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#**PRE-IONIZATION OF PLASMA ON GOLEM TOKAMAK**
##**Research and first-look**
###FUNDAMENTALS OF PRE IONIZATION AND USE IN TOKAMAKS
The pre-ionization is a process, where first plasma is created by various means in order to introduce the first particles which interact with the electric and magnetic field. These particles are later used for ionization of other particles inside a chamber such as Hydrogen and Deuterium atoms, thus creating the plasma needed for the experiments, fusion and research.
There are many techniques regarding the pre-ionization process. Generally, one tries to deposit enough energy to the working gas inside the chamber, that an electron in the electron shells of the atoms is able to break free, thus creating a mixture of still neutral atoms, electrons and positive ions. In the following ionization process, electrons are the main aspect, however the pre-ionization process can be quite complex, including single particle collisionless heating, neutral particle ellastic and inellastic collisions, Coulomb collisions, $\nabla B$ drifts, curvature drifts and $\vec{E} \times \vec{B}$ drifts [4].
###PRE-IONIZATION TECHNIQUES IN TOKAMAK DEVICES
####Electron Cyclotron Resonance and Upper Hybrid Resonance Heating
One of the widely used methods is Electron Cyclotron Resonance Heating ($\omega_\mathrm{c}$) (ECH) and Upper Hybrid Resonant Heating ($\omega_\mathrm{uH}$). In 1981, experiments including the ISX-B tokamak [1] introduced pre-ionization heating of about 80 kW microwave power at frequency of~35 Ghz for up to 15 ms. Results showed reduction of 40 \% in loop voltage, flux savings of about 30 \% in the first 2 ms and 1.4 times more rapid rise of the plasma current. Experimental configuration consisted of unpolarized microwaves from a reflecting-plate antenna located on the high-field side of the torus. Toroidal magnetic field on axis was about 12.5 kG with electron density of $3 \times 10^{12}~\mathrm{cm}^{-3}$. Configuration is shown in Figure 1.
![Figure 1. Configuration of an early ECH experiment on ISX-B Tokamak in 1981. Source: [1].](ISX-B1.png)
ECH technique was also used in 1991 in DIII-D tokamak [2] with addition of low voltage ohmic heating. At that time, a considerable interest was dedicated to the studies of low voltage startup, since it was proposed that ITER would have limits to values up to $E \leq 0.3~\mathrm{V \cdot m^{-1}}$. It was demonstrated that with ECH, assisted startup of $E \sim 0.15~\mathrm{V \cdot m^{-1}}$ was possible. ECH assisted startup gave improved reliability at such low electric fields and permited operation over an extended range of prefill pressures and error magnetic fields. The primary effect of the ECH was also a decrease of the resistive component of the loop voltage during the plasma current ramp-up, where reduction of about 30 \% was achieved. Experimental configuration was based on ten high power (200 kW), 60 GHz gyrotrons providing a total radio frequency power of 2 MW at source. Experiments were carried out in the vicinity of the fundamental electron cyclotron resonance using extraordinary mode launch from the high field side (HFS) of the tokamak. The same launch scenario was used in the ISX-B tokamak. The dee-shaped cross section with some of the magnetic diagnostic locations are shown in Figure 2.
![Figure 2. Configuration of Ohmic heating (OH), shaping and magnetic diagnostic coils in American DIII-D. Source: [2].](DIII-D1.png)
In 2006, researchers at steady-state superconducting tokamak SST-1 [3] showed that Ion Cyclotron Heating (ICH) and ECH are feasible candidates for pre-ionization, breakdown, heating and current drive experiments.
Experiments on Tore Supra [5] in 2007 were also focused on the problem of low electric field in ITER. The system was based on two 118 GHz gyrotrons, with power up to 700 kW, injected into the plasma as a Gaussian beams by an antenna located on the low field side, using actively cooled mirrors inside the Tore Supra vacuum vessel. These mirrors allowed extensive control of both poloidal and toroidal injection angles. Standard plasma start-up was provided by using extra loop voltage with the use of fast switches and a $0.1~\Omega$ resistor with a conduction time of $\sim 80~\mathrm{ms}$. Resulting peak voltage was around 25 V with the pre-magnetization current of 40~kA, which corresponds to a toroidal electric of $\sim 1.7~\mathrm{V \cdot m^{-1}}$ in the center of the vacuum vessel. In ECH assisted discharge, a successful plasma initiation had been obtained with 275~kW of ECH power and the prefill pressure of 5~mPa. At these conditions, ECH produced reliable breakdowns and allowed for plasma ramp-up down to $0.28~\mathrm{V \cdot m^{-1}}$, while Ohmic heating start-up required the electric field of $0.4~\mathrm{V \cdot m^{-1}}$, regarding the Townsend avalanche theory. At the end, it was again proved, that ECH is a very viable and effective way of plasma start-up, located at the resonance, which rapidly expands inside the vessel.
