Since nobody has answered this yet, I'll venture an answer. Hate to monopolize the forum, however.

There are two ways in which the RF can damage the probe. The first is voltage breakdown (arcing), which is an (almost) instantaneous effect. The length of the pulse doesn't really matter except that arcing during a long pulse will inflict more damage than a short one. The effects are also cumulative, in that arcing during many short pulses can be a severe as one long one. Arcing causes the worst damage when it occurs across plastic materials such as teflon capacitors, plastic MAS stators, capacitor wands, etc. In this case the arc creates a carbon track which will cause the probe to arc at a significantly lower voltage in the future. I've seen an MAS probe that should take hundreds of Watts arc at 25 Watts once this has happened. Severe overvoltage can even punch holes in the ceramic and glass/quartz dielectrics used in some capacitors. Another damage mechanism is the formation of rough edges (burrs) on metal surfaces that can create high fields (voltage gradients) and lower the breakdown voltage in the future. Solids probes in general, and narrow bore solids probe especially, tend to be more prone to arcing because they are really pushing the limits on power handling to get the B1 fields required.

Symptoms of arcing include erratic amplitudes and phases when doing pulse calibrations, or even just an array of one-pulse spectra with identical parameters, or severe t1 noise, or erratic pulse calibrations. Note that it does sometimes does take a pulse length of a few us for the breakdown to occur (the first few points in your calibration array will be fine). You can also check for arcing with a directional coupler and a 'scope, but the directional coupler should be in reflected power mode. A gradual lengthing of your 90 deg. pulse over time is not normally a typical symptom of arcing. If this is your problem, the first place I would check is the actual power being deliverd to the probe and the probe tuning. Also check the S:N if it is spacified for that probe channel.

If you operate your probes especially liquids probes within the limits specified by the manufacturer, then arcing shouldn't really be much of a concern, especially for liquids probes. Unless, of course, someone forgets to turn off the solids amplifiers! - Been there, done that :-(

Routinely pushing the probe to the point of arcing is neither a recommended nor a necessary procedure.

The other way in which the RF can damage a probe is through overheating caused by long pulses. Your vendor will usually specify a maximum length that the decoupler or spin-lock field can be on for a given power (or B2 field strength) level. They may also specify a maximum duty cycle.

For gradient coils, the manufacturer will also specify maximum gradient pulse length and duty cycle. Some gradient amplifiers have fuses or other protection for the gradient coils. The best performance tests for gradient coils are those probe performance tests (e.g. recovery time, gradient strength, etc.) specified by the manufacturer.

Remember, you vendor has likely already tested the probes to destruction, and knows what any given design should handle.

Since nobody has answered this yet, I'll venture an answer. Hate to monopolize the forum, however.

There are two ways in which the RF can damage the probe. The first is voltage breakdown (arcing), which is an (almost) instantaneous effect. The length of the pulse doesn't really matter except that arcing during a long pulse will inflict more damage than a short one. The effects are also cumulative, in that arcing during many short pulses can be a severe as one long one. Arcing causes the worst damage when it occurs across plastic materials such as teflon capacitors, plastic MAS stators, capacitor wands, etc. In this case the arc creates a carbon track which will cause the probe to arc at a significantly lower voltage in the future. I've seen an MAS probe that should take hundreds of Watts arc at 25 Watts once this has happened. Severe overvoltage can even punch holes in the ceramic and glass/quartz dielectrics used in some capacitors. Another damage mechanism is the formation of rough edges (burrs) on metal surfaces that can create high fields (voltage gradients) and lower the breakdown voltage in the future. Solids probes in general, and narrow bore solids probe especially, tend to be more prone to arcing because they are really pushing the limits on power handling to get the B1 fields required.

Symptoms of arcing include erratic amplitudes and phases when doing pulse calibrations, or even just an array of one-pulse spectra with identical parameters, or severe t1 noise, or erratic pulse calibrations. Note that it does sometimes does take a pulse length of a few us for the breakdown to occur (the first few points in your calibration array will be fine). You can also check for arcing with a directional coupler and a 'scope, but the directional coupler should be in reflected power mode. A gradual lengthing of your 90 deg. pulse over time is not normally a typical symptom of arcing. If this is your problem, the first place I would check is the actual power being deliverd to the probe and the probe tuning. Also check the S:N if it is spacified specified for that probe channel.

If you operate your probes within the limits specified by the manufacturer, then arcing shouldn't really be much of a concern, especially for liquids probes. Unless, of course, someone forgets to turn off the solids amplifiers! - Been there, done that :-(

Routinely pushing the probe to the point of arcing is neither a recommended nor a necessary procedure.

The other way in which the RF can damage a probe is through overheating caused by long pulses. Your vendor will usually specify a maximum length that the decoupler or spin-lock field can be on for a given power (or B2 field strength) level. They may also specify a maximum duty cycle.

For gradient coils, the manufacturer will also specify maximum gradient pulse length and duty cycle. Some gradient amplifiers have fuses or other protection for the gradient coils. The best performance tests for gradient coils are those probe performance tests (e.g. recovery time, gradient strength, etc.) specified by the manufacturer.

Remember, you vendor has likely already tested the probes to destruction, and knows what any given design should handle.