Diagnostic laser probing of the Alcator Tokamak

I,,/rarnl Phjw\ Vol I’). pp 447 to 454 Prrgamon Press Ltd 1979 Printed m Great Br~tam

DIAGNOSTIC LASER PROBING THE ALCATOR TOKAMAK

OF

JOHN WOLCZOK and BERNARD J. PEYTON Eaton Corporation, AIL Division, Melville, NY 11717, U.S.A Abstract-A 10.6 pm infrared heterodyne receiver with an i.f. response centered at has been employed to measure the spatial, spectral and amplitude distributions of lower-hybrid wave density fluctuations in the Alcator Tokamak. using CO2 laser The heterodyne receiver employs a cooled impedance matching network between Ge:Cu(Sb) photomixer and the SOR i.f. preamplifier, and provides a quantum-noise low as 8.6 dB at if. frequencies between 2000 and 2500 MHz.

2500 MHz the driven scattering. the 12OOQ factor as

INTRODUCTION In an attempt to increase the plasma temperature at the MIT Alcator Tokamak, 50MW of CW microwave radiation is being coupled into the core at a plasma wave resonant a coherent 100 W CO1 laser beam is used to frequency of 2.5 GHz. (l) In addition probe the plasma to determine the ‘depth of the r.f. energy penetration. In support of these activities a wideband infrared heterodyne receiver has been developed and is employed for probing and diagnostic measurements. A simplified representation of the plasma probing technique”’ is shown in Fig. 1. Using a microwave horn, 2.5 GHz radiation is directed toward the plasma. Two CO, laser beams (a 100 W transmitter and a 1 W LO) from a common source are also coupled into the plasma at right angles to the microwave radiation. The two laser beams are adjusted to cross in a small spatial cross section of the plasma.‘3’ The interaction of the microwave radiation and the transmit laser beam in the plasma results in both a spatial and spectral shift in the transmitter laser radiation. The mesurement setup is adjusted so that the shifted laser beam and the unshifted laser LO beam are spatially aligned and fall on a wideband photomixer. The measured intensity of the frequency shifted (2.5 GHz offset) laser beam across the plasma provides information on the r.f. mismatch at the turbulent vacuum plasma boundary layer, and the measured frequency spread of the shifted laser beam provides information on the physics of the boundary layer. The laser beam, used for the plasma probing, permits spatial resolution of about 1 mm. It’should be noted that laser frequency instabilities will not degrade the diagnostic probing measurements because the same laser source is used to provide both the laser signal and LO beams. This paper describes a 10.6pm infrared heterodyne receiver which employs a unique i.f. impedance matching technique which has been designed to optimize the receiver sensitivity at an if. of 2.5 GHz. HETERODYNE

RECEIVER

DESIGN

APPROACH

For the plasma probing measurement application, the infrared heterodyne receiver was designed to operate at 3, = 10.6 pm with an i.f. response peaked at 2.5 GHz and an instantaneous bandwidth of 100 MHz. In addition, the receiver must be capable of detecting weak laser signals in, the presence of high levels of both scattered laser and stray 2.5 GHz microwave radiation. Based on these requirements, the infrared receiver utilizes: -Liquid helium cooled copper doped germanium photomixer. -Cooled 1000-50 Q impedance matching network. -Low noise i.f. preamplifier. --Radiofrequency downconversion network which provides a second, i.f. at 10 MHz. -Fiber optic link for r.f. interference immunity.

JOHN WOLCZOK and

448

BFRNAHD J. PEYTON

IY+

MICROWAVE

-

vLoI

= 2.5 GHz

RADIATION

-A

2.5 GHz

7s

50 kW

“$ (SHIFTED)

“T “LO P=1oow,P=1w

LASER

Fig. I. Plasma

probing

~;UTNSHIFTED)

RADIATION,

X = 10.6 pm

technique.

