I pray five times a day regularly behind the Imam in the mosque. The problem is in the beginning of the "Fard" prayer, I can keep my concentration only in the first Raka'ah; then Satan comes in to my mind and I lose concentration and only after the Imam finishes the prayer I realize that I have lost the good deeds of the Salah. If I close my eyes through out the prayer, I can keep my concentration, but people say that closing eyes is not permitted, kindly advice me if there is any Du'a or so to keep Satan away from me in "Fard" prayer?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Allah praises those who persevere in their prayers. Allah Says (interpretation of meaning): {Successful indeed are the believers. Those who offer their Salât (prayers) with all solemnity and full submissiveness.} [23: 1-2].
Lack of reverence decreases the reward of a prayer. The Prophet (Sallallahu Alaihi wa Sallam) said: "A person turns away from his prayer whereas it is not written in his account except half or third or forth or fifth or sixth till he said tenth of the prayer" [ Ahmad and Abu Dawood ].
There are many reasons for achieving the reverence in prayers some of them are as follows:
1) Preparing to perform prayer right after hearing Azan (call to prayer) or just before it to allow time to perform the Sunnah before the obligatory prayer, and even a little time to recite some Qur'an.
2) Remembering the Greatness of Allah and the importance of standing in front of Allah.
3) Considering the meaning the of verses one recites or that one hears from the Imam.
4) Continuing to recite the supplication of Istiftah (at the beginning of prayer) the Taawwudh (seeking refuge of all from Satan) and on reciting Bismillah.
5) Closing the eyes is permitted if it is useful in achieving reverence in prayer. We do not know any opinion that forbids closing the eyes in prayer though some scholars dislike it, but if there is any benefit then there is no harm in doing so. However, it is desirable to keep one's eyes at the place of prostration.
6) Continuing to perform Sunan al-Ratibah before and after the obligatory prayers. There are Ahadith proving that the defect of the obligatory prayers would be completed by the voluntary prayers.
7) Continuously asking Allah to make you of those slaves who are reverent and submissive.
Allah knows best.