
1uA
High
2
S1_2.tif
10
100nA
High
3
S3_4.tif
10
10nA
High
4
S3_4.tif
10
1nA
High
5
S5_6.tif
10
-1nA
High
6
S5_6.tif
10
-10nA
High
7
S7_8.tif
10
-100nA
High
8
S7_8.tif
10
-1uA
High
9
S9_13.tif
10
10uA
Low
11
S11_15.tif
10
100nA
Low
13
S9_13.tif
10
-10uA
Low
15
S11_15.tif
10
-100nA
Low

8.3.3 Sphere probe bias voltage

The LP spherical probe bias voltage is under RPWSHR processor control. The Bias voltage can be set to 256 levels in four ranges. FUL covers a bias range from –33V to +31V in 256 steps and is used for sweeping and fixed bias. LOW, MID, and HIG have a range of 8 V in 256 steps and are centered around –6V, 0, and +6 respectively. They are used for fine sweeps around zero input current and sweep in low plasma temperatures. This calibration gives the transfer function from Dig Com to Sph Bias (see block diagram).

Figure 15, 16, 17, and 18 show plots of bias voltage vs. digital 8 bit word input from RPWS HP for FUL, LOW, MID, and HIG respectively at 22C.

Figure 15. LP spherical probe full bias range.

Figure 16. LP spherical probe low bias range.

Figure 17. LP spherical probe middle bias range.

Figure 18. LP spherical probe high bias range.

Table 7. List of Spherical probe bias calibration files.

Temp.	File name FUL	File name LOW	File name MID	File name HIG
-20C	A_FUL_-20.CAS	A_LOW_-20.CAS	A_MID_-
20.CAS	A_HIG_-20.CAS
0C	A_FUL_0.CAS	A_LOW_0.CAS	A_MID_0.CAS	A_HIG_0.CAS
22C	A_FUL_22.CAS	A_LOW_22.CAS	A_MID_22.CAS	A_HIG_22.CAS
40C	A_FUL_40.CAS	A_LOW_40.CAS	A_MID_40.CAS	A_HIG_40.CAS

8.4 Cylindrical probes

The LP cylindrical probes (cylp) are two of the three long antenna used by the HFR for E-field measurements from a few Hz to 10MHz. The LP is connected to –cylp and +cylp via two relays, a diode and a 100kohm resistor used for high voltage protection. The bias voltage is restricted to ±10V at the sensor due to satellite environment requirements.

8.4.1 Low frequency calibration

This Calibration is performed using the same test set-up as for the spherical probe dc calibration. The transfer functions from –Cylp and +Cylp to ADC Dig are obtained from this calibration (see block diagram).

8.4.1.1 Converting telemetry values to physical units

The proper physical units for the cylindrical probe measurements will be obtained using a physical parameters model applied to the in-flight measured probe current vs. bias (not given here). The dc probe current to TM unit conversion is given by calibration curves in 9.4.1.2.

8.4.1.2 Cylindrical probe dc calibration tests/results

The calibration output data are stored in files with a unique file name describing the calibration type and at which temperature it was made.

Table 8. File name identification:

Character position	Description
1,2	PC = Positive Cylp.
3,4	PC = positive input current NC = negative input current
5,6	LF = 6Hz filter HF = No filter
7,11	Temperature in deg. Celsius.

Table 9. Data file format:

Column	Description
1	Input current to probe (Amp)
2	Digitized data from LP 12bit ADC (0-4095 -5 to 5 V)
3	Input voltage to ADC (Voltage –5 to 5 V)

Figure 19. + Cylindrical probe dc calibration curves for 22C. 6Hz filter connected.

Figure 20. + Cylindrical probe dc calibration curves for 22C. No filter connected.

Figure 21. - Cylindrical probe dc calibration curves for 22C. 6Hz filter connected.

Figure 22. - Cylindrical probe dc calibration curves for 22C. No filter connected.

