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					「プラズマ科学のフロンティア」2009年9月2-4日 核融合科学研究所   1/31


   マイクロ波の基礎と応用
      ー計測を中心としてー

九州大学産学連携センター              間瀬    淳

             内   容

  1. 電磁波伝搬の基礎方程式

  2. マイクロ波計測の原理と手法

  3. 磁場閉じ込めプラズマにおける
     マイクロ波計測の進展

  4. マイクロ波計測の産業応用
                   1. 電磁波伝搬の基礎方程式
1.1 Electromagnetic Waves in Plasma
Electromagnetic waves in plasma are described by Maxwell’s equations, including current
density J and space charge density r,   E   B t
                                               H  J   0 E t
                                               E   0
                                                                           (1.1)
                                             B  0
                                             B  0 H
and Ohm’s law     J   E          (1.2)
                                                                                         E 
From Eq. (1.1) we obtain the wave equation as     E                    0 J   0 0    0      (1.3)
                                                                        t                t 
When we consider an electric field as        E  E0 exp i (t  k  r )            (1.4)

The Fourier component of Eq. (1.3) is written by           k  k  E  ( 2 c2 ) E  0      (1.5)

or using the refractive index, as      N  N  E  ε E  0              (1.6)

where c is the speed of light, and        1    i 0    (1.7)

is the complex dielectric tensor
The property of the plasma is described by the permittivity through the conductivity [s]. The
conductivity tensor is obtained from the equation of motion of a single electron including a
static magnetic field B0 in z-axis (Fig. 1-1), current density given by
                   dv
               me       e E  v  B0           (1.8)
                    dt
               J  neev
and Ohm’s law.
We ignore thermal particle motions and utilize, so called, the “cold plasma approximation”.
The dielectric tensor is obtained by
           xx  xy  xz   1   2 ( 2  ce )
                                      pe
                                                  2
                                                              i  2 ce  ( 2  ce )
                                                                   pe
                                                                                   2
                                                                                              0    
                           2                                                                    
       yx  yy  yz   i  pece  ( 2  ce )
                                                    2
                                                                1   2 ( 2  ce )
                                                                      pe
                                                                                2
                                                                                              0     (1.9)
                                                                                                
           zx  zy  zz  
                                      0                               0                1 2 2 
                                                                                              pe   
Then the three components of Eq. (1.6) are

     ( N 2   xx ) E x   xy E y  0
                                                                        (1.10)
       yx E x  ( N 2 cos2 q   yy ) E y  N 2 sin q cosqE z  0

      N 2 sin q cosqE y  ( N 2 sin 2 q   zz ) E z  0

where q is the angle between k (wave vector of the incident wave) and z-axis. In order to have
non-zero solutions of Ex, Ey, Ez in Eq. (1.10), the determinant of the matrix of coefficients must
be zero, which gives the dispersion relation of the 4th order of the refractive index N
we obtain the followings:
                            zz [ N 2  ( xx  i xy )][N 2  ( xx  i xy )]
             tan 2 q                                                            (1.11)
                                ( N   zz )[ xx N  ( xx   xy )]
                                    2                 2  2      2

We consider two cases of propagation direction: parallel and perpendicular to the external
magnetic field.

i) Parallel propagation :
When waves propagate parallel to the external magnetic field, tan2q =0, the solutions

            N 2   xx  i xy           (1.12)

From Eq. (1.10) it is shown that the sign “” corresponds to the following relationship between
x and y components of the electric field,

             E y  iEx                  (1.13)

The “+” sign corresponds to the left-hand circular polarized wave, and the “-” sign corresponds
to the right-hand circular polarized wave. Substituting Eq. (1.9) into Eq. (1.12), we obtain
                                             12
                      2            
             Nl ,r  1  pe                                      (1.14)
                         2   ce 
                                    

the subscript l and r of N denote the left-hand and right-hand circular-polarized waves.
                                                                                              5/31

ii) Perpendicular propagation :
When waves propagate perpendicular to the magnetic field, tan2q =∞, the denomination of Eq.
(1.10) has to be zero, which gives two solutions as

             N 2   zz       or       N 2  ( xx   xy )  xx
                                                2      2
                                                                       (1.15)

