Hey! Good call turning it off — those sparks could mean the waveguide cover’s damaged or grease buildup is reflecting the energy. Don’t run it again until someone checks it. You can contact experts offering home appliance repair in Orange County, CA https://akappliancerepairca.com/ to inspect the magnetron or replace the cover safely. I once had AK Appliance Repair fix a similar issue, and they cleaned the cavity coating and replaced the burned part in one visit. It’s way cheaper than buying a new microwave, and now mine works perfectly again.