**3. Causes of microwave tube failures**

There are several well-known causes of microwave tube failure. These include:

#### **3.1 Cathode emission decrease**

A decrease in emission normally results in lowering of both the upper and lower mode boundaries. When this shift downward in current becomes significant, the operating point current has to be adjusted downward to avoid instability and oscillation. The operating point is adjusted periodically during the life of the tube. The tube remains operable until either its output power is low or the shift in mode boundaries due to cathode emission precludes stable operation over the frequency band. Cathode emission degradation is a long-term event that requires regular and careful monitoring.

## **3.2 Loss of vacuum**

Loss of vacuum is a catastrophic failure, which may not be determined with electrical monitors. From experience, it is known that the heater bushing is the predominant failure. Other key vacuum seals include the RF input and output ceramic-to-metal seals.

#### **3.3 Heater failure**

Heater failure is a catastrophic event and results in inability to start cathode emission. Shorted heaters are more typical than open heaters. The cause of this fault cannot be determined with electrical monitors.

*Acoustics of Materials*

time available during the inter-pulse period of the tube for data buffering and fault analysis. The present monitoring systems work well if the microwave tube is operated with 200 or less pulses per second (pps). Normally, the radar tubes are operated at up to 1000 pps with pulse duration of a microsecond. Increasing the A/D conversion speed will, in some cases, make the situation worse, since it increases the amount of data that must be transferred and analyzed during the small time

voltages that can run up to more than 10 kV, while their heat dissipation ranges from 100 W to 10 kW. The complexity of these systems makes them very expensive to produce, maintain, and replace. This provides a motivation for the development of

In recent years, research has established acoustic emission (AE) sensing as a very effective technique for machine condition monitoring and analysis. This technique has been tested and evaluated in a variety of systems as an alternative to conventional techniques. A novel application of this technique is the in-situ performance monitoring of high-power microwave (HPM) tubes. This report addresses two questions: (1) Would the microwave radar tubes operating under normal or abnormal conditions be able to generate AE signals? (2) If so, can the observed signals

–10<sup>−</sup><sup>8</sup>

Torr) have electrode

interval available. These high vacuum devices (10<sup>−</sup><sup>7</sup>

alternative, more effective monitoring and diagnostic techniques.

provide signatures to discriminate among different types of failures?

Condition Assessment System) software currently used by the U.S. Navy.

**2. AE as an advanced nondestructive technique**

Once the output of the sensors was integrated with the ICAS software, a method was developed for integrating a plurality of sensor data in such a way to produce greater information than any individual sensor or combination of sensors. This method is particularly useful for detecting and predicting failures and for life cycle monitoring in microwave vacuum devices. This method has been defined as a

The fundamental principle of this method is based on the phenomenon of the generation of an acoustic pulse when a shock wave is generated inside a solid [1–4].

Acoustic emission (AE) may be defined as stress or pressure waves generated during dynamic processes in materials. AE is elastic energy that is spontaneously released by materials when they undergo deformation and is typically generated in the form of ultrasound waves created by local mechanical instabilities within the material. AE is generally detected by means of ultrasonic transducers coupled to the material with a suitable coupler to decrease impedance mismatch. Among many mechanisms that produce AE activity, the principal mechanisms are crack initiation and growth, magneto-mechanical realignment or growth of magnetic domains (Barkhausen effect), dislocation movements, twining, phase changes, fracture of brittle inclusions, fiber breakage in composite materials, chemical activity, and cavitation. Some stimuli are necessary to trigger acoustic emissions. Stress, a major type of stimuli, may be mechanically applied, thermally generated, or caused by a changing magnetic field. Acoustic emission could thus act as a passive nondestructive technique (NDT) and be used to monitor and analyze normal and abnormal performance of microwave vacuum tubes. The research presented in this paper demonstrates the detection of anomalous RF pulses and system failures using acoustic emission and magneto resistive or inductively coupled current sensors. It also demonstrates the ability to discriminate among the different types of failures. This innovative system has been tested on a klystron as part of an AN/SPS-49(V)5 radar system and on a radar system magnetron (Model 2J55). An added feature of this innovative system is the fact that the outputs from the sensors have been successfully interfaced with the ICAS (Integrated

**44**

virtual sensor.