From Eq. (1.15), we obtain the following polarizations
             E x  E y  0 and E z  0
                                                  (1.16)
             E x , E y  0 and E z  0
Thus the dispersion equations of the ordinary (O-mode) and the extraordinary (X-mode) waves
are given by
                                1/ 2
                     2 pe 
             N o  1  2                                    (1.17)
                       
                           
                                                   1/ 2
                   2                    
                  1  pe     pe 
                               2     2
             Nx                                             (1.18)
                      2  2   2  ce 
                                       2
                                 pe      
Relativistic Effect
When we include the effect of thermal electron motion, the first order of the expansion parameter
  N 2 (Te mec2 ) is considered in the calculation. This assumption is effective when electron
temperature is less than 20 keV since  is less than 0.05. Then, the dispersion relations
become followings

i) Parallel propagation
The dispersion relation of the left-hand and the right-hand circular-polarized waves are given by
                 
                      2       
                                    
                                         2      kTe 
                                                       
                1                  1                          (1.19)
                        pe                  pe
      Nl , r                                        2
                   (  ce ) 
                                    (  ce ) mec 
                                                      
ii) Perpendicular propagation
The dispersion relation of the ordinary wave (E//B0)
            2                           
                           1   pe  kTe 
                                  2
      No  1  2 
                pe                                                (1.20)
                          2   2 m c2 
                                 ce  e 

The dispersion relation of the extraordinary wave (E⊥B0)
           [1  ( pe /  ) 2 ]2  (ce /  ) 2 
                                                
      Nx                                       
           
             1  ( pe /  ) 2  (ce /  ) 2  
                                                                     
                      2 ( 2   2 ) 2  ce (7 2  4 2 )  8ce k Te 
                                                  2                  4
                                                                                
                                                                                    (1.21)
                    1                                                
                            pe          pe                    pe
                                                                                
                    
                                 ( 2  4ce ) ( 2   2  ce ) 2
                                            2
                                                         pe
                                                              2
                                                                         me c 2 
                                                                                
1.2 Electromagnetic Wave Scattering from Plasma

1.2.1 Theory of Scattering

When the electric field of the incident wave is given by

         Ei  E0 exp i it  ki  x                  (1.22)

the equation of motion indicates that an electron oscillates with an acceleration given by

                  E0 exp i it  ki  x 
         dv     e
                                                       (1.23)
         dt    me
The vector potential due to the electron motion
at the position of Q is

                           e    v
         A(r , t )              
                       4 0c 2  R  t  t '   (1.24)

         t 't 
                   1
                     R0  q  x 
                   c
where t’ is retarded time, q=R/R, and |r|=R0.
The scattered wave at the receiving point is
                           A(r , t )
        E s (r , t )                           (1.25)
                             t
Substituting Eq. (1.24) into (1.25), we obtain

                      e            dv 
        Es               q  q                       (1.26)
               4 0c 2 R          dt t '
The scattered wave shown in Eq. (1.26) is the one for a single electron. For the plasma
with many electrons, we must add each value statistically as follows:
                      N
      ne ( x, t ' )    x  xi (t ' )            (1.27)
                      i 1
where  is the Dirac delta function. The total scattered electric field from all the
electrons with electron density ne in a volume V is then

      Es (r , t )   0 E0  q(q  E0 ) dx ne ( x, t ' ) expi(it 'ki  x )
                     r
                                                                                       (1.28)
                      R
where r0=(e2/4pe0mec2) is the classical electron radius.
The electron density in Fourier component is shown by
                                     
                                          d
                                             expi(it  k  x)ne (k ,  )
                             dk
      ne ( x, t )                      2
                                                                              (1.29)
                       (2 ) 
                                  3
We obtain
                                                      
                                                          d
     E s (r , t )   0 E0  q(q  E0 ) dx 
                     r                            dk
                      R                  V   
                                                (2 )3  2
                                                        

                                 R     i                      
                 expi (  i ) t  0     q  x  (k  k )  x  ne (k ,  )           ks=qs/c.       (1.30)
                                     c   c                       
The scattered wave at the center frequency s and bandwidth Ds
                                        s  Ds / 2
                                                       R 
     Es (r , t )   0 E0  q(q  E0 )
                    r
                                               exp is  t  0  ne (ks - ki , s  i )d
                                                              c 
                                                                                                       (1.31)
                                        s  Ds / 2 
                     R

