J. Plasma Fusion Res. SERIES, Vol. 3 (2000) 452455

Measurements and Modeling of Transmitted Power of Ecwaves Propagating for Normal and oblique Injection in FTUTokamak during ECRH Experiments at 140 GHzNOWAK Silvana*, BRUSCHI Alessandro, CIRANT Sante, GANDINI Franco, GRANUCCI Gustavo,
SIMONETTO Alessandro and SOZZI Carlo

Istituto di Fisica del Plasma, 20125 Milano, Via Cozzi 53, Italv(Received: 18 January 2000 / Accepted: 18 April 2000)

Abstract
A continuous monitor of waves absorption during ECRH experiments is performed by a set of threelow-gain probes, installed in the FTU tokamak walls, collecting the residual scattered and depolarized RFradiation. One probe is located in the same sector of the launching system; the other two are toroidally
displaced by 60' and 90" respectively. Two gyrotrons at 140 GHz delivering 0.5 MW each up to 0.5 s
have been used to inject EC waves from the low field side at the fundamental harmonic ordinary mode.

The estimated of the transmitted power is carried out by a 3D ray-tracing code taking into account boththe beam diffraction effects and a statistical model for describing multiple reflections and scrambling ofpolarization at the tokamak walls. Fair agreement is found between the measured and calculated values ofresidual electromagnetic radiation.

Keywords:
tokamak, electron cyclotron resonance heating

1. Introduction

close to the walls is therefore strictly related to single
pass-absorption, and its measure is important both for

Measurements of residual radiofrequency (rf)power are an important monitor of the wave absorption

checking the absorption predictions and for preventing
excessive rf loading of plasma-facing instrumentation. Asystem of three low-gain probes (sniffers) of 2.7 cm

efficiency during plasma heating experiments inmagnetically confined fusion devices. In particular, inFTU tokamak (major radius R=93.5 cm, minor radius
a=30 cm, central magnetic field 8=5-6 T) electron
cyclotron (EC) waves, at high frequency (140 GHz) andhigh power (up to 0.8 MW), are employed to heat theplasma in high density and high temperature regimes.
The central magnetic field of 5-6 tesla allows strong
wave absorption at the fundamental ordinary mode(OM). In most conditions more than957o of the injected
EC power is deposited off/on-axis during the first transit
through the plasma. The fraction of transmitted power isre-absorbed after multiple reflections on the vesselwalls. The amount of rf power in the neutral gas region*diameter, located in three vertical ports, is used tocollect the residual power. One probe is in the samepoloidal section of the EC launcher, while the other two
are toroidally displaced by 60'and 90", respectively(Fig. l) [1]. In this paper we compare the measured
residual power with the linear theory predictions bymeans of a statistical model. which takes into accountmultiple wall reflections at random angles and withpolarization scrambling. Several experimental conditions
have been examined, both in perpendicular and oblique
injection, with full and partial OM polarization.
@2000 by The Japan Society of Plasma
Science and Nuclear Fusion Research

C o rr e sp ondin g autho r' s e - mail : now ak @ ifu . mi. c n r. it452

Nowak S. et al., Measvements and Modeling of Transmitted Power of EC Waves Propagating for Normal and Oblique Injection

3. Experimental results
The Electron Cyclotron Resonance Heating

2. Statistical model
The propagation and absorption of EC waves aredescribed by a fully 3D code, the ray-tracing taking into(ECRH) campaign on FTU tokamak has been performed
up to now by using 2 gyrotrons delivering up to 0.5 MW

account the measured plasma parameter distribution andeach for a maximum pulse length of 0.5s. Eachgyorotron feeds a launching antenna with poloidal andthe diffractive behaviour of the Gaussian beams, which
enter the plasma from the low field side [2]. The beam
propagation is modelled by the quasi-optical approach in
the first transit through the plasma. From the second

toroidal (+10o, +20o, t30') beam steering capability.

During ECRH experiment the FTU plasma ischaracterized by high electron density (central density
up to n"s=I.$ 1020 m-3) and temperature (up to 7-8 keVwith ne0=0.6 1020 m-3, central absorption). We present,
here, three discharges with density neo=6-8 lOle m-3 andelectron temperature T"o=3-5 keV. In Figs. 2-4 theresidual power, collected by the three probes, is shownfor two shots with tl0' of toroidal injection, the plasma

passage on, the beam is replaced by independent rays. In

order to properly describe the amount of the residual
power at the probe's position, a statistical approach isconsidered. The input beam is discretized in a number ofsingle rays, each carrying the amount of power
appropriate to the initial gaussian distribution. A single
ray is randomly selected, with a weight given by thegaussian distribution, and its propagation followed bysingle ray tracing. At the wall, the ray is randomlyreflected. At each reflection the power in the ray is

axis placed at Ru*=!$ cm, Bu*-5.1 T (r^,0"1a0.15),

reduced according to an effective wall reflection
coefficient (0.85-0.87), which has been estimated by"t4Sniffer I1Z

fitting the code predictions with measurements

10

performed by launching EC waves in the empty vessel[3]. When into the plasma, the power in the ray ispropagated and absorbed accordingly to the relativistic
dispersion relation. The ray is abandoned when the levelof the carried residual power becomes less than a fixedthreshold. The residual power of all rays reaching theprobes' positions is counted. Many random selections(up to 50000) are performed, in order to have a large

F8(E=6o.45

0.5

enough statistics.