Another experiments in 2008 including Korean KSTAR [6] reactor were accomplished with positive results on ECH. Experimental setup consisted of one gyrotron at 84 GHz. Toroidal magnetic field coil current was set to 15 kA, giving $B_\mathrm{t} = 1.5~\mathrm{T}$ at second harmonic electron cyclotron resonance at $R_\mathrm{X2} \sim 1.8~\mathrm{m}$. The
available EC power at the output window of the gyrotron was 500 kW, and an EC beam power of up to $350 - 400~\mathrm{kW}$ was launched into the vacuum vessel. The polarization of the EC beam was controlled using two grooved mirrors, allowing pure ordinary (O) or extraordinary (X) modes. In conclusion, ECH pre-ionization and assisted Ohmic start-up had been obtained in the superconducting KSTAR tokamak by second harmonic X-mode waves at a low power value of $P_\mathrm{ECH} \sim 0.35~\mathrm{MW}$. The ECH pre-ionization could provide a reliable breakdown and start-up with a lower loop voltage of 2 V ($\sim 0.24~\mathrm{V \cdot m^{-1}}$) at the innermost vacuum vessel wall.
ECH was also used in 2011 at Frascati Tokamak Upgrade (FTU), which is a Tokamak in Frascati, Italy. One of the reasons for these studies was again the fact, that toroidal electric field on ITER would be only $E \leq 0.3~\mathrm{V \cdot m^{-1}}$. Experimental setup consisted of 0.8 MW of EC power at 140 GHz (resonance at 5T) produced by two of the four gyrotrons of the FTU ECRH system and launched to the plasma as O-mode, first harmonic (O1) by two independent steering mirrors. EC resonance can be seen in Figure 3.
![Figure 3. FTU poloidal field at $t = 0~\mathrm{s}$. Toroidal and poloidal limiters together with the position of the EC resonance are shown. Source: [7].](FTU.png)
One of the key parameters in plasma breakdown is $E / p$ ratio, that must be within a pre-defined range depending mainly on the machine geometry. Sustained breakdown is understood to be when the plasma, formed at breakdown, survives at the peak of radiation losses and the current ramp-up is started. Without ECH, generally lower pressures with higher electric fields are requiered, as with greater pressure comes more radiation losses. Using ECH, one can generally use higher pressures without using greater electric fields. The same was successfully proved with the FTU experiment, as increase by a factor of 4 was achieved regarding the maximum sustainable breakdown pressure. Using 0.4 MW of perpendicular injected EC power a sustainable breakdown was achievable at 9.5 mPa, whereas only 2.3 mPa was achievable using the Ohmic start-up.
####Ultraviolet light emission for Photoionization
Another method, which is used in some cases is the use of UV light for ionization purposes. The work of Martina Žáková [8] (in Czech) is focused primarily on this problem. The results showed that under current conditions, UV pre-ionization in GOLEM Tokamak are not as effective as electron gun.
####Electron Gun / Filaments
Ionization with the use of electron gun is probably the most wildly used method for purposes of pre-ionization, mainly in lower-cost reactors and smaller facilities. The method has some hardly beaten advantages, such as its simplicity, low-cost operation and manufacturing. Principle consists of thermal (or any other emission) of electrons, which are later accelerated by the electric field with pure Ohmic heating. The main disadvantage is the fact that electric field has to be at greater levels with the filling pressure of gas smaller. Also, the device itself is inserted into the Tokamak vacuum vessel, which may create various disruption of plasma and instabilities. For instance, such method is used at ISTTOK Tokamak in Lisbon IST, with the device used in Figure 4.
