3. Theory of Operation
Functional Block Descriptions
139
Cesium Beam Tube Block
The Cesium Beam Tube block consists of the A2 CBT Controller, A16 High
Voltage Supply module, and A17 CBT. It has three inputs and one output.
A2 receives command input from the I-bus to generate five functional signals:
a. control/monitor lines routed to the A16 high voltage supply module,
b. Thermistor signal lines routed to A17 CBT,
c. mass-spectrometer control signal routed to A17,
d. hot-wire ionizer control signal routed to A17, and
e. cesium-oven control lines also sent to A17. A2 also transfers the C-field
control signal generated on A7 to the C-field winding inside A17.
A16 generates supply voltages for the ion pump and electron multiplier in
A17. A16 receives power and control signals from A2. The Ion pump supply
delivers an open-circuit voltage of +3500V, nominal. It is unregulated and
supplies a limited amount of power to A17 under load. The current drawn is
monitored by A2 and A7 and is readable via the front panel interface. If the
ion-pump current exceeds 50 microamps, the instrument cannot warm-up.
The CBT Electron multiplier requires a low-noise supply at a voltage of -900
to -2500 volts dc. The set-point of this supply is determined by a control
current generated by a D to A converter on A2. The Electron-Multiplier supply
is disabled whenever the instrument is the “Fatal Error” or “Standby” modes.
A17 has eight inputs and one primary output. The ion-pump, electron-
multiplier, C-field control, Thermistor-signal lines, mass-spectrometer control,
hot-wire ionizer supply, cesium-oven supply are used to maintain proper CBT
operating conditions, while the 9.19263177138 GHz microwave probe signal
input is used to create a CBT error output signal. The CBT error signal is
routed to one input of the analog signal chain block.
Electronically, A17 is a non-linear resonator that uses the microwave
hyperfine resonance of Cesium-133 atoms in an atomic beam. Close to the
resonance frequency of 9192.63177 MHz, CBT output current changes
rapidly in a predictable manner with the frequency of the applied microwave
probe signal.
Physically, A17 is a sealed, evacuated assembly containing its own vacuum
pump. To operate, A17 requires a number of external power supplies and
signals provided by A2, A7 and A16. A17 contains a read-only memory
(ROM) that stores its operating parameters. The ROM is read at power-up by
A3 using circuits on A2. Up to 30 minutes may be required for A17 to reach
operating temperature.
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