Ishraaq (“ishrak”) prayer is Duha (“chast”) prayer offered at the beginning of its time. These are not two different prayers. It is called Ishraaq because it is done immediately after sunrise (shurooq) when the sun has risen to a certain height.
Shaykh Ibn Baaz said:
Ishraaq prayer is Duha prayer done at the beginning of its time.
Majmoo’ Fataawa al-Shaykh Ibn Baaz, 11/401
The time of Duha prayer is from the time when the sun has risen to a certain height, until just before the time for Zuhr prayer.
Shaykh Ibn ‘Uthaymeen defined it as being from a quarter of an hour after the sun has risen until ten minutes before Zuhr prayer.
Al-Sharh al-Mumti’, 4/122
So all of this period is the time for Duha prayer.
It is better to pray it after the sun’s heat has become intense, because the Prophet (peace and blessings of Allaah be upon him) said: “It is preferable to offer Duha prayer when the sun’s heat has become so intense that even the young of the camels feel it.”
Narrated by Muslim, 748
Ibn Baaz: Majmoo’ Fataawa, 11/395
The scholars defined this as being when one quarter of the day has passed, i.e., halfway between sunrise and Zuhr prayer.
See al-Majmoo’ by al-Nawawi, 4/36; al-Mawsoo’ah al-Fiqhiyyah, 27/224 .