Safety procedure for deuterium DNB operation
                                R. Granetz
                                July, 2003

The short pulse hydrogen DNB that we are currently using is also capable of 
being operated as a deuterium beam, and we are interested in doing this to 
study a number of physics issues.  However, the 50 kV deuterons will create 
copious amounts of 2.45 MeV neutrons from D-D fusion, as detailed below, so 
we need to be sure that personnel are not exposed.

There are actually two distinct cases that we need to address:
 a) Beam into plasma:
      The expected neutron rate is of order 10^13 - 10^14 neutrons/s.  This
      rate is comparable to recent high-power RF discharges, but of course,
      the beam pulse is only 50 ms long.  Therefore a day of deuterium DNB
      operation into plasma yields neutron doses that are similar to a
      single discharge with high-power RF operation.

      Note: personnel exposure is not an issue in this case, since plasma
      operation is already precluded if anyone is in the cell or power
      room.  HOWEVER, during plasma runs it is likely that the DNB will
      also be operated between shots for conditioning purposes, which is
      covered in the following sections.

 b) Beam conditioning (i.e. between plasma shots, or during non-run periods)
      The surfaces of the DNB calorimeter, the beam dump, and the vessel
      wall will effectively be infused with deuterium, since they will be
      operating in a deuterium gas environment.  Therefore the D-D fusion
      neutron production is analogous to operation of a thick-target
      accelerator.  Based on actual experience of the Cockroft-Walton
      accelerator, the expected 2.45 MeV neutron rate will be of order
      10^10 - 10^11 neutrons/s, which is a factor of ~1000 less than
      beam-into-plasma.  Note however, that the calorimeter and beam dump
      are outside of the igloo, so the dose in the cell may not be reduced
      by this full factor.  Fusion neutrons are also generated in the
      neutralizer, but this is about an order of magnitude less than from
      the targets.

Because of the neutron dose, personnel access into the cell must be prevented 
during beam conditioning.  There currently is no hardware-based, foolproof, 
interlock method to do this with the DNB.  THEREFORE WE WILL HAVE TO RELY ON 
A COMBINATION OF CELL LOCKUP AND UNDERSTANDING BY THE DNB OPERATOR THAT THE 
BEAM IS NOT TO BE FIRED UNLESS THE DNB OPERATOR KNOWS THE CELL IS LOCKED AND 
VACATED.  This procedure is deemed to be acceptable as long as deuterium beam 
operation is rare, as is the case so far at C-Mod.

There are two acceptable means for cell lockup and personnel access
control:

  a) During plasma runs, or non-run periods when an engineering operator is
     available, full access control will be enabled, including an initial
     search and clearance of the cell and closure of the big concrete door.
     The engineering operator will then give permission to the DNB operator
     to operate the beam.  If the cell is breached, or someone has been
     granted access to the cell, the engineering operator will immediately
     inform the DNB operator that permission to operate the beam is
     revoked.  When the cell is cleared and locked again, the engineering
     operator will then give permission again to the beam operator to
     continue DNB operation.  (This method has the added advantage that a
     full MDSplus data acquisition cycle can optionally be run during
     non-run periods by the engineering operator, which allows neutron data
     and beam component spectroscopy data to be acquired in synch with the
     DNB pulse.)

  b) If an engineering operator is not available, then the beam operator
     must control access to the cell, as well as controlling DNB operation.
     This is done by first closing the roll-up door, then performing a
     search to verify that the cell is vacated, then locking the downstairs
     cell door and removing the key.  The big concrete door must also be
     closed.  The key to the downstairs cell door remains in the possession
     of the DNB operator.  DNB operation is permitted as long as the DNB
     operator has the door key.  If cell access is required, the DNB
     operator must relinquish the key to unlock the downstairs cell door,
     and therefore is not allowed to operate the DNB until the cell is
     cleared, the door is re-locked, and the key is back in the possession
     of the DNB operator.