2011 IEEEINPSS 24th Symposium on Fusion Engineering SP3-19

42GHz O.SMW ECRH system for Tokamaks SST-1

and Aditya B. K. Shukla, Rajiv Goswami, Rajan Babu, Jatin Patel,

Prabal K. Chattopadhyay, R. Srinivasan, Harshida Patel, Pragnesh Dhorajia and ECRH Group

Abstract- A 42GHz ECRH system would be used to

carry out pre-ionization and start-up experiments on Tokamaks SST-I and Aditya. The system would give reliable start-up in SST-I Tokamak at l.ST operating toroidal magnetic field. Fundamental O-mode would be launched from low field side of tokamak. The same system would also be used in Tokamak Aditya to carry out second harmonic ECRH assisted breakdown experiments at 0.7ST operation. The Gyrotron capable to deliver SOOkW power would be installed such that it will deliver power to both the tokamaks without dismantling any component. It will be achieved by using two waveguide switches in the transmission line. First switch will divert power either to dummy load for Gyrotron testing or to launch power in the tokamaks. The second switch will give the option to transmit power either to tokamak Aditya or SSTI. Approximately SO-meter long transmission line will be used to transmit power from Gyrotron to each tokamak. The transmission line consists of a matching optic unit, DC break, mitre-bends, polarizer, 63.Smm ID circular corrugated waveguide and bellows. The total transmission loss in the line is less than 20%, in this case we can launch - 400kW power to carry out reliable ECRH assisted breakdown experiments at fundamental and second harmonic.

The launcher design is different for both the tokamaks. In Aditya, due to space restriction, a simple waveguide type launcher is used to launch ECRH power in X-mode at second harmonic. However, in SST-I tokamak, a conventional ECRH launcher consisting of two mirrors (one focusing and one plane) is used to launch focused beam for breakdown in tokamak. The VME based data acquisition and control (DAC) system would be used for 42GHz ECRH system. The slow interlocks would be activated through software while fast interlocks would be

hardwired to remove the high voltage within lOllS. The paper discusses the physics and technical aspect of

42GHz ECRH system and preliminary design of launchers for SST-I and Aditya.

Keywords-component; Electron Cye/otron Resonance Heating,

Gyrotron, Transmission line, Launcher, Tokamak

I. INTRODUCTION

A reliable breakdown is an important issue in SST! tokamak. The ECRH assisted breakdown ensures reliable start-up over a wide range of tokamak parameters. The existing 82.6GHz/200kW ECRH system for SST-I [1,2] is suitable for breakdown at fundamental harmonic (3.0 tesla operation), at I.5tesla operation it corresponds to second harmonic. Considering total loss in the transmission line -20%, the maximum power can be launched for breakdown in tokamak is - 160kW. This 160kW-launched power is sufficient for breakdown at fundamental harmonic. However

978-1-4577-0668-41111$26.00 ©2011 IEEE

Institute for Plasma Research, Bhat, Gandhinagar, Gujarat 382428 (India)

shukla@ipr.res.in

looking the experiments on second harmonic ECRH breakdown in various tokamaks [3,4,5], it seems that the 160kW power could be at a threshold for I.5T operation in SST-I. In order to avoid any hassle on start-up in SST-I tokamak, a 42GHz ECRH system would be used to achieve reliable start-up at I.5T operating toroidal magnetic field (fundamental harmonic). The system would be used for heating and other ECRH experiments on SSTI.

In addition to reliable breakdown in SST-I, the 42GHz ECRH system would be useful for Aditya tokamak also. At 0.75T magnetic field, the 42GHz system would be used to carry out various ECRH experiment at second harmonic. The system would help in studying the phenomenon of second harmonic ECRH assisted breakdown on Aditya. The schematic of 42GHz ECRH system in Aditya and SST-I tokamaks is shown in "Fig. 1".

SST· 1

i\DITYi\

SWITCII·2

-<u' CORRUGi\TED TRi\NSMJSSION LINE SYSTEM

SWITCII·I

DUMMY LOi\D

42Gllz GYROTRON

Figure 1 (Schematic of 42GHz ECRH system in Aditya and SSTl)

The system would generate the detailed database on fundamental and second harmonic ECRH assisted breakdown on tokamak SST-I and Aditya. Since the system is mainly required to carry out breakdown and start-up experiments on SST-I and Aditya, long pulse CW system is not preferred. A Gyrotron system capable to deliver 500kW power at 42GHz for - 500milliseconds would be adequate to carry out above­mentioned experiments.

II. 42GHzECRH SYSTEM FOR SST-1 AND ADITYA

A. ECRH system/or SST-J (Fundamental Harmonic):

The 42GHz ECRH system corresponds to fundamental harmonic at loST toroidal magnetic field on tokamak SST-I. At fundamental harmonic, the first pass absorption is better, so even 200kW power at fundamental harmonic would be enough for reliable breakdown in SST-I.

