To   overcome   beam   spreading,   an   axial   magnetic field  is  used.  The  action  of  the  magnetic  field  is  to exert  a  force  on  the  electrons  that  keeps  them  focused into  a  narrow  beam.  The  magnetic  field  may  be  de- veloped   by   a   permanent   magnet   or   by   one   or   more electromagnets.  A  permanent  magnet  is  used  in  tubes that  are  physically  small  or  of  medium  power  rating. Unfortunately,  the  size  and  the  weight  of  a  permanent magnet   are   excessive   for   long   or   high-power   tubes, making   it   necessary   to   use   electromagnets.   In   some large  tubes,  several  separate  electromagnets  are  used. The  current  in  each  coil  is  individually  adjustable  to optimize  the  magnetic  field  shape.  The  magnetic  field is   normally   terminated   a   short   distance   beyond   the output cavity so that the beam can spread before it hits the  collector.  This  tends  to  spread  the  electron  beam interception   over   a   large   surface   on   the   collector, minimizing  collector  cooling  problems  that  otherwise would  result  from  the  beam  remaining  concentrated  at the  time  of  interception. Even  with  an  axial  magnetic  field,  some  electrons stray   from   the   main   electron   beam.   These   electrons are   intercepted   by   the   anode   or   the   klystron   drift tubes.  In  high-power  tubes,  it  is  particularly  important to  minimize  the  number  of  stray  electrons  because  of the   heat   generated   when   they   strike   the   drift   tubes. And  in  a  high-power  klystron,  this  heating  maybe  a very  severe  problem  because  drift  tubes  are  quite  dif- ficult  to  cool.  Temperatures  may  become  high  enough to  melt  the  drift  tubes,  thus  destroying  them. Klystron   amplifiers   normally   have   actual   metal grid  structures  across  the  gaps  in  the  resonant  cavities. Many   low-power   klystrons   have   wire   mesh   grids. However,   most   high-power   klystrons   do   not   have actual  grids  across  the  gaps.  Such  grids  would  inter- cept   sizable   quantities   of   electrons. It  is  very  difficult  to  cool  grid  structures,  and  a large  amount  of  beam  interception  would  melt  the grids,  thus  destroying  the  tube.  Fortunately,  by  proper design,   the   klystron   may   be   made   to   operate   effi- ciently   without   actual   grid   wires   across   the   cavity gaps.  The  absence  of  these  grids  does  not  change  the operating   principles,   but   it   does   have   a   secondary effect   on   klystron   performance.   If   the   electron   beam has  a  small  diameter  compared  to  the  diameter  of  the drift   tube,   the   beam   does   not   couple   energy   to   the cavities  very  well.  Therefore,  the  performance  of  a klystron  amplifier,  which  does  not  have  gridded  gaps, may   sometimes   be   improved   by   permitting   the   elec- tron  beam  to  be  as  wide  as  possible,  while  keeping  the body  current  down  to  the  maximum  specified  for  the tube.  The  width  of  the  beam  may  be  somewhat  con- trolled  by  the  magnetic  field  strength. Body   current   usually   increases   with   RF   input level,   because   it   is   the   RF   input   that   causes   the bunches  to  form.  The  dense  electron  concentration  in the   bunch   causes   mutual   repulsion   of   electrons,   and the  diameter  of  the  bunch  may  become  larger  than  the diameter   of   the   beam   with   no   bunches   present. Consequently,  some  of  the  electrons  in  the  bunch  may be  lost  to  the  drift  tubes,  and  the  body  current  may increase. ADDITIONAL   EQUIPMENT   FOR   KLY- STRON   AMPLIFIERS.—   Additional   equipment   is required   for   a   complete   amplifier   system.   Various power  supplies  are  necessary  to  deliver  required  volt- ages  and  current.  In  high-power  systems,  various  RF circuit  components  are  required  to  control  and  meas- ure  the  RF  input  to  the  klystron  tube  and  to  measure the  RF  output  from  the  tube.  A  large  collection  of meters   and   protective   devices   is   needed   to   monitor performance   and   protect   operating   personnel   and equipment  in  the  event  of  a  malfunction  or  operator error. In   most   klystron   tubes,   the   anode   and   the   RF section  are  connected  inside  the  vacuum  envelope. These  connected  parts  are  called  the  tube  body  and  are generally   operated   at   ground   potential.   It   is   conven- ient   to   operate   the   tube   body   at   ground   potential because   the   input   and   output   connections   (either waveguide  or  coaxial)  are  then  also  at  ground  poten- tial.  This  makes  it  easier  to  connect  the  klystron  into the  rest  of  the  system.  In  addition,  the  cavity  tuners are  at  ground  potential,  eliminating  any  danger  to  per- sonnel  tuning  the  tube. The  beam  supply  provides  the  voltage  required  to accelerate   the   electrons   and   form   the   beam.   It   must also   deliver   the   required   beam   current.   The   crowbar 2-15