A block diagram of the receiving system is shown in Fig. 2. The microwave components are mounted in a r.f.i. shielded enclosure which bolts directly to the cryogenic dewar to minimize signal loss between the photomixer matching network and the 2.5 GHz i.f. preamplifier, and minimize extraneous signal interference. A 2.5 GHz mixer is used to translate the shifted radiation to a center frequency of 10 MHz in order to minimize the effects of 2.5 GHz microwave pickup. A 25 ft fiber optic transmission line is employed to couple the 10 MHz radiation to the control room where a spectrum analyzer is used to interpret the measured data.“’ A key element in the receiver design is the photomixer which must be capable of operating in a high radiation environment. A wideband extrinsic photoconductive Ge:Cu(Sb) photomixer was selected for this receiver and its characteristics are summarized in Table 1. The photomixer cutoff frequency (set by the carrier life time, r) is related to the minority carrier antimony (Sb) doping. (4,5) The photomixer i.f. response is flat up to 300 MHz, is 3 dB down at 800 MHz, and is 10 dB down at i.f. = 2500 MHz.

r I

-----

_-----rI -- I-

I

1I

SPECTRUM ANALYZER

_-----

Fig. 2. CO,

laser infrared

heterodyne

1

RFI SHIELDED

receiver.

I

449

Diagnostic laser probing of the Alcator Tokamak Table 1. Ge:Cu(Sb) photomixer characteristics Mixer element Mixer operator temperature, T,, Mixer 3 dB cutoff frequency,f, Power handling capability Carrier lifetime, r Mixer d.c. resistance, R. Quantum efficiency, n Photo gain, 7/T T= the photomixer

Photoconductor, Ge:Cu(Sb) 4.2K 800 MHz 500 mW 2 x lo-r0 set 1220 0 (with LO applied) 0.55 0.011

transmit time.

The receiver sensitivity for an infrared heterodyne photoconductive mixer is given by:@’

receiver which uses an extrinsic

2hvB + 47-m + TI.,.)B

NEP = -

rl

(1)

CC

where NEP = noise equivalent power, defined as the incident laser power at the detector required to produce an output signal-to-noise ratio of one (W/Hz), h= Planck’s constant, v= infrared operating frequency, k= Boltzmann’s constant, Tlr. = effective if. amplifier noise temperature, G= available photomixer conversion gain, C= impedance matching network efficiency, B= if. bandwidth. As can be seen from equation (l), the principal factors affecting the receiver performance are the photomixer quantum efficiency, the if. preamplifer noise (kT&,B), the conversion gain, and the matching network efficiency. The first term of equation (1) includes the limiting quantum noise (hv) in all infrared receivers, the photomixer quantum efficiency which establishes the number of electrons generated per incident photons, and a factor of two due to the internal photomixer noise mechanism.“’ The second term of equation (1) is the Johnson thermal noise (photomixer plus i.f. preamplifier) as effected by the photomixer conversion gain and matching network efficiency. Furthermore, equation (1) can also be expressed as:

where PT = available thermal noise of the photomixer and i.f. preamplifier P,_, = available photomixer generation-recombination noise.

combination,

At r.f. and microwave frequencies, receiver noise is normalized to kT’,B in order to define a noise factor. In coherent infrared and optical receivers which are fundamentally limited by quantum noise, it is appropriate to normalize the NEP to the quantum noise hvB. Thus, the quantum noise factor (QNF) is a figure of merit to describe quantitatively how closely the sensitivity of an infrared receiver approaches the theoretical minimum. The QNF for a photoconductive heterodyne receiver is given by? (3) and for an ideal receiver has a lower limit of unity (OdBm) and is independent bandwidth.

of

The available

photomixer

conversion

gain is given by:“’

where ~1= electronic charge, I’ = mixer bias voltage. (0 = angular i.f. frequency. The (I + co’ T”)~’ term represents the finite photomixer IF frequency response. The photomixer gain should be maximized in an attempt to obtain optimum receiver sensitivity. The photomixer conversion gain can be increased by increasing the applied d.c. bias voltage but the photomixer must be operated in a linear portion of its current voltage (I-V) char~~cteristic. The photomixer i.f. output resistance. RO. is given by:‘“’

(5) where t = photomixcr inter-electrode spacing, 11 = mobility of principal carriers. P,.(> = incident laser local oscillator power

at the photomixer.

