Controls
Adjusts the retarding field. More negative = stronger repulsion.
Accelerates electrons from the cathode.
Determines the number of RF cycles in the transit time.
Calculated Parameters
Applegate Diagram (Space-Time)
● Early/Slow ● Late/Fast ● Bunch Center
Mode Characteristics
Power vs. Repeller Voltage
Cavity Gap Interaction
Velocity Modulation & Bunching
Theory of Operation
1. Introduction
The Reflex Klystron is a single-cavity microwave oscillator. It generates RF power by converting the kinetic energy of an electron beam into microwave energy. It relies on the principles of velocity modulation and electron bunching.
2. Velocity Modulation
Electrons emitted from the cathode are accelerated by the Beam Voltage ($V_0$). As they traverse the cavity gap, they interact with the RF field.
- Accelerated: Electrons passing when the RF field is positive speed up.
- Decelerated: Electrons passing when the RF field is negative slow down.
This differential velocity creates the condition for bunching.
3. The Applegate Diagram
This diagram plots distance (X-axis) versus time (Y-axis).
1. Electrons enter the repeller space at different times (vertical lines).
2. Due to velocity differences, their paths diverge.
3. They form a dense bunch at a specific distance.
4. The repeller voltage ($V_r$) is tuned so this bunch returns to the cavity exactly when the RF field is negative, delivering maximum energy to the cavity.
4. Mode Condition
For oscillation to build up, the transit time $\tau$ must satisfy:
$$ f_0 \cdot \tau = n + \frac{3}{4} $$
where $n$ is the mode number (1, 2, 3...)
The $3/4$ factor accounts for the phase reversal needed for positive feedback (electrons must see a retarding field upon return).
Experimental Procedure
Initialization
Set the Beam Voltage ($V_0$) to 300V. Set the Mode selector to $n=2$. Observe the Applegate diagram. You should see electrons entering from the left.
Mode Tuning
Slowly vary the Repeller Voltage ($V_r$) from -50V to -300V.
- Watch the Applegate Diagram: Notice how the "bunch" forms and moves.
- Watch the Power Meter: Identify the peaks. These are the operating "Modes".
Data Logging
Record the Repeller Voltage ($V_r$) and Output Power for at least 15 points to plot the Mode Characteristic curve manually on graph paper.
Mode Analysis
Switch to Mode $n=1$ and $n=3$. Observe how the optimal Repeller Voltage shifts. Higher modes (larger $n$) require a longer transit time, which means a weaker repeller field (less negative voltage).
Lab Report Guidelines
Required Sections
- Aim of the experiment
- Theory (Velocity Modulation)
- Schematic Diagram
- Observations Table
- Graph: Power vs. $V_r$
- Result & Conclusion
Viva Questions
- 1. Why is it called a "Reflex" Klystron?
- 2. What is the significance of the $3/4$ factor in the mode equation?
- 3. Why does power decrease if $V_r$ is too negative?
- 4. What is electronic tuning?
- 5. Compare efficiency of Mode 1 vs Mode 2.
Sample Data Table
| S.No | Repeller Voltage (V) | Output Power (mW) | Remark |
|---|---|---|---|
| 1 | -80 | 0.0 | No oscillation |
| 2 | -120 | 45.2 | Peak Power (Mode 2) |
| 3 | -180 | 0.0 | Between modes |