Quit[] can be used to complete the kernel, and then everything that is evaluated in the notebook will start it again.
If the reason you want to do this is to make sure that what is running on the laptop does not conflict with previously defined characters, you can set the menu instead:
Evaluation > Notebook Default Context > Unique to This Notebook
If you want Quit[] to set the kernel often, then you may need to adjust the keyboard shortcut. For this see this answer:
Customizing Mathematica Shortcuts
If you find that Mathematica slows down in the middle of processing a long laptop, and the last parts of this laptop are independent of the previous parts, you can paste this at the beginning of your laptop:
Needs["Utilities`CleanSlate`"]
and then paste:
CleanSlate[]
where the Quit[] effect is required Quit[]
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