For sorting in descending order, you'll need to use '-1' instead of 'desc'. This is how it's done in the shell:
db.collection.find({}).sort({'showed._i':-1}).limit(30)
Also, the field "showed._i" is stored as a string, rather than a Date. The values will need to converted to Date type for the correct results to be returned by Sorting.
Check this post for an approach to convert string to Date type