28 U.S.C. § 1821 : US Code - Section 1821: Per diem and mileage generally; subsistence

      (a)(1) Except as otherwise provided by law, a witness in
    attendance at any court of the United States, or before a United
    States Magistrate Judge, or before any person authorized to take
    his deposition pursuant to any rule or order of a court of the
    United States, shall be paid the fees and allowances provided by
    this section.
      (2) As used in this section, the term "court of the United
    States" includes, in addition to the courts listed in section 451
    of this title, any court created by Act of Congress in a territory
    which is invested with any jurisdiction of a district court of the
    United States.
      (b) A witness shall be paid an attendance fee of $40 per day for
    each day's attendance. A witness shall also be paid the attendance
    fee for the time necessarily occupied in going to and returning
    from the place of attendance at the beginning and end of such
    attendance or at any time during such attendance.
      (c)(1) A witness who travels by common carrier shall be paid for
    the actual expenses of travel on the basis of the means of
    transportation reasonably utilized and the distance necessarily
    traveled to and from such witness's residence by the shortest
    practical route in going to and returning from the place of
    attendance. Such a witness shall utilize a common carrier at the
    most economical rate reasonably available. A receipt or other
    evidence of actual cost shall be furnished.
      (2) A travel allowance equal to the mileage allowance which the
    Administrator of General Services has prescribed, pursuant to
    section 5704 of title 5, for official travel of employees of the
    Federal Government shall be paid to each witness who travels by
    privately owned vehicle. Computation of mileage under this
    paragraph shall be made on the basis of a uniformed table of
    distances adopted by the Administrator of General Services.
      (3) Toll charges for toll roads, bridges, tunnels, and ferries,
    taxicab fares between places of lodging and carrier terminals, and
    parking fees (upon presentation of a valid parking receipt), shall
    be paid in full to a witness incurring such expenses.
      (4) All normal travel expenses within and outside the judicial
    district shall be taxable as costs pursuant to section 1920 of this
    title.
      (d)(1) A subsistence allowance shall be paid to a witness when an
    overnight stay is required at the place of attendance because such
    place is so far removed from the residence of such witness as to
    prohibit return thereto from day to day.
      (2) A subsistence allowance for a witness shall be paid in an
    amount not to exceed the maximum per diem allowance prescribed by
    the Administrator of General Services, pursuant to section 5702(a)
    of title 5, for official travel in the area of attendance by
    employees of the Federal Government.
      (3) A subsistence allowance for a witness attending in an area
    designated by the Administrator of General Services as a high-cost
    area shall be paid in an amount not to exceed the maximum actual
    subsistence allowance prescribed by the Administrator, pursuant to
    section 5702(c)(B) (!1) of title 5, for official travel in such
    area by employees of the Federal Government.

      (4) When a witness is detained pursuant to section 3144 of title
    18 for want of security for his appearance, he shall be entitled
    for each day of detention when not in attendance at court, in
    addition to his subsistence, to the daily attendance fee provided
    by subsection (b) of this section.
      (e) An alien who has been paroled into the United States for
    prosecution, pursuant to section 212(d)(5) of the Immigration and
    Nationality Act (8 U.S.C. 1182(d)(5)), or an alien who either has
    admitted belonging to a class of aliens who are deportable or has
    been determined pursuant to section 240 of such Act (8 U.S.C.
    1252(b)) (!1) to be deportable, shall be ineligible to receive the
    fees or allowances provided by this section.
      (f) Any witness who is incarcerated at the time that his or her
    testimony is given (except for a witness to whom the provisions of
    section 3144 of title 18 apply) may not receive fees or allowances
    under this section, regardless of whether such a witness is
    incarcerated at the time he or she makes a claim for fees or
    allowances under this section.