The scattered power averaged over the observation time T is given by
                                                     c 0 Nr0
                                                                                       
                                                            2
         c                                                                                                        D s
                                                                E0 1  sin 2 q s cos2  S k s  ki ,  s  i 
                 1                         2
                             DE s (r, t ) dt 
                                                                 2
     Ps  0 lim
          2 T  T                                     2R 2                                                         2
                           V                                                                          (1.32)
where                                            2
                           2       ne (k ,  )
      S (k ,  )  lim                                        (1.33)
                  T ,V  TV           Ne
                                                                           2            2
                                                                                         
 is the power spectral density of the density fluctuations, E0  q(q  E0 )  E0 1  sin q s cos  ,
                                                                               2                2
                                                                                                            
 is the angle between E0 and ks-ki plane.
It is noted that the scattered power is observed when following matching conditions are satisfied.
      k  k s  k i ,    s  i                         (1.34)
                                                                                             10/31
1.3 Electromagnetic Wave Radiation from Plasma
1.3.1 Radiation process in plasma
The radiation process is described by the equation of transfer which includes the emission
and absorption in plasma.
The energy absorbed along the distance is given by
            I dS drdd            (1.35)
The radiation energy is given by
            j d Sdrdd               (1.36)

The energy difference between entering and leaving the small volume corresponds to the
difference between Eqs. (1.35) and (1.36), that is,

           I  d I  d S d d  I dS d d           (1.37)

that is,   d I
                 I  j             (1.38)
            dr
When the refractive index of the plasma is inhomogeneous and anisotropic, the equation of
transfer is given by

             2   d  I 
                           I  j
            Nr                                   (1.39)
                     N2 
                 dr  r 
If the plasma is in thermal equilibrium, Kirchhoff’s law is worked out;

       j   I B                              (1.40)

 I B is the black-body radiation written by

               2    h 3         1
       IB    Nr                               (1.41)
                   8 3c 2 exp(h / Te )  1

In microwave region,   Te , Eq. (1.41) becomes

                    2
              2
       I B  Nr         T
                     3 2 e
                                                (1.42)
                   8 c

By use of (1.42) the solution of the transfer equation is written by

       I  I BO 1  exp   0              (1.43)
              L
        0    dr                            (1.44)
              0

 0 is called as “optical thickness”.

 When  0 ≫1, I equals to the intensity of black body radiation
1.3.2 Bremsstrahlung

In a plasma there exists electromagnetic radiation due to collisions of electrons with ions and
neutral particles since the electrons deaccelerated in the electric field. For example, the
radiation power due to the electron-ion collision is given by

      dP ei  1.09 1051ne ni Z 2Te1 / 2 Gd     [W  m3sr-1]     (1.45)

Where G d , the Gaunt factor averaged over velocities, takes
                  ,       3   4Te         
       G d (, Te )        ln       0.577
                             
                                             

 When  Te  1 . The absorption coefficient becomes

        ei  7.0 1011ne ni Z 2 Te3 / 2  2G    [m-1]            (1.46)

 The total radiation power is obtained from integration in  as

        P  1.6  1040 neni Z 2Te / 2 [W  m3 ]
         ei
                                 1                                      (1.47)

 Meanwhile for low-temperature weakly-ionized plasma, the radiation power occurs due to
 the collision between electron and neutral particles, and is given by

        dP ea  3.9  1062 nenaTe3 / 2 Fd   [W  m3 ]              (1.48)
1.3.3 Cyclotron emission

A plasma in an external magnetic field radiates as a result of acceleration of electrons in their
orbital motions around the magnetic field lines. This emission is called as electron cyclotron
emission. The cyclotron emission power is also calculated from the integration of the
coefficient of self emission over the distribution function. The equation of motion in the
magnetic field is
       dP
           ev  B0 
                                       m0
                             P                 v                               (1.49)
       dt                         1       2


The value of  at the angle q from the external magnetic field is obtained by

            e2 2    cosq  //                                    
                                              2
                                             2
         2  c                         J n ( X )    J n( X ) Y 
                                                            2 2
                                                                                (1.50)
           8 0 n 1 sin q
                                                                    