The photomixer if. output resistance varies inversely as the square of the photomixer height. Accordingly, a photomixer height of 250pm was selected for this application in an attempt to ease the requirements on the cooled impedance matching network. Substituting equation (5) into equation (4). the photomixer conversion gain can be expressed by :

(6) As indicated in equations (5) and (6), both the photomixer conversion gain and the resistance decrease as the laser LO power is increased. Therefore. a tradeoff is required to select the optimum value of incident LO power which will provide sufficient photomixer gain and a photomixer resistance which can also be efficiently matched to the 50R if. preamplifier. It should also be noted that the photomixer conversion gain increases linearly with increasing applied d.c. bias power (equation 6). The photomixer resistance is typically 12 MR at P,.,, = 0. The incident laser LO power reduces the photomixer resistance to approximately IZOOR and a cooled microwave impedance matching network is used to obtain maximllm power transfer between the photomixer and i.f. preamplifier. MEASUREMENTS

As has been indicated, the photomixer resistance is a key parameter in the heterodyne receiver design. Measurements were performed to obtain values of the mixer rcsistancc as a function of incident laser LO power. (The laser LO spot size is larger than the active area of the photomixer and only one-third of the incident laser LO power strikes the photomixer.) The measured data, given in Fig. 3, show that values of R, appronching 4OOR are obtainable for incident LO power levels of 400 mW. However, the high values required of laser LO power wili reduce the photomixer conversion gain and further increase the liquid helium boil-off rate. A mixer resistance value of R,, : 12OOi2 was selected for a P,_. 2 180 mW and a P,.,. : 100 mW. The total applied power of 280 mW resulted in a 2 I. dewar hold time of greater than 8 hr. The measured dewar hold time is compatible with the typical Toknmak experimental run time.

Diagnostic

laser probing

INCIDENT

LO INDUCED

Fig. 3. Measured

Matching

network

variation

and frequency

of the Alcator

LASER

LO POWER

PHOTOMIXER

of photomixer

response

IN mW

CURRENT

resistance

451

Tokamak

IN mA

with incident

laser LO power.

measurements

After selecting a mixer resistance of R, z 1200R, a cooled microwave matching network which operates near i.f. = 2500 MHz was incorporated between the photomixer and the input to the 50 R i.f. preamplifier for maximum power transfer. A helium cooled impedance matching network which operated between an i.f. of 200 and 1500 MHz was previously developed”’ at this laboratory for a CO2 laser radar application. The matching network, as shown in Fig. 4, is a quarter-wave transformer type microstrip circuit which employs an alumina substrate. The matching circuit is directly connected to the photomixer and designed to operate at liquid helium temperature (T= 4.2K). A kovar housing is employed because of its similarity to the thermal characteristics of alumina, and its low thermal expansion which is important in this application A. EQUIVALENT

CIRCUIT I_

I

I

L*

Ll

1

I

e-

l

I--

21

I

22

I

H 71 Ll Ro = 5OOTO140052

co = 0.035

TO 0.050 pF

L2 Cl

= 3.1 nH

21

= 2oa

= 5.2 nH

22

=29Sl

= 0.05 pF

20

= 5orl

B. PHOTOMIXER/MATCHING

Fig. 4. Cooled

NETWORK

mixer impedance

matching

MOUNT

network.

ZO

TOIF AMPLI FIER 0

452

JOHN WOLCZOK and BERNARD J. P~YTW

2.1

2.5

2.3

IF FREQUENCY IN GHx Fig. 5. Reflectometer

measurement

of impedance

matching

(9.dB return

loss reference).

because of the large amount of temperature cycling that is required between room temperature and liquid helium temperature. Matching network circuit measurements were carried out using a reflectometer technique and resulted in a measured VSWR < 1.3: 1 over a 200 MHz bandwidth centered at 2400 MHz. The measured results are shown in Fig. 5. A reference return loss of 9 dB, corresponding to VSWR z 2.1: 1 is shown in Fig. 5 for comparison. The measured matching network insertion loss is L < 0.5 dB and frequency tuning can be accomplished by adjusting the L, inductor at room temperature. The effective thermal noise of the photomixer-i.f. preamplifier combination has been measured as a function of if. frequency. The results, given in Fig. 6. indicate that the effective thermal noise (and, therefore, the heterodyne receiver sensitivity) is minimum in the 1900-2500 MHz frequency range due to the effects of the cooled impedance matching network. Receiver

sensitivity measurements

The receiver sensitivity (NEP) has been determined using both a noise measurement and a signal-to-noise ratio (SNR) measurement technique. The noise measurement technique consists of irradiating the photomixer with the laser LO and measuring the ratio of the d.c. bias dependent y-r noise and the receiver thermal noise as a function of applied d.c. bias.(6) Equation (2) is then used to calculate the receiver NEP. The quantum efficiency of the photomixer must be accurately known when using the noise measurement technique. The SNR measurement technique(“) utilizes a spectrally broadband blackbody source and a laser LO in a heterodyne configuration. The blackbody measurement technique makes use of the measured signal to noise ratio of the receiver and THERMAL NOISE

IF FREQUENCY IN GHr

Fig, 6. Measured

effective thermal

noise as a function

of IF frequency.

