Tecnai on-line help manual -- Options - UT Southwestern

<strong>Tecnai</strong> on-line help Options 132Options Software version 210 TEM AutoGun (FP5460/20)10.1 AutoGun proceduresThe AutoGun Control Panel provides a number of automated procedures. A short description of theseprocedures is given below.10.1.1 Auto ConditoningConditioning can be used to stabilize the high tension. It is mostly used after an exchange of a filamentor on higher-voltage (200kV or higher) instruments. Conditioning takes the high tension up in controlledsteps while monitoring the vacuum pressure in the gun. If the pressure remains more than 2 log stepsabove the previous attained level, the procedure will wait for it to drop again. If the pressure exceeds thepreviously attained value by a large amount, the procedure will step the high tension back a bit and wait.Conditioning goes up to the specified level with the delay step, both as defined by the user.On 100 and 120kV conditioning takes place by slowly increasing the high tension up to the maximum ofthe instrument (these microscopes do not have the ability to increase the high tension beyond thehighest value). Once the user-defined goal has been reached, the conditioning procedure is finished. Ifthe currently active high-tension setting is not one of the fixed high-tension steps, the high tension will beset to the closest fixed high-tension step below the current value.On 200 and 300kV instruments, the conditioning will take place in the normal regime at low high tensionsettings, but switch to true conditioning when an overvoltage (a high tension value beyond the normalmaximum of 200 or 300kV) is required. After the user-define goal has been reached, the procedure willkeep the high tension at that value for 10 more minutes for stabilization. Afterwards the high tension isset to the closest fixed high-tension step equal to or below the current value.The starting point for the conditioning depends on the current high-tension status :• If the high tension is off, it will be changed to lowermost fixed high-tension step.• If the high tension is on, the current high tension value defines the starting point.During conditioning the control panel displays how much time conditioning is estimated to take. Theestimate is based on the assumption that there will be no delays due to vacuum pressures increases.Conditioning is accessible only to Expert user level or higher.10.1.2 Auto SaturationThe saturation of thermionic filaments is often problematic. Many microscope users tend to try andremove all structure visible in the focused beam (while especially LaB6 filaments tend to have a structurethat cannot be removed or only by strongly oversaturating the filament). The resulting overheating leadsto considerable reductions in filament life time. The correct saturation point of a filament is the pointwhere further increases in heating current do not lead to significant increases in beam current. Once thebeam is defocused for normal operation, any structure visible in the focused beam disappears.The AutoGun software has a procedure for automatically saturating the filament to the correct point. Thefinal measurement is always made using the screen current as a function of filament step, combined withrefinement of the gun tilt. The typical pattern seen is an S-shaped curve when the filament setting is