                                                                      
         X   / 0   sin q
        Y  n0   1   // cosq                                             (1.51)
        0  ce (1   2 )1 2

 From Eq. (1.51), it is shown that  has discrete line spectra with its peaks at Y=0, that is,
                n0
                          n  1, 2, 3,                      (1.52)
            1   // cosq

 The total emission power is obtained by the integration of Eq. (1.50) over the
 distribution function.
The spectrum of electron cyclotron emission exhibits the broadening due to the physical
processes in plasmas. There are several possible mechanisms for the broadening.
i) Doppler broadening:                  Dn  2 1/ 2 nce (kTe / mec 2 )1 2 cosq                         (1.53)

ii) Relativistic broadening: Dn  2  nce (kTe / mec )
                                        1/ 2            2                                                   (1.54)

It is seen that the relative importance of relativistic effect and Doppler effect is determined by
the angle q. We now consider two cases
1) For the case of N cosq  vte c
   i) n  1
                                   2  v 2
                        pe         te  (1  2 cos q ) sin q  LB
                                                       2 2      4
     1 o )   2 N o 
      (
                       
                                                                                             (O-mode)          (1.55)
                       ce           c        (1  cos2 q )3    0
                                         
                                       2              2          2
                   2                     ce          v te 
       1   N x 1  2                                      cos2 q  LB
        ( x) 2         pe
                                                                                              (X-mode)          (1.56)
                                          pe         c              0
                      ce                                    
   ii) n  2
                                                            2( n 1)
                    2n 2( n 1)   pe   vte 
                                          2
                                                                     sin q 2(n 1) (1  cos2 q )no, x) (q ) LB
                                
                                    c 
        ( o, x )                                                                                     (
                  n 1                                                                                                  (O, X-mode) (1.57)
        n         2 (n  1)! ce                                                                             0
                                                
   where                                                                                                              2
                      pe 
                               2
                                            2(ce   2 )
                                                2                                                ce   2 
                                                                                                   2
       N o, x  1  
                                                      pe                                                 pe 
                     
         2
                                                                                    sin q  4
                                                                                    2     4
                                                                                                               cos2 q             (1.58, 59)
                     ce          2(ce   2 )  ce (sin2 q   )
                                      2             2                                            2         
                                             pe                                                      ce     
                                                                                                                   15/31


2) For the case of N cosq <vte / c (qc<q  90 )

  i) n 1
                              1/ 2
                        2                    2
       o    2 1   pe           pe   vte  2
                                     
                                                              LB
      1
                   2                 c 
                                                             0
                                                                                              (O-mode)    (1.60)
                        ce          ce         

                                       3/ 2                        2
                            2                          ce   vte  4 LB
        x   5 2 2 1   pe 
      1                                      B  z1        
                                                         pe   c  0
                                                                                              (X-mode)    (1.61)
                       2 2 
                             ce                                  

 ii) n  2
                               2 n 1 / 2
                 2n2n 1 
                                                    2
        n  n 1
         (o)            1   pe           pe   vte 2n LB
                                                 
                         n2 2              c                                          (O-mode)   (1.62)
             2 (n  1)!        ce         ce             0


                                                   n 1 / 2
                                                               pe   vte  2n 1
                 2 2( n 1)    2                                      2
              n        1   pe                                                       LB
       n  n 1                                              
                                                                 c 
        ( x)
                       A                                                                      (X-mode)   (1.63)
             2 (n  1)!  n 2ce 
                        
                                2
                                                              ce                    0
      内 容



1. 電磁波伝搬の基礎方程式

2. マイクロ波計測の原理と手法

3. 磁場閉じ込めプラズマにおける
   マイクロ波計測の進展

4. マイクロ波計測の産業応用
2.1 Interferometry
2.1.1 Principle
Measurements of refractive index are often made by O-mode interferometry given by
                   1/ 2
         2 (r )         ne (r ) 
                                     1/ 2
  No  1  pe          1       
                             nc 
                                             (2.1)
           2                   

where nc   0me / e is the “cutoff” density.
                   2

The interferometry measures the phase difference between the
waves propagating in the plasma and in the outside of the plasma,
which is given by
            y                  2 y2
   ( x)   2 (k0  k p )dy     (1  No )dy (2.2)
              y1                 y1