Table 10. List of files for the dc Cylindrical probe calibration:

Temperature	Cylp.	Positive Current	Negative Current
22C	neg	NCPCLF_22.CAS	NCNCLF_22.CAS
neg	NCPCHF_22.CAS	NCNCHF_22.CAS
pos	PCPCLF_22.CAS	PCNCLF_22.CAS
pos	PCPCHF_22.CAS	PCNCHF_22.CAS
-20C	neg	NCPCLF_-20.CAS	NCNCLF_-20.CAS
neg	NCPCHF_-20.CAS	NCNCHF_-20.CAS
pos	PCPCLF_-20.CAS	PCNCLF_-20.CAS
pos	PCPCHF_-20.CAS	PCNCHF_-20.CAS
0C	neg	NCPCLF_0.CAS	NCNCLF_0.CAS
neg	NCPCHF_0.CAS	NCNCHF_0.CAS
pos	PCPCLF_0.CAS	PCNCLF_0.CAS
pos	PCPCHF_0.CAS	PCNCHF_02.CAS
40C

ADC input

voltage only	neg	NCPCLF_40.CAS	NCNCLF_40.CAS
--	--	--
pos	PCPCLF_40.CAS	PCNCLF_40.CAS
--	--	--

8.4.2 High frequency calibration.

As for the spherical probe m will be the main input parameter for all cylindrical probe HF calibrations. Unlike the spherical probe logarithmic processor the cylindrical processor have a constant logarithmic factor for the range of interest and the transfer functions can be obtained form the flat part of the NC and PC frequency response plots. The test set-up is the same as for the spherical probe (see Figure 5). The cylindrical probes frequency response is depending on the input current and range from a few Hz to tens of kHz.

8.4.2.1 Converting telemetry values to physical units.

The conversion from m to Vrms output voltage is covered by this calibration. The estimate of signal power from m values requires a scientific model, which is not included here.

8.4.2.2 Cylindrical probes HF calibration tests/results.

The output voltage from the cylindrical probe to WBR is given by the same equation as for the spherical probe. Figure 23 and 24 show the frequency response for +100nA and –100nA current input to the negative cylindrical probe and Table 10 lists the frequency response obtained from all NC and PC plots. The relationship is given by

log(Vout) = k + log(m)

where k = -0.648 obtained from plot NC5, NC6, PC5 and PC6.

Figure 23. –Cylindrical probe frequency response at 100nA input current.

Figure 24. –Cylindrical probe frequency response at -100nA input current.

Table 10. List of NC and PC plots.

Plot No	File name	3dB cut off freq.	Input current
NC8	NC8.tif	2750Hz	1uA
NC5	NC4_5.tif	1650Hz	100nA
NC2	NC1_2.tif	216Hz	10nA
NC1	NC1_2.tif	20Hz	1nA
NC7	NC6_7.tif	3050Hz	-1uA
NC6	NC6_7.tif	1070Hz	-100nA
NC3	NC3.tif	148Hz	-10nA
NC4	NC4_5.tif	31Hz	-1nA
PC8	PC8_7.tif	3160Hz	1uA
PC5	PC5_6.tif	1290Hz	100nA
--	--	--	10nA
PC1	PC1_4.tif	26Hz	1nA
PC7	PC8_7.tif	2950Hz	-1uA
PC6	PC5_6.tif	1080Hz	-100nA
--	--	--	-10nA
PC4	PC1_4.tif	35Hz	-1nA

8.4.3 ±Cylindrical probe bias voltage.

The cylindrical probe bias can be operated in two ranges, ±10 V and ±5V, with 256 steps in each range. To find the actual probe bias the voltage drop over the 100kohm resistor in series with the diode need to be subtracted. This calibration gives the bias voltage at the driver end. Figure 25 and 26 show plots of the two ranges at 22C.

Figure 25. ±Cylindrical probe full bias range at 22C

Figure 26. ±Cylindrical probe middle bias range at 22C.

Table 11. List of Cylindrical probe bias calibration files.

Temp.	File name FUL	File name MID
-20C	B_FUL_-20.CAS	B_MID_-
20.CAS
0C	B_FUL_0.CAS	B_MID_0.CAS
22C	B_FUL_22.CAS	B_MID_22.CAS
40C	B_FUL_40.CAS	B_MID_40.CAS