Diagnostic

laser probing

of the Alcntor

Tokamak PDc

IF FREOUENCY

Fig. 7. Measured

uses the following

equation

453

=104mW,V=11.2V

-

BLACKBODY MEASUREMENT

c-r-w

g-rNOlSE MEASUREMENT

IN GHz

heterodyne

receiver

to obtain

sensitivity

the receiver

versus d.c. hius power

NEP:

hva(B7)“2

NEP

=

SNR[exp(/z~/kT)

-

l]

(7)

where SNR = postdetection signal-to-noise ratio, 7 = postdetection integration time, T = blackbody temperature (I 300 K), CI = infra-red receiver transmissivity (0.4). The resultant heterodyne receiver NEP is given in Fig. 7 as a function of i.f. frequency for the two measurement techniques. The effects of the impedance matching network are apparent. As can be seen, an NEP : 2 x lo-l9 W/Hz was obtained at an i.f. z 2.3 GHz f 200 MHz and the data is in good agreement for the two measurement techniques. Improved heterodyne receiver sensitivity and higher photomixer gain are obtained when the dc. bias power is increased (see equations 1 and 6). The maximum applied

DC

Fig. 8. Measured

BIAS POWER heterodyne

IN mW

receiver

sensitivity.

454

JOHN

WOLC-zorc and

BLKNAKU

J.

PCYTON

d.c. bias power

(Pd,c, : 300mW) is fixed by the diameter the photomixer and the impedance matching network. The receiver sensitivity as a function of d.c. bias power is given the receiver NEP increases from 2 x lo- l9 W/Hz (QNF = to 1.35 x lo- l9 W/Hz (QNF = 8.6 dB) for Pd,c, = 250 mW.

of the gold wire between calculated and measured in Fig. 8 and shows that 10 dB) for Pd.‘. = 100 mW

SUMMARY

The development of an extremely high i.f. frequency infrared heterodyne receiver with a quantum noise factor as low as 8.6 dB at i.f. = 2.5 GHz has permitted the CO, laser diagnostic probing measurements of a Tokamak plasma. Initial measurements have resulted in the determination of the r.f. penetration depth, and information which is being used to model the interaction mechanism at the turbulent plasma boundary.“’ The extension of this infrared receiver technology to higher i.f. frequencies ( - 5 GHz) for lower-hybrid r.f. heating experiments at MIT’s Alcator Tokamak are planned for late 1979. In addition, a wideband 1.2 GHz response infrared heterodyne receiver is under development for the remote measurement of the ‘ion temperature’ of the PDX Tokamak at Princeton’s Plasma Physics Laboratory. Acknow/edyrm4nts-This work was supported by the U.S. Energy under a contract with MIT’s National Magnet Laboratory

Research

and

Development

Administration

REFERENCES 1. SLUSHER R. E. et al., Bull. phys. Sot. 23, 765 (1976). Nuclear Fusion Research, Proc. 5th Int. Conf. Tokyo 2. Alcator Group in Plasma Physics and Controlled (1974), IAEA (1975). (1978). 3. SLUSHER R. E. & C. M. SURKO, Php Rev. Lrtt. 40, 4W403 4. PICUS G. S., J. Phps. Chem. Solids 23, 1753~1761 (1962). 5. YARDLEY J. T. & C. B. MOORE, Appl. Phys. Left. 7, 311-312 (1965). Ek~trw~. QE-3. 484492 (1967). 6. ARAMS F. R.. E. W. SARI), B. J. PEYTON& F. P. PA(-~, IEEE J. Quuntm (1958). 7. VAN VLIET K. M., Proc. I.R.E. 46. 10041018 8. SVELTO 0.. P. D. COLEMAN.M. DIDOMENICO JR & R. H. PANT~LL. Appl. Phn. 34. 3182-3 186 (1963). Ektrom QE-9. 669-670 (1973). 9. LANGE R. A.. P. KALISIAK. 1. RUBINSTEIN& F. P. PACI:. IEEE J. Qumtm IO. PEYTON B. J., A. DINARDO. S. COHEN. J. MCEI.ROY & R. COAT~S. IEEE J. Qucrnfum Elrctrotl. QE-1 I. 569-574 (1975).