Assuming ne ≪ nc , (x) is shown as the following formula.
             y2               2 a                2 1 / 2
   ( x)      y1 ne (r )dy      x ne (r )(r  x ) rdr, r  x (2.3)
                                               2
           nc                 nc
When radial profile of the density is axisymmetric, we can obtain
the density profiles by Abel inversion
                nc a d 2
  ne (r )              ( x  r 2 )1 / 2 dx , x  r    (2.4)
                2 r dx
Now we assume the plasma has parabolic distribution given by the following formula, as it is
known empirically, the phase difference is given by
                               r 2                          2  a ne 0
             ne r   ne 01       (2.5)        0                        (2.6)
                              a                              3    nc
                                     
2.1.2 Choice of incident wavelength
The density gradient along the diameter causes a refractive effect, when the frequency of the
incident wave becomes close to the electron plasma frequency. The value of the refraction
angleδis maximum when the incident beam propagate at the chord of x / a  0.7
              sin 1 ne 0 / nc   ne 0 / nc                  (2.7)
Taking Gaussian beam theory into consideration, the beam expands along the distance y.
                                  1/ 2
                  2 42 y 2 
             d   d0  2  2                  (2.8)
                       d0 
                                
Let us take the distance to the first collecting optics as L, and assuming d0  2L /  1 / 2 we obtain
d  2d0 . Then, the conditions to allow measurement are

            L m  L ne 0 / nc  d  2L /  1/ 2                        (2.9)

The lower wave length limit is determined that parasitic fringe shift has no effect on measurement
accuracy. If it is 1% and below, F is fringe number due to the plasma density. Equation (2.9) leads
range of incident wavelength as D /   10 2 F . Therefore we obtain

                                                  
            4.1 108 a ne 0D    1.2 1010 Lne 0
                                                    2
                                                           1 / 3
                                                                             (2.10)
2.1.3 Phase Detection

An example of interferometer system and phase detector




    Heterodyne interferometer using upconverter.         Quadrature-type phase detector.
                                                                                                      20/31
2.2 Reflectometry
2.2.1 Density profile measurements
A reflectometer consists of a probing beam propagating through a plasma and a reference beam. The
microwave beam in the plasma undergoes a phase shift with respect to the reference beam given by
                        rc ( )
                                                   
         ( )  2k              N (r ,  )dr 
                                                    2
                                                        within the WKB approximation.        (2.11)
                            a
The refractive indexes of the O-mode and the X-mode propagations are given by
                                            12                                          12
               ck o   pe                         
                                , N  ck x  1   pe   x   pe 
                                                                               
                             2                             2         2    2
         No         1                                                              (2.12)
               o  o                        x   x  x   2  ce 
                             2           x                  2    2           2
                                                                    pe      
In an ultrashort-pulse reflectometer, a very short pulse is used as a probe beam. The time-of-flight for
a wave with frequency  from the vacuum window position rw to the reflection point at rp is given by
                        rp                 1 2
                    2           2 
                            1  pe 
          ( ) 
                    c          2 
                                       
                                                   dr     (2.13)
                        rw 
In order to obtain the density profile from the time-of-flight data, the Eq. (2.43) can be Abel
inverted to obtain the position of the cutoff layer,
                         pe
                      c           ( )
          r ( pe ) 
                                            d
                                    ( 2   2 )1 2
                                                            (2.14)
                                  0    pe
By separating different frequency components of the reflected wave and obtaining time-of-flight
measurement for each component, the density profile can be determined.
2.2.2 Fluctuation measurements
Reflectometry has also been used in order to study plasma fluctuations. The instataneous
phase shift  between the local beam and the reflected beam is expressed as   0  

In a simple homodyne reflectometer, the mixer output is given by

                           
              V  El Er cos( 0   )  El Er (cos0 cos  sin 0 sin  )
                                                                                    (2.15)
                                       El Er (cos0   sin 0 )

The time varying component of the mixer output depends on both amplitude and phase modulations.
In general, the radial fluctuations of the cutoff layer produce the phase modulations and the poloidal
(azimuthal) fluctuations cause amplitude modulations.
It is important to identify both phase and amplitude fluctuations using, such as, heterodyne detection
or quadrature type mixer.