0.55

0.6

Time (sec)

0.65

0.70.75

Fig. 2 Rf power collected by the sniffer 1 for EC injection
at -10' toroidally away from perpendicular (a),opposite to probes 2 and3, and at +10' (b).

Sniffer 2
1.5

P(El=0!
0.45

port 70.5

0.s5 0.6 0.65
Time (sec)

0.7

0.75

Fig. 3 Same as Fig.2 for the sniffer 2. The model
estimates 4 W for the case (a) and 1 W for caseport 8(b).

Fig. 1 Scheme of the sniffer locations in FTU tokamak
453

Nowak S. et al., Measurements and Modeling of Transmitted Power of EC Waves Propagating for Normal and Oblique Injection
0.3

Sniffer 2

Snlfter 3
0.8
o.2

$

P
(!

0'6

0.1s
0.1

=

0.4

=

00.45

0.5

0.55

0.6

Time (sec)

0.65

0.7

00.75

0.55

Fig. 4 Same as Fig.2 for the sniffer 3. Model estimate:0.01 W for the case {a) and .001 W for case (b)0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95
Time (sec)Fig.

6 Same as Fig.S for the sniffer 2.

0.1

Sniffer 3
0.08

Sniffer IPP(EP60= 0.04(E=0.06

40o.02

0LU.J)
0.55

0.6 0.6s 0.7 0.75 0.8 0.85 0.9 0.95
Time (sec)

0.6 0.65 0.7 0.75 0.8 0.85
Time (sec)

0.9

o.95

Fig. 7 Same as Fig. 5 for the sniffer 3.

Fig. 5 Level of the sniffer 1 for an ECRH pulse with twogyrotrons (200 ms) and only one (100 ms).

4. Comparison of calculated and measured
rt-=n"6=$ 10le m-3 and /*=Z"s=J ksv.

dataSince the wave propagation from symmetric

We compare the residual power measured by thethree probes with model estimates. The traces of Figs.

launching conditions in the same (co) and in theopposite (counter) magnetic field direction ischaracterized by different values of the parallel
refractive index because of the poloidal fieldcomponents, different levels of the residual power are

2-4 show that about 4 and 8 W are measured by thesniffer l, 0.5 and 1.0 W by the sniffer 2,0.15 and 0.08
by the sniffer 3. The simulations give l0 and 30 W, I
and 4 W, 0.01 and 0.001 for the three antennas,

expected to be measured. In particular, in our examples,respectively, showing an agreement within a factor =2
with the experimental data, which is well within theorder of magnitude. Also for the third shot of Fig. 5-7
the comparison is acceptable. During the first 200 ms ofECRH with two gyrotrons the sniffer I collects about 30W, the second probe 0.5 W and the third one 0.04 W.

at the resonance layer the counter-case parallel index islarger (10.2241) than the co-case one (0.2).
The higher levels of the transmitted power refer tothe rf injection in counter-B direction, away from probes
2 and 3, while the lower traces are measured for co-B
direction, or towards the measuring probes 2 and 3.
In Figs. 5-7 results are shown for a discharge with
Ru=96.7 cm, B*-5.4 T, nu-7.6 10le m-3 and To-3 keV,perpendisular injection. One gyrotron is on for 200 ms,
and the second one for 100 ms.

The statistical model gives residual power levels of
100 W (probe l), I W (probe 2) and 0.009 (probe 3),

respectively with 2 gyrotrons on. The largest
discrepancies are found for the farthest probe, likely due

to the low statistic of the rays with enough power left toreach the probe 90" away from the launchers.
454

Nowak S. et al., Measutements and Modeling of Transmitted Power of EC Waves Propagating for Normal and Oblique Injection

There is a systematic overestimate by a factor =2 ofwith a statistical approach, the wave propagation andthe value of the power coupled to the sniffer probes,

absorption after multiple reflections and polarization
scrambling. Experimantal and calculated data are inagreement within a factor 2. Extension of the theoreticalwhich might be due either to an effective wall
reflectivity somewhat lower than assumed (0.85-0.87),
with zero reflectivity at large wall ports), and/or to anmodel to more complicate geometries could be made.

inadequate modelling of the effects of the upper hybrid

layer on X-mode propagation and absorption.
Improvements on both issues are under way, inparticular on the description of the ray tracing onreaching the upper hybrid. A correction of the averagewall reflectivity is also considered, mostly determinedby a less stringent condition of port reflectivity, still
consistent with the measurements made with the empty

References

tll C. Sozzi et al., High power system for ECRH at
140 GHz, 2 MW, 0.5s on FTU tokamak. Proc. ofthe l3th Topical Conf. on Radio Frequency Powerin Plasmas, AIP 485, 462(1999).
[2] S. Nowak and A. Orefice, Phys. Fluids B 5, 1945(1993); Phys. Plasmas l, 1242 (1994).
[3] S. Nowak, A. Airoldi, A. Bruschi, S. Cirant et al.,vessel.

Measurements of residual power of EC wavespropagating in FTU tokamak during ECRH
experiments at 140 GHz, Proc. of llth Joint

5. Conclusions
In FTU tokamak a set of three low-gain probes
allows during ECRH experiment a continuous
monitoring of EC absorption by collecting the notabsorbed power. A 3D code has been used to follow,

Workshop on ECE and ECRH, 4-8 October 1999,
Oh-arai, Japan.

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