求翻译：获得美国签证需要很长的周期。这里，我想请您发给我一个邀请函以确保我能够及时拿到签证，按时参加会议是什么意思？

Get United States visas take a very long period. Here, I would like to invite you to send me a invitation to ensure I can get a visa in time, on time to attend the meeting