In a simple one-dimensional model, the phase changes in the O-mode and the X-mode propagations
due to the small perturbations of the density and the magnetic field, at the critical density layer are
given by

              o  2ko Ln (ne / ne )                              (2.16)


             x 
                         
                   2k x ne ne  (cex  2 ) B B
                                           pe                       (2.17)
                        1 Ln  (cex  2 ) LB
                                        pe
                    Reflectometry-Principle
                                                                        Cutoff layer
   Reflectometer utilizes                                    Ls
   reflected wave from the
                                Source
   cutoff layer of plasmas.                Lr
                       12
                        
            1  e2ne (r)
       fp   m                          Detector                                      Plasma
           2  e 0 
                       
                                                                                    rc
                                                (t)                         rant
● Measure the group delay, or the return phase, as a function of frequency
 
                                                 1 2
          d   () d ,  () 2
                                   r 
                                    c 2                   Ls  Lr
                                                                c   p()  w ()
                                            
          dt         dt          c  1 pe 
                                        2 
                                                       dr
                                  r 
                                   ant     

● Deduce the distance to the cutoff as a function of cutoff density
  -A simple inversion procedure can be used for O-mode radiation
   
                     
                    c pe  p( ) d
        rc  rant   
                                    1 2
                       0  2   2 
                         
                          pe       


 We can obtain reflected waves from each cutoff-layer corresponding to each
    
 radial position by injecting an incident wave with wide frequency region.
                  Various Types of Reflectometry
Fast-Sweep FM Refletometer              AM Reflectometer
○high resolution with simple hardware   ○minimal effect of density fluctuations
●phase runaway                          ●parastic reflections from wall and window

                          f
    Source
                              f+

                   (t)


Short Pulse Reflectometer               Ultrashort Pulse Reflectometer
○measurement of real-frozen plasma      ○an impulse generator
●many sorces or sweep source with       ●ultrashort pulse (<10 ps) for high density
  wideband switches                        plasmas
             f    Pin         f±Df/2
    Source                                    Pulse
                 Switch                      Generator

       Trigger
                                                                        BP
                                                                       Filter
                              Time
                              Delay
                                                    Digitizer and/or TAC
2.3 Thomson Scattering
2.3.1. Collective scattering
By using microwave as an incident wave, scattering parameter is usually larger than unity, so called
collective scattering. Most laboratory plasmas have density fluctuations caused by various types of
instability. These fluctuations generally have wavelengths exceeding the Debye length and the
fluctuation levels encountered far exceed the thermal levels.
The scattered power per steradian and per radian frequency at the scattering angle qs is written by
         P (ks ,s )  pi neVsT S (k ,)
          s                                                                     (2.18)
where pi is the power density of the incident wave, Vs is the scattering volumn, T is the cross section
of Thomson scattering, and S(k,w) is the power spectral density of the density fluctuation given by
                                             ~ 2
                    1                         nk
        S (k ) 
                  2     S (k ,  )d  neVs
                                              ne
                                                                                (2.19)
                       
       ~
where nk is the amplitude of density fluctuations with wave number k. The wave number spectrum
can be obtained by changing the scattering angle qs. The density fluctuation level is then determined
from the integration of k as.
                                    4
           ~ (r , t ) 2   1  n S (k ,  )dkd
           ne                e
                          2 
                                                                              (2.19)

For the thermal fluctuations, S(k)~1, however, S(k)>>1 for the non-thermal fluctuations. Assuming
ne / ne  102  103 ne=1019 m-3, and Vs=10-5 m3, S(k)=108-1010.
~                   ,
                                                   25/31
            Microwave Scattering

Apparatus                          Frequency spectra for
                                   various scattering
                                   angles




Wavenumber spectrum

                                         Dispersion
                                         curve of
                                         ion-wave
                                         turbulence
    Far-Infrared Laser Scattering


                 Measurable wavenumber is 3<k<50 cm-1.
                 Resolution is Δk<3 cm-1.