The maximum power required for breakdown in SST-1 at fundamental harmonic can be calculated using reference [6]. In ECR region electron gets energy in order of keY. These high­energy electrons become uncoupled with the neutral. So in this case it is needed to estimate an upper boundary on required microwave power for successful breakdown. For ECR assisted breakdown, the microwave E field should be less than [6]:

Eoev / cm) < 5.85xlO-27 (Oo (bRBTlaSH Y' 2

Where b is minor radius (cm), R is major radius (cm), B is magnetic field (Gauss), no is neutral density, SH is ionization rate coefficient and lQ) is frequency of microwave. For SST-1 tokamak, the parameters are: (b=20cm, R=llOcm, B=15kG, neutral density at 5xlO-5torr is 1.7xlO12/CC, the ionization rate coefficient at 1keV electron temperature - 2xlO-8cm3/s and frequency 42GHz). The upper bound of microwave electric field for SST-1 tokamak E is - 1835 V/cm.

At this E field (1835 V/cm), the energy of an electron at (() = (()ce layer can be calculated as:

The electron energy at E - 1835 V/cm is - 5.6keV. This will generate many runaway electrons, which would not couple to the neutrals for the reliable breakdown. So in order to avoid the generation of runaway electrons, the wave E field should be less than 1835 V/cm. As the ionization rate coefficient is -2xlO-8cm3/s at 1keV electron temperature. So in order to achieve the 1keV electron temperature E should be - 140V/cm.

The wave electric field can be considered - 300 V/cm and at this field (E-300V/cm) the electron will get energy -1. 67keV. Considering the total beam area at ECR location -0.10 m2, the ECH power required for breakdown is - 120kW. Hence the Gyrotron capable to deliver 500kW is suitable for ECRH assisted breakdown in SST-I.

B. 42GHz system/or Aditya (Second harmonic):

In addition to reliable breakdown in SSTl at fundamental harmonic, the same 42GHz system would be used in Aditya for various experiments at second harmonic ECRH assisted breakdown and start-up at 0.75T operating troidal magnetic field. On Aditya, other various experiments like heating and MHD would also be carried out using second harmonic electron cyclotron resonance heating.

The study on second harmonic breakdown in Aditya using 42GHz system is being done and preliminary calculation

indicates that the launched power - 300kW would be adequate to carryout successful second harmonic ECRH breakdown experiment on tokamak Aditya. The ionization Growth rate [3] at second harmonic ECRH breakdown is compared for tokamak Aditya (300kW power at 42GHz) with other machines (�Aditya/�DIllD(60GHz) -1.5, �Aditya/�DlIID(llOGHz) 0.81, �Aditya/�KSTAR = 0.82 and �Aditya/�T-lO = 0.36). As the Growth rate for tokamak start-up with second harmonic ECRH breakdown in Aditya is comparable with other tokamaks, it is anticipated that 42GHz / 500kW system would give reliable ECRH assisted breakdown in Aditya at second harmonic.

III. TECHNICAL PARAMETERS OF 42GHz ECRH SYSTEM

The 42GHz ECRH system consists of a high power microwave source (Gyrotron) and -75 meter long transmission line to launch power in SST-I and Aditya. The quasi-optical launchers are used to launch power in the tokamaks. The technical details of Gyrotron and transmission line are following:

A. Microwave Source (Gyrotron):

The Gyrotron delivers 500kW power at 42GHz, the detailed features of Gyrotron are mentioned in Table 1.

TABLE 1 (SPECIFICATIONS OF GYROTRON)

Parameters Values

Frequency 42GHz

Power 500kW

Pulse duration 500ms

Type of Gyrotron Depressed collector type Gyrotron

Gyrotron with internal mode Yes

converter

Gyrotron output HEll (Gaussian output)

Mode Purity after MOO >95%

Efficiency better than 45%

Magnet of Gyrotron Single Cryomagnet

Gyrotron with external Suitable to connect the waveguide

matching optic unit

B. Transmission Line system:

The transmission line for tokamak Aditya and SST-I consists of 63.5mm diameter corrugated waveguides, mitre-bends with bi-directional coupler, ordinary bends, polarizer, bellows and DC break. The length of transmission line for one tokamak is - 50 meter, while for other tokamak is - 25 meter. The transmission line consists of two waveguide switches. The first switch diverts power in the dummy load for Gyrotron testing, while second switch gives option to launch power either in SST-I or in Aditya. The total loss in the transmission line is less than 20%. The layout of the ECRH system in SST-1 and Aditya tokamak is shown in "Fig. 2".

