For  example,  in  a  20-kilowatt,  10-GHz  klystron  am- plifier,  approximately  1  kilowatt  of  heat  is  generated by   the   circulating   RF   currents   in   the   output   cavity. Since  the  cavity  is  approximately  a  1-inch  cube,  it  is apparent  that  removing  the  heat  is  a  formidable  prob- lem. Another  problem  associated  with  cavity  heating  is not   immediately   apparent.   The   resonant   frequency   of a  cavity  depends  on  its  physical  size.  The  cavities  are made  of  metal,  which  expands  as  its  temperature  in- creases.  This  effect  tends  to  change  the  resonant  fre- quency  of  the  cavities  and,  thereby,  detune  the  tube. As  the  tube  detunes,  the  power  output  drops.  This,  in turn,  reduces  the  RF  heating  and  allows  the  tube  to come  back  into  tune.  If  this  problem  were  not  con- sidered   in   the   initial   tube   design,   the   resulting   tube would   be   unstable   in   its   operation.   This   situation exists  in  some  tubes  that  are  external  cavities.  These external   cavities   are   cooled   by   air,   rather   than   by liquid,  and  the  cavity  tuning  is  seriously  affected  by the   ambient   air   temperature. All   high-power   klystrons   are   liquid-cooled, including  cavities  and  tuners.  The  cavities  are  main- tained  at  a  stable  temperature  by  controlling  the  tem- perature  of  the  cooling  liquid;  therefore,  thermal  de- tuning  is  no  longer  a  problem. Drift-tube   heating   is   a   serious   problem   in   very- high-power   and   medium-power,   high-frequency   kly- strons.  The  drift  tubes,  which  are  inside  the  vacuum envelope,   are   physically   small,   and   it   is   difficult   to conduct  the  drift-tube  heat  into  the  region  outside  the vacuum   envelope.   In   some   high-power   tubes,   it   is necessary  to  bring  the  cooling  liquid  inside  the  vac- uum   envelope   and   around   the   drift   tubes   to   remove the  heat. In  some  high-power,  high-frequency  systems,  it  is necessary   to   cool   the   output   waveguide.   A   10-GHz waveguide   carrying   a   5-kilowatt   signal   becomes   too hot  to-touch  in  normal  ambient  air.  Fortunately,  wave- guides   may   be   cooled   easily   by   soldering   copper tubing  along  the  sides  of  the  guide  and  running  cool- ing  liquid  through  the  tubing. Systems  that  use  blowers  for  cooling  usually  have an  airflow  switch;  if  the  blower  fails,  the  switch  opens and  removes  power  from  appropriate  power  supplies. Systems   that   use   liquid   cooling   normally   distribute the  liquid  into  a  large  number  of  paths,  since  the  flow requirements   are   quite   dissimilar.   Each   path   has   a low-flow   interlock.   If   one   of   the   liquid   cooling   cir- cuits   becomes   plugged,   the   low-flow   interlock   opens and  removes  power  from  the  system. Distilled   water   is   the   best   medium   for   cooling klystron   amplifiers.   Some   very-high-power   amplifiers specify  that  only  distilled  water  may  be  used.  Un- fortunately,  water  freezes  at  a  temperature  that  could be   encountered   under   normal   operating   conditions. Many   low-   and   medium-power   klystrons   permit   the use  of  ethylene  glycol  and  water  as  the  cooling  liquid. However,  since  ethylene  glycol  reacts  with  certain types  of  metals  and  hoses  that  might  be  used  in  the system,  special  care  must  be  taken  in  working  on  a system   that   uses   ethylene   glycol.   Only   nonferrous metals  should  be  used  in  a  cooling  system  for  a  kly- stron   amplifier. NOISE   IN   KLYSTRON   AMPLIFIERS.— Volumes  have  been  written  about  noise  in  microwave systems.   However,   this   chapter   covers   only   the   high points.  The  output  of  a  klystron  amplifier  contains harmonics  primarily  because  the  output  cavity  is excited  by  bunches  of  electrons  that  pass  through  the output  gap  once  every  cycle.  Since  the  driving  energy supplied   to   the   output   cavity   is   not   continuous,   but occurs   in   quick   pulses,   it   is   evident   that   the   output current   may   not   be   purely   sinusoidal.   Therefore,   the output   contains   harmonic   components. In  general,  the  harmonic  output  of  a  klystron  am- plifier  is  largest  when  the  tube  is  operated  at  or  above saturation.  Harmonic  content  decreases  when  the  tube is   operated   below   saturation.   Also,   harmonics   in   the output  may  be  reduced  by  the  use  of  harmonic  filters. Another  source  of  distortion  is  the  nonlinearity  of the  klystron.  If  the  RF  signal  is  amplitude  modulated, 2-18