                     Wavenumber and frequency spectra




Apparatus
                   Far-Forward Scattering

                                Frequency spectra for
                                various values of
                                end-plate bias.




                                                           Detector array




Fluctuation level vs. ambipolar field                   Dispersion relations
                                                        for various values of
                                                        end-plate bias
2.4 Electromagnetic Wave Radiation from Plasma
2.4.1 Determination of electron temperature
In experiments, the plasma is produced in a metal chamber. If we consider the effect of the
reflection from the metal wall, it is known that the radiation intensity is modified as
                             1  e n
          I n ( )  I B 0              n
                                                                (2.21)
                             1  ree
where re is the wall reflectivity (1> re > 0.9). When 1  re ≪ n , In becomes nearly equal to the
black body radiation, then we call as “plasma is optically thick”.
On the other hand, when  n ≪1  re , In in optically thin case becomes
                        I B0
          I n ( )           n              (2.22)
                       1  re
Let us consider a tokamak plasma, where B0 is
the magnetic field intensity at the plasma center,
R is the major radius. It is know that the toroidal
magnetic field is a function of x as,
                        R
          BT  B0                             (2.23)
                       Rx
Therefore ce also varies accordingly. ECE
appears resonantly in width

          Dxn  Dn d nce  / dx1                 (2.24)

with centering on x=x() which corresponds to   nce
                                                                                                 30/31
When the plasma is optically thick, the radiation power becomes proportional to its local electron
temperature.

On the other hand, when plasma is optically thin  ≪1  re ,

                        I B0      1
          I n ( )                  n e (Te ) n                       (2.25)
                       1  re   1  re
The radiation power is proportional to both ne and Te profiles. Therefore, when Te is obtained
by different methods, we can determine ne profile, and vise visa.
Furthermore observing the ECE at the optically thin n and n+1 th harmonics, we can determine
the electron temperature using the following formulas,

                   M n I n  I ( )
          k Te                                                          (2.26)
                    I n ( )m0 c   2

                                             n 1
                2en 2n 1         1     
          Mn                2n      3                              (2.27)
               n  12n 1       2n    

Similarly, observing the ECE at the optically thin O-mode and X-mode waves, we obtain the
electron temperature as

          kTe  mec 2 I n0) ( ) / I nx) 
                        (            (
                                                                         (2.28)
2.4.2 ECE radiometry

There are several types of diagnostic systems for ECE measurements, such as , i) Heterodyne
radiometer, ii) Fourier-transform spectrometer, iii) Grating polychromator, iv) Fabry-Perot
interferometer, and v) Multichannel mesh filter

i) Heterodyne radiometer
Conventional heterodyne technique is often used for 2ce ECE. This technique has good frequency
resolution. In the initial stage this could not be used to monitor the entire 2ce spectrum, however,
wideband mixers having almost full band responsibility have been developed, and most of the
spectrum can be covered by a few mixers.




  110-196 GHz
  ECE                                                                      96 channels IF system
                                                                           with MIC technology




                              Heterodyne Radiometer
                                                                                                        31/31
iv) Fourier-transform spectroscopy
When electric filed of incident wave on interferometer is given by
       E(t )  i Ei cosit                                           Interferometer
                                                                         Detector

The electric field entering a detector is written by
                                                             Scanning Mirror
       En (t )   i cosit  cosit  Di 
                   E
                                                  (2.31)
                 i
                   2                                                 Grid
                                                                                       Grid
                                                                                              Rdaition from
Therefore, if we take mean square of En                                                         Plasma

        1 T 2              1 2    Ei2
             En (t )dt   Ei  
        T 0
                                      cos Di       (2.32)       Fixed Mirror

                         i
                           4    i
                                   4
       Di  i , and   X / c
                                                                   Monitor Detector
The second term of right-hand side of Eq. (2.28) is proportional to the auto-correlation function
                   1 T                      1
                        E (t )E (t   )dt   Ei2 cos i
                   T 0
       RE ( )                                                     (2.33)
                                            2 i
According to Wiener-Khinchine theorem, In is eventually obtained by the Fourier transform of RE()
                    
       I ( )  4 RE ( ) cos d                                 (2.34)
                    0

In this method, the frequency resolution Df is determined from maximum  m  X m / c as Df  1 /  m

				
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