82.6GHz G�H(

... To Adilya tcl<amak

--"1OlIO,. Aditya Hall

Figure 2 (42GHz system layout in tokamak Aditya and ST-I)

IV. SUBSYSTEM FOR 42GHZ GYROTRON

The subsystems of Gyrotron like power supplies, data acquisition and control (DAC) , mechanical (cooling and transmission line support) and low power microwave for diagnostics are finalized and in advance stage of procurement. The power supplies and DAC part is explained below:

A. Power Supplies for 42GHz Gyrotron:

The Gyrotron delivers 500kW power at 42GHz at - -5SkV beam voltage and - 20A beam current. A regulated high voltage power supply (RHVPS) SOk V lISA is in advance stage of commissioning. Another high voltage power supply (+30kV/IOOmA) for anode is also procured. The cryomagnet power supply (l00A-5V) is available for the cryomagnet. A filament power supply is also finalized and under procurement.

B. Data Acquisition and Control (DAC) system:

A VME based data acquisition and control (DAC) would be used for 42GHz ECRH system. Around 20 analog signals would be used for remote monitoring and setting of auxiliary power supplies. Approximately 30 digital (TTL and PFC) signals would be used for status monitoring and control of supply parameters. The monitoring and interlock of cooling parameters would be done through Data Logger. All slow interlocks would be executed through VME and operated within lOOms however the fast interlocks (like arc, beam over current and dIldt etc.) are hardwired and operates within lOlls.

V. LAUNCHERS FOR SST-l AND ADITYA

The launchers are designed based on quasi-optical Gaussian beam theory. The output of transmission line is 63.5 mm diameter waveguide. The beam waist radius (lie) at the mouth of waveguide is 14.432mm. The microwave beam emerges out from waveguide diverges as per the Gaussian beam, which is focused at plasma center using focusing mirror. The launcher details separately for SST-l and Aditya is mentioned below:

A. Launcher for Aditya:

In Aditya, the plasma is close to radial port, the distance between plasma center and radial port is - 350mm. In this case the beam divergence will not be more (lie beam radius at the plasma center is - 42mm). Due to space constraint near tokamak radial port, it is difficult to install mirrors to focus the microwave beam. So ordinary waveguide connected with the help of boron nitride window would be used as a launcher for Aditya. The schematic of launcher showing the beam trajectory inside the tokamak is shown in "Fig. 3".

Plasma Center

Window

HE 11 CORRUGATED \

Figure 3 (ECRH launcher in Aditya)

B. Launcher for SST-l:

In SST-I, the distance between plasma center and radial port is - 1100mm. In this case the divergence is significant and focusing and plane mirror combination is used to transmit the microwave power in the tokamak. The distance between plasma center and focusing mirror is - 1300mm. The lie beam radius at the plasma center is - 40mm. The focal length of focusing mirror is - 642mm. The tentative mirror size of launcher is - 240mm x lS0mm. The material for the mirror is SS304L I OFHC Copper. The schematic of ECRH launcher for SST-l is highlighted in "Fig. 4".

Window

Figure 4 (ECRH launcher in SST-I)

VI. CONCLUSION

The 42GHz ECRH system would be used in SST -1 and Aditya tokamak to carry out ECRH assisted breakdown. The system would give reliable breakdown in SST-1 at l.ST operation (fundamental harmonic) and in Aditya at O.7ST operation (second harmonic). The transmission line consists of two waveguide switches to launch power either in SST-l or in Aditya without dismantling any system. In Aditya tokamak, the launcher is simple waveguide connected to tokamak with the help of a boron nitride window, while SST-11auncher consists of focusing and plane mirror combination to launch focused beam. The power supplies are in advance stage of commissioning. The VME based DAC system is tested and commissioned with dummy signal.

The system would generate detailed database for ECRH assisted breakdown at fundamental and second harmonic in tokamak SST -1 and Aditya.

REFERENCES

[l] D. Bora, K. Sathyanarayana, B.K.Shukla et. al. "Cyclotron Resonance Heating systems for SST-I" Nuclear Fusion 46 (2006) sn - S84

[2] D Bora, K Sathyanarayana, B K Shukla, Prabal Chattopadhyay, Y S S Srinivas, et a!. "Test and Commissioning of 82.6 GHz ECRH system on SST-I" Journal of Physics 25 (2005) 96-102

[3] Jackson G L et. a!. "Second Harmonic Electron Cyclotron pre­ionizatioon in DIII-D Tokamak" Nuc!. Fusion 47, 2007, 257-263

[4] Bae Y S et. a!. "ECH pre-ionization and assisted start-up in the fully superconduction KSTAR tokamak using second harmonic" Nuc!. Fusion 49,2009,022001 (5pp)

[5] Kirneva N A et. a!. "Plasma Start-up oprtimization with 2nd harmonic ECR pre-ionization in T-IO tokamak" 34th EPS conference on Plasma Phys. Warsaw, 2-6 July 2007 ECA Vo!. 31F, P-I, 164 (2007)

[6] Peng M. et. ai, "Microwave start-up of tokamak near electron cyclotron and upper hybrid resonances" Nuclear fusion 18 11 (1978) 1489.