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MAGNETRON   OPERATION
Electron  Gun

Fire Controlman Volume 02-Fire Control Radar Fundamentals
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ments   or   heaters).   With   a   cold   cathode,   sufficient electron  emission  is  available  to  form  a  space  charge when   an   electric   field   is   applied.   The   electrons   will not   achieve   sufficient   velocity   to   escape   the   region close   to   the   cathode   without   an   RF   field   applied. Initially,   when   an   RF   field   is   applied,   the   electrons gain  some  energy  from  the  RF  and  move  farther  out from   the   cathode.   While   some   will   gain   even   more energy  and  reach  the  anode,  others  will  return  to  the cathode   and   strike   with   sufficient   energy   to   cause secondary  emission  and  increase  the  electrons  in  the space  charge.  Therefore,  the  major  source  of  electrons is  secondary  emission  from  the  reentrant  electrons. Secondary  emission  generates  heat  in  the  cathode and   further   increases   emission.   If   the   cathode   be- comes   too   hot,   thermionic   emission   may   become sufficient  to  allow  the  tube  to  oscillate  or  to  produce noise,  even  though  an  RF  field  has  not  been  applied. To  prevent  the  spurious  generation  of  noise,  the cathode   is   usually   water-cooled.   Another   means   of reducing  noise  output  is  to  make  the  RF  pulse  slightly wider  than  the  high-voltage  pulse. Further  control  is  found  on  some  tubes  with  a  bias electrode  (a  cutoff  electrode).  The  bias  electrode  has a  positive  pulse  (with  respect  to  the  cathode)  applied just  before  the  end  of  the  RF  pulse.  The  cutoff  pulse allows  the  electrode  to  collect  the  electrons  in  the drift-tube  region  and  the  tube  to  shut  off.  The  pulse must  be  wide  enough  to  extend  past  the  end  of  the  RF pulse. CFAs  do  not  have  the  gain  of  some  of  the  linear- beam  tubes.  However,  they  do  have  three  advantages. 1.   The   CFAs,   when   used   in   a   multistage   chain, can  produce  equal  or  greater  overall  gain  at  lower high-voltage  requirements  than  a  linear-  beam  ampli- fier,   such   as   a   multicavity   klystron.   For   example,   a typical  CFA  can  deliver  a  1-megawatt  RF  peak  power output   with   a   40-kilovolt,   50-ampere   peak   pulse, where  a  klystron  would  require  a  90-kilovolt,  40-am- pere   peak   pulse   to   produce   the   same   RF   output. However,  the  CFA  would  require  a  higher  power  level input  drive  signal  than  the  klystron. 2-12 2.  The  CFA  is  a  cold  cathode,  which  normally has  a  much  longer  operating  life  than  a  heated  cath- ode. 3.  The  CFA  produces  far  less  X-rays  than  linear- beam  tubes;  therefore,  lead  shielding  is  not  required. LINEAR-BEAM   TUBES A  linear-beam  tube  uses  a  magnetic  field  that  is parallel  to  the  electron  beam  and  is  used  to  focus  the beam.   Some   tubes   do   not   use   a   magnetic   field;   in- stead,  electrostatic  focusing  is  used  to  hold  the  beam together  while  it  travels  the  length  of  the  tube.  The two  most  common  types  of  linear-beam  tubes  used  in fire-control   equipment   are   the   klystron   amplifier   and the   traveling-wave   tube. Klystron   Amplifiers The  basic  theory  of  a  klystron  amplifier  is  quite simple.   The   klystron   amplification   principle   may   be readily  explained  with  an  analogy  to  a  simple  triode amplifier  with  tuned  plate  and  grid  circuits.  Klystron amplifiers   include   two-cavity   power   klystrons   and multicavity   power   klystrons. TWO-CAVITY   POWER   KLYSTRON   AM- PLIFIERS.—   Figure   2-8   shows   a   simplified   sche- matic   of   a   triode   amplifier   with   resonant   circuits   at both  the  input  and  the  output.  Such  resonant  circuits restrict   the   bandwidth   of   the   amplifier   and   increase the  gain. Figure 2-8.—Simplified schematic of a triode amplifier with resonant circuits at both the input and the output.






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