tection  for  the  body  current  and  the  collector  current. In  most  systems,  the  collector  and  the  body  operate  at nearly  the  same  potential.  Any  potential  difference  is usually  only  the  difference  in  voltage  drop  across  the various  metering  circuits. MULTICAVITY   POWER   KLYSTRON   AM- PLIFIERS.—   The  simple,  two-cavity  power  klystron amplifier   is   not   capable   of   high-gain   or   high-power output,  or  suitable  efficiency.  However,  with  the addition   of   intermediate   cavities   and   other   physical modifications,   the   basic   two-cavity   klystron   may   be converted   into   a   multicavity   power   klystron,   capable of   both   high-gain   and   high-power   output. In  addition  to  the  intermediate  cavities,  there  are several   physical   differences   between   the   basic   two- cavity   klystron   and   the   multicavity   klystron.   The cathode  of  the  multicavity  power  klystron  must  be larger  to  be  capable  of  emitting  large  numbers  of  elec- trons.   The   shape   of   the   cathode   is   usually   concave, which   aids   in   focusing   the   electron   beam.   The   col- lector   must   also   be   larger   to   allow   for   greater   heat dissipation.   In   a   high-power   klystron,   the   electron beam  may  strike  the  collector  with  sufficient  energy to   cause   the   emission   of   X-rays   from   the   collector. Many   klystrons   have   a   lead   shield   around   the   col- lector   as   protection   against   X-rays.   Most   high-power klystrons  are  liquid-cooled  and  must  be  constructed  to facilitate   cooling   the   collector. Klystron  amplifies  have  as  many  as  seven  cavities, including  five  intermediate  cavities.  The  intermediate cavities   improve   the   bunching   process,   resulting   in increased   efficiency.   Adding   more   intermediate   cavi- ties  is  roughly  analogous  to  adding  more  stages  to  an IF  amplifier;  that  is,  the  overall  amplifier  gain  is  in- creased  and  the  overall  bandwidth  is  reduced—if  all the  stages  are  tuned  to  the  same  frequency. The  same  effect  occurs  with  klystron  amplifier tuning.   A   given   klystron   amplifier   tube   will   deliver high  gain  and  narrow  bandwidth  if  all  the  cavities  are tuned  to  the  same  frequency-this  is  called  synchron- ous   tuning.   If  the  cavities  are  tuned  to  slightly  dif- ferent  frequencies,  the  gain  of  the  klystron  amplifier will  be  reduced  and  the  bandwidth  may  be  appre- ciably  increased—this  is  called  asynchronous    tuning. Most   klystron   amplifiers   that   feature   relatively   wide bandwidths    are    stagger-tuned. The   klystron   is   not   a   perfectly   linear   amplifier; that  is,  the  RF  power  output  is  not  linearly  related  to the   RF   power   input   at   all   operating   levels.   In   other words,   the   klystron   amplifier   will   saturate,   just   as   a triode  amplifier  will  limit  if  the  input  signal  becomes too  large.  In  fact,  if  the  RF  input  is  increased  to  levels above   saturation,   the   RF   power   output   will   actually decrease. To  better  understand  the  reason  for  this  decrease, remember  that  electron  bunches  are  formed  by  the action  of  the  RF  voltage  across  the  input  cavity  gap. This  RF  voltage  accelerates  some  electrons  and  slows down  other  electrons,  resulting  in  the  formation  of bunches   in   the   drift   tube   region.   Obviously,   this speeding  up  and  slowing  down  effect  is  increased  as the  RF  drive  power  is  increased. The  saturation  point  is  reached  when  the  bunches are  perfectly  formed  at  the  instant  they  reach  the  out- put   cavity   gap.   This   results   in   the   maximum   power output   condition.   When   the   RF   input   is   increased beyond   this   point,   the   bunches   are   perfectly   formed before  they  reach  the  output  gap;  that  is,  they  form  too soon.  By  the  time  the  bunches  have  reached  the  output gap,   they   tend   to   debunch   because   of   the   mutual repulsion  of  electrons  and  because  the  faster  electrons have  overtaken  and  passed  the  slower  electrons.  This causes  the  output  power  to  decrease. FOCUSING    KLYSTRON    AMPLIFIERS.— One  very  important  item  that  is  required  for  high- power   klystron   amplifier   operation   is   an   axial magnetic  field  (a  magnetic  field  parallel  to  the  axis  of the  klystron).  In  klystron  amplifiers,  which  are  physi- cally   long,   it   is   difficult   to   keep   the   electron   beam properly   formed   during   its   travel   through   the   RF section.   The   mutual   repulsion   between   electrons causes  the  beam  to  spread  in  a  direction  perpendicular to the axis of the tube. If this is allowed to occur, elec- trons  will  strike  the  drift  tube  and  be  collected  there, rather  than  passing  through  the  drift  tube  to  the collector. 2-14