exciting TEM mode threw outer casing slot of coax cable

[yefj]

Hello , I have created a coaxial cable which has a short circuit from both sides.
I think this is a TEM mode.
I closed this cable from both sided and made a eigen mode solver simulation in CST, so it created all the possible modes in the cavity .
(dissregarding the need for excitation)

however i want to excite the TEM mode by creating a slotas shown below on the outer casing(keeping both sides closed)
what kind of excitation method you reccomend to excite this TEM mode threw that slot?
Thanks.

 

[yefj]

Hello,I used a method which is using a coax probe as shown below.
It excited the TEM mode in my resonator as shown in the photos below.
Currently the coax probe is very very under coupled as shown in the S params and smith chart.
Is there a way to see how can I improve the coupling threw the E-field generated?
Thanks.