GOLEM Tokamak currently uses the thermal electron emission method, however with much more less complexity as ISTTOK Tokamak.
###Conclusion
Current pre-ionization scheme for GOLEM Tokamak is the use of thermal electron emission via a filament and a new solution has to be found, either by improving current method, or creating a completely different one. As the UV pre-ionization is deemed not efficient [8], the most probable step would be the introduction of ECH ionization process to the GOLEM Tokamak, and its study. However, as I was able to achieve the same device which is currently used in ISTTOK Tokamak, a study of this technically more cleaner solution also takes place. That would be the goal of my upcoming work.
###REFERENCES
[1] R.M. GILGENBACH, M.E. READ, K.E. HACKETT, R.F. LUCEY, V.L. GRANATSTEIN, A.C. ENGLAND, CM. LORING, J.B. WILGEN, R.C. ISLER, Y-K.M. PENG, K.H. BURRELL, O.C. ELDRIDGE, M.P. HACKER, P.W. KING, A.G. KULCHAR, M. MURAKAMI, R.K. RICHARDS. *Electron cyclotron/upper hybrid resonant pre-ionization in the ISX-B tokamak.* 1981 Nucl. Fusion 21 319.
[2] B. LLOYD, G.L. JACKSON, T.S. TAYLOR, E.A. LAZARUS, T.C. LUCE, R. PRATER. *Low voltage Ohmic and electron cyclotron heating assisted startup in DIII-D.* 1991 Nucl. Fusion 31 2031.
[3] D. Bora, Sunil Kumar, Raj Singh, K. Sathyanarayana, S.V. Kulkarni, A. Mukherjee, B.K. Shukla1, J.P. Singh, Y.S.S. Srinivas, P. Khilar, M. Kushwah, Rajnish Kumar, R. Sugandhi, P. Chattopadhyay, Singh Raghuraj, H.M. Jadav, B. Kadia, Manoj Singh, Rajan Babu, P. Jatin, G. Agrajit, P. Biswas, A. Bhardwaj, D. Rathi, G. Siju, K. Parmar, A. Varia, S. Dani, D. Pragnesh, C. Virani, Harsida Patel, P. Dharmesh, A.R. Makwana, P. Kirit, M. Harsha, J. Soni, V. Yadav, D.S. Bhattacharya, M. Shmelev, V. Belousov, V. Kurbatov, Yu. Belov and E. Tai. *Cyclotron resonance heating systems for SST-1.* 2006 Nucl. Fusion 46 S72.
[4] G.L. Jackson, J.S. deGrassie, C.P. Moeller and R. Prater. *Second harmonic electron cyclotron pre-ionization in the DIII-D tokamak.* 2007 Nucl. Fusion 47 257.
[5] J. Bucalossi, P. Hertout, M. Lennholm, F. Saint-Laurent, F. Bouquey, C. Darbos, E. Traisnel and E. Trier. *First experiments of plasma start-up assisted by ECRH on Tore Supra.* 2008 Nucl. Fusion 48 054005.
[6] Y.S. Bae, J.H. Jeong, S.I. Park, M. Joung, J.H. Kim, S.H. Hahn, S.W. Yoon, H.L. Yang, W.C. Kim, Y.K. Oh, A.C. England, W. Namkung, M.H. Cho, G.L. Jackson, J.S. Bak and the KSTAR team. *ECH pre-ionization and assisted startup in the fully superconducting KSTAR tokamak using second harmonic.* 2009 Nucl. Fusion 49 022001.
[7] G. Granucci, G. Ramponi, G. Calabr\`o, F. Crisanti, S. Nowak, G. Ramogida, O. Tudisco, W. Bin, A. Botrugno, P. Buratti, O. D’Arcangelo, D. Frigione, G. Pucella, A. Romano and FTU team. *Plasma start-up results with electron cyclotron assisted breakdown on Frascati Tokamak Upgrade.* 2011 Nucl. Fusion 51 073042.
[8] Martina Žáková. *Předionizace s pomocí UV lampy.* Online: [http://golem.fjfi.cvut.cz/wiki/TrainingCourses/FTTF/2013-2014/MartZak/index](http://golem.fjfi.cvut.cz/wiki/TrainingCourses/FTTF/2013-2014/MartZak/index). ČVUT FJFI, 2014.
