Cal. Pen. Code § 2621
When the testimony of a material witness is required in a criminal action, before any court in this state, or in an examination before a grand jury or magistrate in a felony case and such witness is a prisoner in a state prison, an order for the prisoner's temporary removal from such prison, and for the prisoner's production before such court, grand jury or magistrate, may be made by the superior court of the county in which such action or examination is pending or by a judge thereof; but in case the prison is out of the county in which the application is made, such order shall be made only upon the affidavit of the district attorney or of the defendant or the defendant's counsel, showing that the testimony is material and necessary; and even then the granting of the order shall be in the discretion of said superior court or a judge thereof. The order shall be executed by the sheriff of the county in which it is made, whose duty it shall be to bring the prisoner before the proper court, grand jury or magistrate, to safely keep the prisoner, and when the prisoner is no longer required as a witness, to return the prisoner to the prison whence the prisoner was taken; the expense of executing such order shall be a proper charge against, and shall be paid by, the county in which the order shall be made. Such orders shall recite the purposes for which said person is to be brought before the court, grand jury or magistrate, and shall be signed by the magistrate or judge making the order, and sealed with the seal of the court, if any.
Such order must be to the following effect:
County of ____ (as the case may be).
The people of the State of California to the warden of ____:
An order having been made this day by me, that A. B. be produced in this court as witness in the case of ____, you are commanded to deliver the prisoner into the custody of ____ for the purpose of (recite purposes).
Dated this ____ day of ____, 19__.
When a prisoner is removed from a state prison under this section the prisoner shall remain in the constructive custody of the warden hereof. During the prisoner's absence from the prison, the prisoner may be ordered to appear in other felony proceedings as a defendant or witness in the courts of the county from which the original order directing removal issued. A copy of the written order directing the prisoner to appear before any such court shall be forwarded by the district attorney to the warden of the prison having protective custody of the prisoner.
Ca. Pen. Code § 2621